Friday, July 9, 2010

ENHANCEMENT-MODE MOSFETs

An enhancement-mode MOSFET is normally off because it requires a gate bias signal to cause current flow because of the high impedance of its substrate source-to-drain channel.In the N-channel enhancement-mode MOSFET shown in Fig. 1a, the substrate is P-type silicon and both the source and drain regions are heavily doped N-type silicon. The metal gate, the insulation layer, and the channel act like a capacitor, so if a bias is placed on the gate, a charge of opposite polarity will appear in the channel below it. For example, if the drain voltage is positive with respect to the source voltage, and the bias on the gate is zero, no current will flow.

But, if the gate is then made positive, negative charge carriers (electrons) are induced in the channel between the source and drain regions. Further increases in positive bias induce more electrons into the channel, where they accumulate to form an N-type channel between source to the drain. The value of drain current depends on channel resistance, so gate voltage controls drain current. Because channel conductivity is enhanced by a positive gate bias, the transistor is called an enhancement-mode MOSFET.

Figure 1b shows the schematic symbol for an N-type enhancement-mode MOSFET. The vertical line connected to the gate pin represents the gate, and the broken lines connected to the drain and source pins indicate that a channel does not exist until a gate voltage is applied. The arrowhead representing conventional current points from the P-type substrate to the induced N-type channel

A P-channel enhancement-mode MOSFET has the same geometry as the N-channel enhancement-mode MOSFET except that both the material dopants and the applied voltage polarities are reversed. Its schematic symbol is identical except that the direction of the arrowhead is reversed.


Figure 1 Enhancement-mode N-channel MOSFET: (a) section view, and (b) symbol.





METAL-OXIDE SEMICONDUCTOR FETs (MOSFETs)

The metal-oxide semiconductor FET (MOSFET) offers a higher input impedance than a JFET. A section view of an N-channel MOSFET is shown in Fig. 1a. An insulating layer of silicon dioxide is grown on top of the region between the N-type source and the N-type drain. The gate is electrically isolated from the source and gate contacts and the source to-drain channel beneath it. The schematic symbol for an N-channel MOSFET is shown in Fig. 1b. The two kinds of MOSFETs are enhancement mode and depletion mode. The depletion-mode MOSFET has a lightly doped source-to-drain channel, whereas the enhancement-mode version does not.



Figure 1 Metaloxide semiconductor FET (MOSFET): (a) section view, and (b) symbol.

JUNCTION FETS (JFETs)

The N-channel junction FET (JFET), shown in section view Fig.1a, has an N channel diffused into a P-type substrate and a P-type region diffused or implanted into the N channel to form the P-type gate. Metal deposited directly on the gate, source, and drain regions forms their contacts. Because a JFET has a symmetrical structure, the drain and source are interchangeable. Thus, depending on the location of the ground and the +V power source, the JFET will work in either direction.



Figure 1 Junction field-effect transistors (JFETs): (a) N-channel section view

If a positive voltage is applied at the drain contact and a negative voltage is applied at the source contact with the gate contact open, a drain current flows. If the gate is then biased positive, channel resistance decreases and drain current increases. However, if the gate is biased negative with respect to the source, the PN junction is reverse biased and a depletion region depleted of charge carriers is formed. Because the N-type channel is more lightly doped than the P-type silicon, the depletion region penetrates into the channel, effectively narrowing it and increasing its resistance. If the gate bias voltage is made even more negative, drain current is cut off completely. A gate bias voltage value that will cut off the drain current is called the pinch-off or gate cutoff voltage. The schematic symbol for an N-channel JFET is shown in Fig. 1b. The arrow points from the P-type gate to the N-type channel.


(b) N-channel symbol, (d) P-channel symbol

The P-channel JFET, shown in Fig. 1c, has characteristics similar to those of the N-channel JFET except that the polarities of the voltage and current are reversed. A P-type channel is diffused into an N-type substrate and then an N-type gate region is diffused or implanted into the P-channel to form the N-type gate. If a negative voltage is applied to the drain and a positive voltage is applied to the source, current flows between source and drain. But if the gate is made more negative more current will flow, while if it is made positive with respect to the source, current will be cut off.


(c) P-channel section view

The schematic symbol for a P-channel JFET is shown in Fig. 1d. The arrow points from the P channel to the N gate.

Field-Effect Transistors

A field-effect transistor (FET) is a voltage-operated transistor. Unlike a BJT, a FET requires very little input current, and it exhibits extremely high input resistance. There are two major classes of field-effect transistors: junction FETs (JFETs) and metal-oxide semiconductor FETs (MOSFETs), also known as insulated-gate FETs (IGFETs). FETs are further subdivided into P- and N-type devices. FETS are unipolar transistors because, unlike the BJT, the drain current consists of only one kind of charge carrier: electrons in N-channel FETs and holes in P-channel FETs.

FETs and MOSFETs are both made as discrete transistors, but MOSFET technology has been adopted for manufacturing power FETs (see “Power Transistors” later in this section) and ICs. There are both NMOS and PMOS ICs. When both P- and N-channel MOSFETs are integrated into the same gate circuit, it is a complementary MOS (CMOS).

Darlington Transistor Pairs

A Darlington pair, as shown in the schematic Fig.1, is a pair of BJTs in which the emitter of the first transistor is connected to the base of the second transistor. This configuration provides far higher current gain than a single transistor through direct coupling. The pair can be made on a single die and it is packaged in a three-terminal transistor case. The pairs are often used in linear ICs, such as operational amplifiers, and in power amplifier output stages. Its most common application is that of an emitter follower. The output is taken across a resistor from the emitter of the second transistor to ground. The input resistance at the base of the first transistor is raised to a higher value than that of a singletransistor emitter-follower circuit.



Figure 1 Darlington transistor pair symbol.

Tuesday, July 6, 2010

Bipolar Junction Transistors (BJTs)

The term transistor implies a silicon bipolar junction transistor (BJT) unless modified by an adjective such as JFET or MOSFET. BJTs can be can be made in two different configurations: NPN and PNP. Figure 1 shows a section view of an NPN BJT transistor. Here the letter N indicates silicon doped with an N-type material, which, by convention, means that it contains an excess of negatively charged electrons. The letter P indicates silicon doped with a P-type material, which means it has an excess of positively charged holes.


Figure 1 NPN bipolar junction transistor (BJT) structure.

A voltage applied to the P-type base in the NPN transistor causes electrons to flow from the N-type emitter through the base to the N-type collector. (Conventional current is considered to flow in the opposite direction). This BJT has vertical topology, so its metal base contact is deposited on the P-type base next to the metal emitter contact on the N-type emitter, while the collector contact is a metal layer on the bottom of the N-type collector.

Electrons in an NPN transistor cannot flow from the emitter to the collector through the P-type base unless a positive bias is placed on the base contact and a positive voltage is applied to the collector contact. Then holes, repelled by the positive bias, enter the emitter region while electrons flow from the emitter region to the base region. Most of the injected electrons complete the transit through the base region into the N-type collector region and are collected at its contact.



Figure 2 NPN bipolar junction transistor: (a) section view, and (b) symbol.

Figure 2a shows a simplified section view of the NPN BJT, and Fig. 2b shows its schematic symbol. The direction of the arrow represents conventional current flow directed from its P-type base to its N-type emitter.



Figure 3 PNP bipolar junction transistor: (a) section view, and (b) symbol.

Figure 3a shows a simplified section view of a PNP BJT, and Fig. 3b shows its schematic symbol. It can be seen that the polarities and doping of NPN and PNP transistors are reversed. The PNP BJT schematic symbol has its arrow directed from its P-type emitter to its N-type base.

Monday, July 5, 2010

Signal-Level Transistors

A transistor is a three-terminal semiconductor device capable of amplification and switching. It is essentially the solid-state analogy of the triode vacuum tube. There are two principal classes of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs). These transistors are made as discrete small-signal and power devices. Variations of them are integrated into digital and analog or linear ICs. Small-signal discrete BJTs remain popular in low-frequency circuits, while small-signal discrete FETs meet the requirements for high-input impedance transistors. Discrete power BJTs are still popular in low-frequency and linear circuits, but discrete metal-oxide semiconductor (MOSFET) transistors are preferred for high-frequency switching.

Rectifier Diodes

A rectifier diode is a diode capable of converting AC into DC. It can conduct 1 A or more or dissipate 1 W or more of power. Most rectifier diodes are now made from silicon. The dies have large PN junctions to eliminate or minimize damage from heat produced by power dissipation. Typically packaged as discrete devices, the rectifiers can be paralleled to increase their power-handling ability. Rectifiers rated for less than 6 A are usually packaged in axial-leaded glass or plastic cases. However, those with 8- to 20-A ratings are usually packaged in flat plastic cases with copper tabs that can act as heat sinks or metal-to-metal interfaces with larger heat-dissipating busbars. Rectifiers rated from about 12 to 75 A are usually packaged in metal cases. Some have threaded base studs for fastening the case directly to a larger heat-dissipating surface.

The most important electrical ratings for rectifier diodes are:
  • Peak repetitive reverse voltage VRRM
  • Average rectified forward current IO
  • Peak repetitive forward surge current IFSM
Standard PN junction rectifiers are specified for linear power supplies operating at input frequencies up to 300 Hz, but they are inefficient in switching power supplies that switch at frequencies of 10 kHz or higher because of their slow recovery time. This is the finite amount of time required for the minority and majority carriers—electrons and holes—to recombine after a polarity change of the input signal. The minority carriers must be removed before full blocking voltage is obtained.

Despite their slow recovery time, standard PN junction rectifiers have lower reverse currents, can operate at higher junction temperatures, and can withstand higher inverse voltages than faster rectifiers designed to overcome this speed limitation.

Three types of fast silicon rectifiers perform more efficiently at the higher-frequency switching rates:

  1. Fast-recovery rectifiers.
  2. Ultrafast- or superfast-recovery rectifiers.
  3. Schottky rectifiers.

FAST-RECOVERY RECTIFIERS

A fast-recovery rectifier is a PN junction rectifier made by diffusing gold atoms into a silicon substrate. The gold atoms accelerate the recombination of minority carriers to reduce reverse recovery time. These rectifiers can be switched in 200 to 750 ns. They have current ratings of 1 to 50 A and voltage ratings to 1200 V. Forward voltage drop is typically 1.4 V, higher than the 1.1 to 1.3 V of the standard PN junction. The maximum allowable junction temperature is about 25°C. This value is lower than that for a standard PN junction. The maximum reverse voltage for a fast-recovery rectifier is about 600 V.

ULTRAFAST- OR SUPERFAST-RECOVERY RECTIFIERS

An ultrafast- or superfast-recovery diode is a PN junction rectifier whose reverse recovery time is between 25 and 100 ns. Gold or platinum is also diffused into the silicon wafers from which the rectifier is made to speed up minority carrier recombination. These rectifiers are specified for power supplies with output voltages of 12, 24, and 48 V.

SCHOTTKY RECTIFIERS

A Schottky rectifier has a metal-to-semiconductor junction rather than a PN junction, so it does not have minority charge carriers. The die is in direct contact with one metal electrode, so recovery time, although not specified, is typically less than 10 ns. Recovery current is principally caused by junction capacitance. Schottky rectifiers provide lower forward voltages (VF) than the PN rectifiers (0.4 to 0.8 V vs. 1.1 to 1.3 V). Hence power dissipation is lower and efficiency is higher. One drawback of the Schottky rectifier is its low blocking voltage, typically 35 to 50 V. However, Schottky rectifiers with maximum blocking voltages of 200 V are available. These rectifiers require transient protection, and they have inherently higher leakage current (IRRM) than PN junction rectifiers. This makes them more susceptible to destruction by overheating (thermal runaway). Schottky rectifiers can be paralleled in the output stages of switching power supplies, where they are usually used with output terminals rated for 5 V or less.

VARACTOR DIODES

A varactor diode, also known as a voltage-variable capacitor diode or varicap, is a reversebiased PN junction whose operation depends on the variation of junction capacitance with reverse bias. Special dopant profiles are grown in the depletion layer to enhance this capacitance variation and minimize series resistance losses.

The varactor is made from a semiconductor material whose dopant concentration is graded throughout the device, with the heaviest concentration in the regions adjacent to the junction. The junction region is small to take advantage of the variation of junction capacitance with reverse voltage. Varactor diodes have very low internal resistance so that the PN junction, when reverse biased, acts as a pure capacitor. Because the junction is abrupt, junction capacitance varies inversely as the square root of the reverse voltage.

Most varactor diodes are made from silicon, but gallium-arsenide varactors offer higherfrequency response. Low-power varactors serve as voltage-variable capacitors in electronic tuners, and do phase shifting and switching in the VHF and microwave circuits. They also function as very low frequency multipliers in solid-state transmitters and do limiting and pulse shaping.

Standard varactors can provide 12 W at 1 GHz, 7 W at 2 GHz, 1 W at 5 GHz, and 50 mW at 20 GHz. Efficiencies of 70 to 80 percent have been obtained at 1 and 2 GHz. The dimensions of a varactor’s package depend on its operating frequency and power dissipation.

SCHOTTKY BARRIER DIODES

A Schottky barrier diode is a semiconductor diode formed by a semiconductor layer and a metal contact that provides a nonlinear rectification characteristic. Hot carriers (electrons for N-type materials or holes for P-type materials) are emitted from the Schottky barrier of the semiconductor and move to the metal coating that is the diode base. Majority carriers predominate, but there is essentially no injection or storage of minority carriers to limit switching speeds. These diodes are also called hot-carrier or Schottky diodes.

Schottky-clamped transistors used in some transistor-transistor logic (TTL) IC families include Schottky barrier diodes to prevent transistor saturation, thereby speeding up transistor switching. Also, the gates of gallium-arsenide MESFET transistors are actually Schottky barrier diodes.

ZENER DIODES

A zener or reference diode is a silicon PN junction made to operate only under reverse bias or voltage conditions. At a known reverse voltage an avalanche breakdown occurs, indicated by the knee in the curve shown on the left side of Fig. 1a. Beyond that point the reverse voltage remains constant enough to serve as a useful reference voltage. Zener diodes exhibit sharp reverse knees at less than about 6 V. Large quantities of electrons within the depletion region break the bonds with their atoms, causing a large reverse current to flow, as indicated by the vertical dropoff of the curve.



Figure 1 Characteristic curves for a PN diode: (a) forward bias (right) and reverse bias (left), and (b) symbol for zener diode.


Zener diodes are stable voltage references because the voltage across the diode remains essentially constant for wide variations of current. These diodes are used as generalpurpose voltage regulators and for clipping or bypassing voltages that exceed a specified level. Variations of the zener diode called transient voltage suppressors (TVSs) serve as circuit-protective devices because of their ability to bypass unwanted high-input voltage transients.

The schematic symbol for a zener diode is shown in Fig. 1b. It differs from the conventional diode schematic symbol because of its S-shaped anode representation. Zener diodes have nominal reference voltage values from 1.8 to 200 V and power ratings from 250 mW to as high as 50 W. They are packaged in a variety of glass, metal, and plastic cases, some for surface mounting. TVS diodes have ratings from 5 to 300 V, and can handle up to 5 W steady-state or 1,500 W peak power. Although both of these diodes can operate in the small-signal region, they are considered to be regulator and suppressor diodes rather than small-signal diodes.

Small-Signal Diodes

A small-signal diode is a two-terminal silicon PN junction that can rectify and clip signals. Rated to handle up to 1 W, these diodes are made by growing an N-type region on a P-type wafer so that there is a direct interface or junction between the two different materials. The wafer is then diced and packaged with terminals attached to both sides of the die. The P-type material is the anode and the N-type material is the cathode, as shown in the section view Fig.1a. The P-type anode contains a surplus of “holes,” or vacant sites that can be filled by electrons to conduct current, and the N-type cathode contains a surplus of electrons. The schematic symbol for a diode is shown in Fig.1b. The arrowhead indicates the direction of conventional current flow, but this is opposite to electron flow, indicated by the arrow pointed in the opposite direction.



Figure 1 PN diode: (a) functional diagram, and (b) schematic symbol.

If a positive voltage is applied to the anode and a negative voltage is applied to the cathode, or it is connected to ground, the diode is forward biased. Electrons flow from the cathode across the PN junction to the anode, but conventional current is considered to flow in the opposite direction. However, if a negative voltage is applied to the anode and a positive voltage is applied to the cathode, or it is connected to ground, the diode is reverse or back biased, as shown in Fig. 2. Under these conditions there will be little or no electron flow across the PN junction. A reverse-biased diode effectively becomes an insulator with resistance measurable in megohms because of the expansion of the highly resistive depletion region that forms around the PN junction.



Figure 2 Depletion region of PN junction diode.


Surface Acoustic Wave (SAW) Filters

A surface acoustic wave (SAW) filter is a solid-state filter that can replace a conventional passive inductive-capacitive LC filter. It offers excellent amplitude and phase response over wide bandwidths and frequency ranges. SAW filters are made from piezoelectric materials such as lithium niobate (LiNbO3) and quartz. A filter made from quartz offers excellent temperature stability over wide temperature ranges, and a lithium-niobate filter simplifies electromagnetic-to-acoustic coupling. These filters have relatively high insertion losses, so they typically require an amplifier in series with the SAW to recover lost signal strength.



Figure 1 Crystal in holder.

Crystal Frequency Standards

Crystals used as frequency standards are made from piezoelectric materials that resonate at high frequencies when subjected to an alternating current. Selectively cut quartz crystals generate more stable frequencies than coil-and-capacitor tank circuits. Crystals for generating frequencies for timing or other purposes are packaged in radial-leaded metal cases, as shown in Fig. 1.

Quartz wafers are ground to precise thicknesses, and metal-film electrodes are deposited on both sides. The electrodes are connected to the leads that extend through the base. When powered by AC, the quartz wafer vibrates at a frequency determined by its thickness. Thin crystals resonate at higher frequencies than thick crystals. The highest fundamental frequency of a quartz crystal wafer is 15 to 20 MHz. Harmonics or multiples of this frequency provide higher radio frequencies. Quartz crystals in holders serve as oscillator tank circuits. Crystals can also serve as selective filters because of their high Q factors.

Wednesday, June 30, 2010

Power Supply Filters

A power supply filter is a passive filter for linear or switching power supplies to smooth ripples or pulsations in the raw DC output. A line filter, as shown in Fig. 1, suppresses RF interference (RFI) induced into or transmitted on the AC power line or induced into or conducted from within the host product. These filters are required in products powered by switching power supplies, such as personal computers, that must comply with Federal Communications Commission (FCC) regulations limiting EMI/RFI above 10 kHz.



Figure 1 Line filter for a power supply.

CHARACTERISTIC FILTER CURVES

The characteristic curves of the four basic types of filters are shown in Fig. 1. The frequency values on the horizontal axes are typical operating frequencies for the filters shown, and the positions on the curves labeled fC are the cutoff frequencies.




Figure 1 Filter characteristics:

(a) low-pass filter, (b) high-pass filter,
(c) bandpass filter, and (d) band-reject filter.


Passive Filters

A passive filter is a network of resistors, capacitors, and inductors configured to pass specific frequency bands while suppressing others. The upper and lower limits of the band are called cutoff frequencies. Filters are designed so that their input and output impedances match their source and load impedances. Roll-off or attenuation at the cutoff frequency is measured in decibels.A filter with high attenuation has a steep roll-off curve that is nearly a vertical slope.

Filters are configured by connecting capacitors and inductors in networks, and their schematics suggest letters or other familiar symbols. The four most common configurations are the L, T, pi, and ladder. The positions of the elements are determined by the desired function of the filter (e.g., low pass or high pass). The L filter schematic is shaped like an inverted letter L, and the T filter is shaped like the letter T. The pi filter schematic looks like the Greek letter π, as shown in Fig. 1, and the ladder filter looks like a ladder.

All capacitors can pass AC, and high frequencies pass with less opposition than low frequencies. (Capacitive reactance is inversely proportional to frequency.) But because a capacitor has conductive plates separated by an insulating dielectric, DC is completely blocked. By contrast, inductors, basically coils of wire, easily pass DC and very low frequency AC, but their ability to oppose AC is directly proportional to frequency because inductive reactance is proportional to frequency. Thus, passive filters exploit the frequency-response characteristics of capacitors and inductors.



Figure 1 Pi filter for a power supply.

Friday, June 25, 2010

FILTER DESIGNATIONS

  • The constant-k filter is so named because the product of its series and parallel impedances remains a constant designated k at all frequencies. These impedances can be inductive or capacitive reactances. A constant-k filter can be configured as any of the basic filter types.
  • The m-derived filter is a modified form of a constant-k filter based on a constant called m, the ratio of the cutoff frequency to the infinite attenuation frequency. An m-derived filter exhibits a sharper attenuation or roll-off curve than a constant-k filter because it has more poles. It can also be configured as any of the basic filter types.
  • The Butterworth filter exhibits an essentially flat ripple response in the passband and a sharp attenuation or roll-off curve at its cutoff frequency. It has a wide operating frequency range that extends from DC into RF. These filters can be configured as low-pass, high-pass, and bandpass. Their transient responses are much better than those of Chebyshev filters.
Filters can be identified by one or more of the following classifications:

  • The Chebyshev filter has characteristics that are similar to those of the Butterworth filter, but it trades off higher amplitude ripple response to obtain an even sharper frequency roll-off curve at its cutoff frequency. Because these are constant-k filters, they can be configured as low-pass, high-pass, and band-reject.
  • The Bessel filter is named for the mathematical functions used to design it. Its frequency cutoff characteristics are not as sharp as those of the Butterworth filter.
  • The elliptical filter is similar to a Chebyshev filter, but its passband contains even higher amplitude ripple response.
  • A filter can be further characterized by its number of poles, as determined by the number of reactive components (inductors or capacitors) within the filter. (Resistors do not count as poles because they are not reactive.) The steepness of the attenuation curve or roll-off is determined by the number of poles. For example, a six-pole filter has a steeper attenuation curve than a two-pole filter.

Filters

A filter is a circuit that passes certain frequencies while suppressing others. This property isuseful for eliminating unwanted frequencies and separating wide frequency bands into multiple channels. A passive filter does not require a power source, but because it dissipates input power it cannot provide either current or voltage gain. Moreover, it has a limited frequency range. Signal loss caused by filtering with a passive filter is called insertion loss.

By contrast, an active filter can perform the same functions as a passive filter, but it can perform those functions over a wider frequency range, and it can provide current or voltage gain. Although an active filter requires a power source, it does not need a bulky inductor.

BASIC FILTER TYPES

There are four basic types of filter:

  1. A low-pass filter can pass all frequencies from zero to its cutoff frequency, and block all frequencies above the cutoff.
  2. A high-pass filter can block all frequencies below its cutoff frequency, and pass all frequencies above the cutoff. Its response is the inverse of the low-pass filter.
  3. A bandpass filter can pass all frequencies within a band defined by lower and upper cutoff frequencies, and block all frequencies above and below that band.
  4. A band-reject or notch filter can block all frequencies between its lower and upper cutoff frequencies, and pass all frequencies above and below that band. Its response is the inverse of the bandpass filter.

POWER TRANSFORMERS

A power transformer can transform 50- to 60-Hz AC line power to voltages suitable for rectification to regulated DC. They are made in volume as standard products for the linear power supplies in such products as TV sets, VCRs, and stereos. Their laminated iron or steel cores are made from stacks of E- and I-shaped stampings assembled around toroidal bobbins. Power transformers intended for use in switching power supplies that switch at 400 Hz to 50 kHz are wound on ferrite cores because the reactance losses from laminated iron cores limit efficient operation to about 400 Hz.



AUDIO OR VOICE TRANSFORMERS

An audio or voice transformer is similar to a power transformer, but it operates over a wider frequency range. These transformers can conduct DC in one or more windings, transform voltage and current levels, and act as impedance matching and coupling devices, or as filters. A limited range of voice frequencies within the 20 Hz to 20 kHz audio band can be passed by audio transformers.

PULSE TRANSFORMERS

A pulse transformer is a miniature transformer that generates fast-rising output pulses for timing, counting, and triggering such electronic devices as thyristors (silicon controlled rectifiers) [SCRs] and triacs) and photographic flash lamps.

CIRCUIT-BOARD TRANSFORMERS

A circuit-board transformer is made for circuit-board mounting. Classed in this group are miniature power, audio, and pulse transformers. Some have low profiles, as shown in Fig. 1-21, to permit circuit cards in card cages to be stacked closely together. Typically, these transformers are dipped in epoxy resin to seal them from dirt and moisture. Some windings have pin terminations for circuit-board insertion, and others have pads for surface mounting.



Figure 1-21 Transformer for circuit-board mounting.

RADIO-FREQUENCY TRANSFORMERS

A radio-frequency transformer is designed to function efficiently at radio frequencies. Unlike low-frequency transformers, they are wound on air-core bobbins because neither ferrite nor laminated iron cores are efficient at radio frequencies.

TOROIDAL TRANSFORMERS

A toroidal transformer is wound on a ring-shaped core made by winding long thin continuous sheet metal strips around a cylindrical form. Both the primary and secondary windings are wound on the core by special machines designed to be able to pass wire through and around the open core. Toroidal transformers are more efficient and lighter than comparably rated laminated-core transformers, and they do not emit an audible chatter.

Inductors

An inductor provides a known amount of inductance in an AC circuit. It is made by winding a length of copper wire around a cylinder or other form to make a coil or toroid. The value of inductance can be increased by inserting a core of high magnetic permeability material such as iron or ferrite within the coil. Factory-made standard inductors have values that range from less than 1 μH to about 10 H. Small inductors are used in tuned RF circuits, and large inductors are widely used in tuned audio circuits. However, the inductors with the largest values are used as filter chokes in linear power supplies. A perfect inductor would have only pure inductive reactance, but real inductors have a finite resistance. The inductance value of a variable inductor can be adjusted over a finite range by changing the number of turns in the coil or moving a permeable core in or out of the coil. At high UHF and microwave frequencies, short lengths of copper or aluminum wire serve as inductors.

Transformers

A transformer transfers electrical energy from one or more primary circuits to one or more secondary circuits by means of electromagnetic induction. It consists of at least one primary winding and one secondary winding of insulated wire on a common core. No electrical connection exists between any primary or input circuit and any secondary or output circuit, and no change in frequency occurs between the two circuits.


Figure 1-20 Transformer schematic symbols: (a) step-up transformer, (b) step-down transformer, and (c) multiple-wound transformer.

If an AC voltage is applied to the primary winding of a transformer, an electromagnetic field forms around the core and expands and contracts at the input frequency. This changing field cuts the wires in the secondary winding and induces a voltage in it. The voltage that appears across the secondary winding depends on the voltage at the primary winding and the ratio of turns in the primary and secondary windings. Schematic diagrams for three commonly specified transformer configurations are shown in Fig. 1-20.

A step-up transformer, as shown in Fig. 1-20a, has twice the number of turns in its secondary winding as it has in its primary winding, so the voltage across the secondary winding will be twice that of the voltage across the primary winding. Similarly, a step-down transformer, as shown in Fig. 1-20b, has half as many turns in its secondary as in its primary, so the secondary voltage will be half that of the primary voltage. A multiple-winding transformer, as shown in Fig. 1-20c, provides three separate output voltages that also depend on the ratios between primary and secondary windings.

All of these transformer configurations obey the law of conservation of energy. In transformers this can be interpreted as the equality of the products of voltage and current or power in both primary and secondary windings, except for losses. Thus, the power input at the primary winding is nearly equal to the power output at the secondary winding or the sum of the secondary windings if there are more than one.

If, for example, the voltage at the secondary terminals of the transformer is twice that
of the primary terminals, the current at the secondary terminals must be about half that at the primary terminals to keep the product of voltage and current, which is equal to power, constant. An ideal transformer would be 100 percent efficient because the power output would be equal to the power input. But, because losses reduce the efficiency of most transformers to about 90 percent, output power is about 10 percent less than input power. The total loss is the sum of ohmic resistance loss, eddy-current induction loss, and hysteresis (molecular friction) loss, all caused by the changing polarity of the applied current.

Most transformers transform voltage or current up or down, but an isolation transformer provides secondary voltage and current that are essentially the same as the primary voltage and current (except for resistive losses) because both windings have the same number of turns. These transformers prevent the transfer of unwanted electrical noise from the primary to the secondary windings, thus providing isolation.

The transformers closely associated with electronics are the power, audio, pulse, and RF transformers. They are rated according to the products of their secondary voltages and current in voltamperes (VA) or watts. The transformers specified for most electronic applications are rated for less than 100 VA or 100 W, but some switching power supplies have transformers rated to 1 kW.

Military Standard MIL-T-27 is the mandatory guide for workmanship on mil-spec transformers, but it is also widely used as a guide in the manufacture of commercial units. Commercial transformers that are connected to the AC power line are usually certified by a national organization for conformance to recognized safety guidelines because faults or failures in these transformers could cause electrocution or fires.

VARIABLE CAPACITORS

A variable capacitor is a capacitor whose capacitance value can be adjusted by turning a shaft or screw. Used almost exclusively in RF circuits, there are two classes: tuning and trimmer. Their dielectrics can be plastic, ceramic, glass, or air.

TUNING CAPACITORS

A tuning capacitor is a variable air-dielectric capacitor with plates that move within other plates to change the overall capacitance value. A single gang-tuning capacitor, as shown in Fig. 1-19, has a set of aluminum plates called the rotor mounted on a shaft so that the plates interleave with a matching set called the stator mounted on a rigid spacer. When the rotor shaft is turned by a knob, the rotor plates move in or out between the stator plates without touching them. A change in knob position alters the capacitance value, which is directly proportional to the area of the interleaved plates. Capacitance values can be from 1 to 500 pF. They are used to tune radio receivers, transmitters, and oscillators.



Figure 1-19 Tuning capacitor.

TRIMMER CAPACITORS

A trimmer capacitor is a small variable capacitor with air, ceramic, plastic, glass, or other dielectric that is used for fine-tuning RF circuits. They have capacitance values from 2 to about 100 pF. Made in many different styles, plate spacing is changed to alter the capacitance value by turning an adjustment screw.

TANTALUM ELECTROLYTIC CAPACITORS

Tantalum electrolytic capacitors are made in three styles: (1) wet foil, (2) wet anode, and (3) solid anode. Tantalum capacitors typically have higher CV ratings than aluminum electrolytic capacitors with the same capacitance values. The dielectric formed, tantalum oxide (Ta2O5), has nearly twice the dielectric constant of aluminum oxide. All tantalum capacitors are inherently polarized. As a group, they offer long shelf life, stable operating characteristics, high operating temperature ranges, and higher CV ratios than aluminum electrolytic capacitors. However, they are more expensive than comparably rated aluminum capacitors and have lower voltage ratings.

WET-FOIL TANTALUM CAPACITORS

A wet-foil tantalum capacitor is made by a process similar to that used in making an aluminum electrolytic capacitor. These capacitors can withstand voltages of up to 300 VDC. Packaged in tantalum cases, they are primarily specified for military/aerospace and highreliability applications.


Figure 1-16 Wet-slug tantalum electrolytic capacitor.

WET-ANODE TANTALUM CAPACITORS

A wet-anode tantalum capacitor, as shown in Fig. 1-16, is made from a porous tantalum pellet that is formed by pressing finely ground tantalum powder and a binder in a mold and firing it in a vacuum furnace at about 2000°C. Heat welds or sinters the powder into a solid spongelike pellet with a large effective surface area. A thin film of tantalum oxide is grown electrochemically on the pellet and electrolyte is added. Packaged in silver or tantalum cases, their CV ratios are about 3 times those of wet-foil tantalum capacitors.


Figure 1-17 Epoxy-dipped solid-slug tantalum capacitor.

SOLID-ANODE TANTALUM CAPACITORS

A solid-anode tantalum capacitor, as shown in Fig. 1-17, is also made from a porous pellet anode. A thin film of manganese dioxide that is chemically deposited on the tantalum oxide dielectric serves as a solid electrolyte and cathode. Then a layer of carbon and conductive paint is applied to complete the cathode connection. The most popular and lowest-cost tantalum capacitors, they are available with either radial or axial leads. They are dipped or molded in plastic resin to form protective jackets. Some are also enclosed in tantalum cases for further environmental protection. These capacitors have the longest lives and lowest leakage current of any tantalum capacitors. They can have capacitive values of 0.10 to 680 μF, capacitive tolerances of +- 10 to 20 percent, and maximum voltages of 50 V. The popular ratings are 1 to 10 μF.



Figure 1-18 Tantalum chip capacitor.

SOLID-ANODE CHIP TANTALUM CAPACITORS

A solid-anode chip tantalum capacitor, as shown in Fig. 1-18, is made by the same methods as the radial-leaded version, but it is packaged in a leadless molded epoxy case for bonding to surface-mount cards or hybrid circuits. They can have capacitive values of 100 pF to 100 μF, capacitive tolerances of +- 5 to 20 percent, and maximum voltages of 50 V.

ALUMINUM ELECTROLYTIC CAPACITORS

An aluminum electrolytic capacitor is made by sandwiching a paper separator soaked in electrolyte between two strips of etched aluminum foil, as shown in Fig. 1-15. The paper spacer prevents a short circuit between the cathode and anode foils. The layers of materials are wound in jelly-roll fashion and inserted in an aluminum case. External connections are made from the electrodes to the outside terminals of the case. Direct current is passed through the terminals of the capacitor, causing a thin dielectric layer of aluminum oxide to form on the anode. The electrolyte in contact with the metal foil is the cathode. A plus sign marks the positive terminal of an aluminum electrolytic capacitor.

These capacitors offer high CV ratios and are low in cost. but they exhibit high DC leakage and low insulation resistance. They also have limited shelf lives, and their capacitance values deteriorate with time. Standard units are available in radial- or axial-leaded cases in a wide range of sizes and values. The most commonly specified values are between 4.7 and 2200 μF with working voltages up to 50 VDC. These capacitors are polarized, and this property must be observed when connecting the capacitor in a circuit or it will be destroyed,


Figure 1-15 Aluminum electrolytic capacitor.

Nonpolarized aluminum electrolytic capacitors are available for use in AC circuits for such applications as speaker crossovers and audio filtering. Two polarized capacitors are placed in series with their cathode terminals connected. The anode terminals form the external circuit connections, and the cathode terminals are isolated from the external circuit by an insulator. These capacitors are rated from 1 to 10 μF with maximum working voltages of 50 VDC.

Sunday, May 9, 2010

ELECTROLYTIC CAPACITORS ตัวเก็บประจุชนิดอิเล็กทรอไลติก



Electrolytic capacitors are specified where high values of capacitance are required in the least amount of space (high volumetric efficiency). This property is called high CV ratio. They are formed by electrochemical processes in which oxide dielectrics are grown in and on porous aluminum and tantalum foil and pellets. The metal foils are acid etched to make them porous, increasing their effective exposed areas from 6 to 20 times. High CV ratios are made possible by the thin oxide layers formed on the plates of the capacitors. The pellets are also made so that they are porous or spongelike and have large exposed surfaces. However, electrolytic capacitors have higher leakage current than electrostatic capacitors because of the impurities embedded in the foil and the electrolyte. This current increases with temperature while voltage breakdown decreases with temperature. Electrolytic capacitors also have higher power factors than electrostatic capacitors, causing losses called equivalent series resistance (ESR).

MONOLITHIC MULTILAYER CERAMIC (MLC) CAPACITORS



A monolithic multilayer ceramic (MLC) capacitor, as shown in cutaway view Fig. 1-14, is a multilayer ceramic chip capacitor that offers high volumetric efficiency because a large capacitor area is compressed into a small block. Preformed metallized layers are stacked and fired to form MLCs in a wide range of sizes and values with different properties. Originally developed for hybrid circuits, MLCs are widely used in surface mounting because they can substitute for larger capacitors with comparable capacitance values. They offer low residual inductance values and low resistance, a wide range of capacitance values in a given size, and a wide selection of temperature coefficients. They also exhibit lower inductance and resistance values than tantalum capacitors with comparable ratings. MLCs are used for timing and frequency selection.



MLCs are made as sandwiches of “green” (unfired) barium-titanate ceramic strips 0.8 mils (20 μm) thick that have been imprinted with silver-palladium ink to form plates. Up to 40 layers of the soft doughlike strips are stacked, compressed, diced, and furnace fired to form the monolithic chips.

End terminals for solder bonding MLCs to a circuit board or attaching leads are made by plating successive layers of silver-palladium, nickel, and tin or lead-tin on the ends of the chips. The process used depends on whether the chip is to be leaded and coated with insulation or is to remain bare for bonding directly to a circuit board.

Bare MLCs are used on hybrid microcircuits and in surface-mount assembly. They will withstand the 232°C reflow-soldering temperatures and the 282°C wave-soldering temperatures. Bare MLC chip sizes are standardized. Examples include 0.08 × 0.05 in (2.0 × 1.3 mm), designated 0805; 0.125 × 0.063 in (3.2 × 1.6 mm), designated 1206; and 0.225 × 0.05 in (5.7 × 1.3 mm), designated 2225. Standard MLCs have capacitance values of 10 pF to 3.5 μF, capacitance tolerances of +- 1 to 20 percent, and maximum voltages of 50 V.

CERAMIC TUBULAR CAPACITORS



A ceramic tubular capacitor is a length of ceramic tube whose inner and outer surfaces are painted with silver ink to form its plates. They have replaced ceramic disk capacitors in surface-mounted circuits to save board space and permit automatic placement. They are protected with a coat of protective resin.

CERAMIC CAPACITORS เซรามิก คาปาซิเตอร์

Ceramic dielectric capacitors are classified by dielectric constant k, as Classes I, II, and III. Class I dielectrics exhibit low k values, but they have excellent temperature stability; Class II dielectrics have generally high k values and volumetric efficiency but lower temperature stability; and Class III dielectrics are prepared for the lower-cost disk and tube capacitors.



Class I dielectrics include negative positive zero (NPO) ceramics, which are designated COG and BY. These ceramics are made by combining magnesium titanate (with a positive coefficient) and calcium titanate (with a negative coefficient) to form a dielectric with excellent temperature stability. Their properties are essentially independent of frequency, and they have ultrastable temperature coefficients of 0 +- 30 ppm°C over the range of −55 to 125°C. These dielectrics show a flat response to both AC and DC voltage changes. Low-k multilayer ceramic capacitors (MLCs) are used in resonant circuits and filters.

Class II dielectrics are high-k ceramics called ferroelectrics made from barium titanate. The addition of barium stannate, barium zirconate, or magnesium titanate lowers the dielectric constant from values as high as 8000. These compounds stabilize the capacitor over a wider temperature range. Class II dielectrics include the general-purpose X7R (BX) and Z5U (BZ). X7R is stable but its capacitance can vary +-15 percent over the temperature range of −55 to 125°C. Its capacitance value decreases with DC voltage but increases with AC voltage. Z5U compositions exhibit maximum temperature-capacity changes of +22 and −56 percent over the range of 10 to 85°C.

Class III dielectrics, developed for ceramic-disk capacitors, give high volumetric efficiency but with the tradeoff of high leakage resistance and dissipation factor. Capacitors made with Class III dielectrics have low working voltages. Ceramic dielectric capacitors are constructed in three styles: (1) single-layer disk, (2) tubular, and (3) monolithic multilayer

Friday, May 7, 2010

MICA CAPACITORS ไมก้า คาปาซิเตอร์




A
mica capacitor has dielectrics of thin rectangular sheets of mica, a natural mineral. Mica has a dielectric constant from 6 to 8. The electrodes are either thin sheets of metal foil interleaved between mica sheets, or thin films of silver that have been screened and fired on the mica. Silvered mica capacitors (ซิลเวอร์ไมก้าคาปาซิเตอร์ เหมาะที่จะใช้กับวงจรความถี่สูง) have greater mechanical stability and offer more uniform properties than foil and mica capacitors. Both are used primarily in RF applications. Mica capacitors perform satisfactorily over temperature ranges as wide as −55 to 150°C, and they have high insulation resistance. Their capacitance values range from about 1 pF to 0.1 μF. However, they have a low ratio of capacitance to volume or mass.

FILM DIELECTRICS

Polyester film (tradenamed Mylar) is the most popular general-purpose dielectric in filmtype capacitors. It permits smaller capacitors than comparably rated units made from other films, and these capacitors exhibit low leakage, moderate temperature coefficients over the −55 to 85°C range, and moderate dissipation factors. Capacitance tolerance is typically +-10 percent. The film-and-foil versions are widely used in consumer electronics products while the metallized units perform general blocking, coupling, decoupling, bypass, and filtering functions.



Polypropylene film provides capacitor characteristics that are superior to those of polyester. Polypropylene capacitors have both high- and low-frequency applications. The plastic has properties that are similar to those of polystyrene, but capacitors made from it have higher AC current ratings. Polypropylene capacitors can operate at 105°C, and their volumetric efficiency is better than those made of polyester. Foil and polypropylene capacitors are used in CRT deflection, pulse-forming, and RF circuits. The capacitance tolerance for polypropylene capacitors is +-5 percent, and their temperature coefficients are linear.

Polystyrene film has characteristics that are similar to those of polypropylene. Capacitors made from the film exhibit a low dissipation factor, small capacitance change with temperature, and very good stability. But they are larger than comparably rated polypropylene units. Used in timing, integrating, and tuning circuits, their maximum operating temperature is 85°C.

Polycarbonate film capacitors offer dissipation factors and capacitance stability which approaches those of polystyrene capacitors. They also offer high insulation resistance stability. Operating temperatures are −55 to 125°C with capacitance tolerances of +-5 percent. These capacitors are widely used in military applications.

PLASTIC-FILM CAPACITORS

A plastic-film capacitor, as shown in Fig. 1, is typically made by rolling a thin film of plastic dielectric with metal foil or a metallized dielectric film into a cylindrical form and attaching leads. The dielectrics include polyester, polypropylene, polystyrene, and polycarbonate. Film thickness can range from 0.06 mil (1.5 μm) to over 0.8 mil (20 μm). The most popular film capacitors have capacitance values of 0.001 to 10 μF, although values from 50 pF to 500 μF are available as standard products. Working voltages range from 50 to 1600 VDC, and capacitance tolerance is from +-1 to +-20 percent.



Fig. 1

In film-and-foil construction, tin or aluminum foil about 0.00025 in (0.00635 mm) thick is wound with the dielectric film, but in metallized-film construction, aluminum or zinc is vacuum deposited to thicknesses of 200 to 500 Å (20 to 50 nm) on the film. Film capacitors can also be made by cutting and stacking metallized foil with attached leads. A capacitor with metallized film is smaller and weighs less than a comparably rated film-and-foil unit. Moreover, metallized-film capacitors are self-healing; that is, if the capacitor dielectric is pierced by a transient overvoltage, the metal film around the hole will evaporate, effectively lining the hole with molten plastic dielectric. This prevents short-circuits between adjacent metal layers and preserves the capacitor.

After rolling or stacking is complete, the capacitor is dipped in or conformally coated with an insulating plastic jacket. Some units are also hermetically sealed in tubular or rectangular metal cases for added environmental protection. Both film-and-foil and metallizedfilm capacitors are available with axial or radial leads in a wide variety of case styles.

ELECTROSTATIC CAPACITORS

An electrostatic capacitor has a dielectric made from plastic film, mica, or glass, and its plates or electrodes are made from metal foil or metal deposited on the dielectric. Ceramic capacitors have plates formed from precious-metal inks that have been screened on the raw ceramic prior to furnace firing.

Capacitors,คาปาซิเตอร์, ตัวเก็บประจุ

A capacitor, as shown in Fig. 1, is an electronic component capable of storing electrical
energy. The simplest form of capacitor is two metal plates insulated from each other by some dielectric. Capacitors are the second most widely purchased passive components next to resistors. There are both fixed and variable capacitors for electronics, and their capacitance values vary from a few picofarads (pF) to thousands of microfarads (μF). The schematic symbol for a fixed capacitor is shown in Fig. 2 and that for a variable capacitor is shown in Fig. 3.




Fig. 1




Fig. 2


Fig. 3


Capacitors are classified as either electrostatic or electrolytic. Electrostatic capacitors have dielectrics that are either air or some solid insulating material such as plastic film, ceramic, glass, or mica. (Paper dielectric capacitors are no longer specified in electronics.)

Electrolytic capacitors are further classified as aluminum or tantalum because those metals form thin oxide film dielectrics by electrochemical processing. They can have wet-foil, wet-slug, or dry-slug anodes.

The capacitance value of fixed capacitors remains essentially unchanged except for small variations caused by temperature changes. By contrast, the capacitance value of variable capacitors can be set to any value within a preset range of values. Variable capacitors are usually used in RF circuits.

Variable Resistors ตัวต้านทานปรับค่าได้

POTENTIOMETERS

A
potentiometer is a variable resistor whose resistance value can be changed by moving a sliding contact or wiper along its resistive element to pick off the desired value. A potentiometer has terminals at each end of its fixed resistive element, and the third terminal is connected to a moveable wiper. If the wiper is moved back to the beginning of the resistive element, the potentiometer’s resistance value is minimal, but if it is moved across the full length of the element, the value reaches its maximum. There are three different mechanisms for moving the wiper along the resistance element:



1. Sliding the wiper by finger pressure
2. Turning a leadscrew on the case to drive the wiper back and forth
3. Rotating a screw or knob attached to the wiper to sweep it around a curved element

Potentiometers for electronic circuits are classified as follows:
  • Precision
  • Panel or volume-control
  • Trimmer
The common abbreviation for potentiometer is pot, so there is a precision pot and a panel or volume-control pot. However, a trimmer potentiometer is usually called a trimmer (to be distinguished from a trimmer capacitor). These variable resistors share the same schematic symbol and are made from many of the same kinds of materials.

Fixed Resistors ตัวต้านทานแบบค่าคงที่

A resistor is a circuit component that provides a fixed value of resistance in ohms to oppose the flow of electrical current. Resistors can limit the amount of current flowing in a circuit, provide a voltage drop in accordance with Ohm’s laws, or dissipate energy as heat.

Fixed resistors are discrete units typically made in cylindrical or planar form. The most common cylindrical style is the axial-leaded resistor, as shown in Fig. 1. The resistive element is wound or deposited on a cylindrical core, and a cap with a lead wire is positioned on each end. The resistive elements include
resistive wire (wirewound), metal film, carbon film, cermet, and metal oxide. Resistor networks and chip resistors are examples of planar resistors. All fixed resistors are rated for a nominal resistance value in ohms over the range of fractions of an ohm to thousands of ohms (kilohms), or millions of ohms (megohms). Other electrical ratings include:



Figure 1

Some resistors also have additional ratings for electrical noise, parasitic inductance, and parasitic capacitance. Resistors exhibit unwanted parasitics of inductance and capacitance because of their construction. These effects must be considered by the designer when selecting resistors for unusual or specialized applications such as their use in instrumentation. A resistor’s ability to dissipate power is directly related to its size. With the exception of those specified for power supplies, most resistors for electronic circuits are rated under 5 W, usually less than 1 W. A 5-W cylindrical resistor is about 1 in (25.4 mm) long with a diameter of 1⁄4 in (6.4 mm). The 1⁄2-, 1⁄4-, and 1⁄8-W resistors are correspondingly smaller.
  • Resistive tolerance as a percentage of nominal value in ohms
  • Power dissipation in watts (W)
  • Temperature coefficient (tempco) in parts per million per degree Celsius of temperature
  • change (ppm/°C)
  • Maximum working voltage in volts (V)

CARBON-COMPOSITION RESISTORS

A carbon-composition resistor, as shown in Fig. 2, is made by mixing powdered carbon with a phenolic binder to form a viscous bulk resistive material, which is placed in a mold with embedded lead ends and fired in a furnace. Because their resistive elements are a bulk material, they can both withstand wider temperature excursions and absorb higher electrical transients than either carbon- or metal-film resistors. These qualities are offset by their typically wider resistive tolerances of +-10 to 20 percent and tendency to absorb moisture in humid environments, causing their values to change. However, the benefits of carboncomposition resistors are less important in low-voltage transistorized circuits, so demand for them has declined. These resistors have ratings of 1 ohm to 100 megohms, but values in the 10- to 100-ohm range were most popular. Power ratings are 1⁄8 to 2 W.



Figure 2

CARBON-FILM RESISTORS

A
carbon-film resistor, as shown in Fig.3, is made by screening carbon-based resistive ink on long ceramic rods or mandrels and then firing them in a furnace. The rod is then sliced to form individual resistors. After leaded end caps are attached, the resistance values are set precisely in a laser trimming machine that trims away excess resistive film under closed-loop control. The trimmed resistors are then coated with an insulating plastic jacket. Resistive tolerances of carbon-film resistors are typically +-10 percent. Standard resistors have power ratings of 1⁄2, 1⁄4, and 1⁄8 W.



Figure 3

WIREWOUND RESISTORS

A
wirewound resistor, as shown in Fig.4, is made by winding fine resistive wire on a plastic or ceramic mandrel. The most commonly used resistance wire is nickel-chromium (nichrome). The axial leads and end caps are attached to the ends of the wire winding and welded to complete the electrical circuit. There are both general-purpose and power wirewound resistors. General-purpose units have resistive values of 10 ohms to 1 megohm, resistance tolerances of +-2 percent, and temperature coefficients of +-100 ppm/°C. Power
units rated for more than 5 W have tolerances that can exceed +-10 percent.



Figure 4

Wirewound resistors are generally limited to low-frequency applications because each is a
solenoid that exhibits inductive reactance in an AC circuit, which adds to its DC resistive value. The inductive reactance can be reduced or eliminated at low or medium frequencies by bifilar winding. This is done by folding the entire length of resistive wire back on itself, hairpin fashion, before winding it on the mandrel. As a result, opposing inductive fields cancel each other, lowering or eliminating inductive reactance.

Wirewound resistors are made with both axial and radial leads. Epoxy or silicone insulation is applied to some low-power wirewound resistors, but high-power units are encased in ceramic or placed in heat-dissipating aluminum cases. This reduces the danger of the hot resistor igniting nearby flammable materials or burning fingertips if accidentally touched.



Figure 5

METAL-FILM RESISTORS

A metal-film resistor, as shown in Fig. 5, is made by the same general method as a carbon-film resistor. A thin metal film is sputtered or vacuum deposited on an alumina (aluminum-oxide) mandrel in a vacuum chamber, or a thick metal film is applied in air. Tin oxide or nickel-chromium are widely used thin films, and a thick film made from powdered precious metal and glass (frit) in a volatile binder is a common cermet resistive ink. These resistors are laser trimmed to precise values under closed-loop control after firing. Metal-film resistors are offered in two grades: (1) those with resistive tolerances of +-1 percent and temperature coefficients of 25 to 100 ppm/°C, and (2) those with resistive tolerances of +-5 percent and temperature coefficients of 200 ppm/°C. Demand is highest for 1⁄4- and 1⁄8-W units, but 1⁄20-W units are available. Resistive values up to 100 megohms are available as catalog items, but they are generally rated for less than 10 kilohms.


Figure 6

RESISTOR NETWORKS

A resistor network, as shown in Fig. 6, consists of two or more resistive elements on the same insulating substrate. These networks are specified where 6 to 15 low-value resistors are required in a restricted space. Most commercial networks contain thick-film resistors, and they are packaged in dual-in-line packages (DIPs) or single-in-line packages (SIPs). Standard DIPs have 14 or 16 pins, and standard SIPs have 6, 8, or 10 pins. Resistor networks are used for “pull-up” and “pull-down” transitions between logic circuits operating at different voltage values, for sense amplifier termination, and for light-emitting diode (LED) display current limiting.

Alumina ceramic is the most widely used network substrate. Conductive traces are formed by screening an ink made from a powdered silver-palladium mix in a volatile binder on the bare ceramic substrate. After firing, the ink bonds with the ceramic to form hard, lowresistance paths. Resistive inks made from a powdered ruthenium-cermet mix with a powdered glass frit and a volatile binder are then screened over the ends of the conductors to form the resistive elements. This ink is also fired, and when it bonds with the ceramic it forms a hard, resistive element. Network resistors are laser trimmed under closed-loop control to precise resistance values. Standard network resistance values are from 10 ohms to 10 megohms with tolerances of +-2 percent. Most networks can safely dissipate less than 1⁄2 W.

Where more precise resistance values are required, thin-film networks are specified. They are made formed from compositions that include nickel-chromium, chrome-cobalt, and tantalum nitride, deposited or sputtered on alumina ceramic substrates. Unpackaged thin-film resistor networks are also sold as hybrid-circuit substrates. Thin-film resistivecapacitive (RC) networks are also packaged in metal and ceramic flatpacks.



Figure 7

CERAMIC-CHIP RESISTORS

A ceramic-chip resistor, as shown in Fig. 7, is made by screening and firing cermet resistive inks or sputtering tantalum nitride or nickel-chromium on an alumina substrate. The deposited resistive surface is then coated with glass for protection. The substrate is then diced into individual chips, and a silver-based ink is applied to the end surfaces and fired as the first step in forming leadless terminals. A barrier layer of nickel plating is then applied to prevent the migration of silver from the inner electrode. Finally, the terminations are coated with lead-tin solder for improved adhesion during reflow soldering.

Chip resistors were originally made for hybrid circuits, but surface-mount technology (SMT) has increased demand for them. Surface-mount chip resistor dimensions have been standardized to 1.6 × 3.2 mm for handling by automatic pick-and-place machines. (This is the same size as the 1206 chip capacitor that measures 0.063 × 0.125 in.) Chip resistors are typically rated for 1⁄8 W or less. An alternative form of SMT resistor is the leadless cylinder with solder-coated bands around each end for reflow solder bonding.

Thursday, May 6, 2010

Magnetism

Electric currents and magnetic fields are closely related. Whenever an electric current flows—that is, when charge carriers move—a magnetic field accompanies the current. In a straight wire that carries electrical current, magnetic lines of flux surround the wire in circles, with the wire at the center, as shown in Fig.1. (The lines of flux aren’t physical objects; this is just a convenient way to represent the magnetic field.) You’ll sometimes hear or read about a certain number of flux lines per unit cross-sectional area, such as 100 lines per square centimeter. This is a relative way of talking about the intensity of the magnetic field.


Fig. 1. Magnetic flux lines around a straight, current-carrying wire. The arrows indicate current flow.

Magnetic fields are produced when the atoms of certain materials align themselves. Iron is the
most common metal that has this property. The atoms of iron in the core of the earth have become aligned to some extent; this is a complex interaction caused by the rotation of our planet and its motion with respect to the magnetic field of the sun. The magnetic field surrounding the earth is responsible for various effects, such as the concentration of charged particles that you see as the
aurora borealis just after a solar eruption.

When a wire is coiled up, the resulting magnetic flux takes a shape similar to the flux field surrounding the earth, or the flux field around a bar magnet. Two well-defined
magnetic poles develop, as shown in Fig. 2.



Fig. 2. Magnetic flux lines around a current-carrying coil of wire. The flux lines converge at the magnetic poles.

Power and the Watt กำลังงานและวัตต์

Whenever current flows through a resistance, heat results. The heat can be measured in watts (symbolized W) and represents electrical power. (As a variable quantity in equations, power is denoted by the uppercase italic letter P.) Power can be manifested in many forms, such as mechanical motion, radio waves, visible light, or noise. But heat is always present, in addition to any other form of power, in an electrical or electronic device. This is because no equipment is 100 percent efficient. Some power always goes to waste, and this waste is almost all in the form of heat.



Fig. 1. Whenever current passes through a component having resistance, a voltage exists across that component.

Look again at Fig. 1. There is a certain voltage across the resistor, not specifically indicated.
There’s also a current flowing through the resistance, and it is not quantified in the diagram, either. Suppose we call the voltage E and the current I, in volts (V) and amperes (A), respectively. Then the power in watts dissipated by the resistance, call it P, is the product of the voltage in volts and the current in amperes:

P = EI

If the voltage E across the resistance is caused by two flashlight cells in series, giving 3 V, and if the current I through the resistance (a light bulb, perhaps) is 0.1 A, then E = 3 V and I = 0.1 A, and we can calculate the power P in watts as follows:


P = EI = 3 × 0.1 = 0.3 W

Suppose the voltage is 117 V, and the current is 855 mA. To calculate the power, we must convert the current into amperes: 855 mA = 855/1000 A = 0.855 A. Then:

P = EI = 117 × 0.855 = 100 W