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.