Power transformer:

Power transformers can be categorized as step down or step-up. The output, or secondary, voltage of the step down unit is lower than input, or primary, voltage. The reverse is true for a step up transformer.

Step-down

Most solid state electronic devices, like radios, require only a few volts. The power supplies for such type equipment use step down power transformers. The physical size of transformer depends on current.

Some devices require a small current and a low voltage. The transformer in a radio receiver, for instance, can be quite small physically. A ham radio transmitter or hi fi amplifier requires much more current. This means that secondary winding of the transformer should be of heavy gauge wire, and the core should be bulky to contain the magnetic flux. Such type of a transformer is massive.

Step-up

Some circuits requires high voltage. The picture tube in a TV set needs many hundred volts. Some of the ham radio power amplifiers use vacuum tubes working at kilovolts direct current. The transformers in these appliances are step up types. They are moderate to large in size due to the number of turns in secondary, and because high voltages can spark, or arc, between wire turns if windings are very tight.

If a step up transformer needs to supply a small amount of current only, it need not be large. But for ham radio transmitters and radio/TV broadcast amplifiers, the transformers are big and heavy and expensive.

Transformer ratings

Transformers are rated according to the output voltage and current. For the given unit, the volt-ampere (VA) capacity is specified often. This is product of voltage and current. A transformer having a 12-V output, capable of delivering 10 A, would have 12 V 10 a 120 VA of capacity.

A high-quality, rugged power transformer, which is capable of providing the required currents and/or voltages, is crucial in power supply. The transformer is the most expensive component to replace. When designing a power supply, it is wise to spend extra to get a reliable transformer. Engineers call this as maintenance insurance.