Transformers

A "transformer" takes one voltage and changes it into another.

What is a transformer, and why should I care?
Types of transformers
- Power transformers
- Signal transformers
Where can I buy one?

What is a transformer, and why should I care?

A "transformer" changes one voltage to another. This attribute is useful in many ways.

A transformer doesn't change power levels. If you put 100 Watts into a transformer, 100 Watts come out the other end. [Actually, there are minor losses in the transformer because nothing in the real world is 100% perfect. But transformers come pretty darn close; perhaps 95% efficient.]

A transformer is made from two coils of wire close to each other (sometimes wrapped around an iron or ferrite "core"). Power is fed into one coil (the "primary"), which creates a magnetic field. The magnetic field causes current to flow in the other coil (the "secondary"). Note that this doesn't work for direct current (DC): the incoming voltage needs to change over time - alternating current (AC) or pulsed DC.

The number of times the wires are wrapped around the core ("turns") is very important and determines how the transformer changes the voltage.

If the primary has fewer turns than the secondary, you have a step-up transformer that increases the voltage.
If the primary has more turns than the secondary, you have a step-down transformer that reduces the voltage.
If the primary has the same number of turns as the secondary, the outgoing voltage will be the same as what comes in. This is the case for an isolation transformer.
In certain exceptional cases, one large coil of wire can serve as both primary and secondary. This is the case with variable auto-transformers and xenon strobe trigger transformers.

Types of transformers

In general, transformers are used for two purposes: signal matching and power supplies.

Power Transformers

Power transformers are used to convert from one voltage to another, at significant power levels.

Step-up transformers

A "step-up transformer" allows a device that requires a high voltage power supply to operate from a lower voltage source. The transformer takes in the low voltage at a high current and puts out the high voltage at a low current.

Examples:

You are a Swiss visiting the U.S.A., and want to operate your 220VAC shaver off of the available 110 VAC.
The CRT display tube of your computer monitor requires thousands of volts, but must run off of 110 VAC from the wall.

Step-down transformers

A "step-down transformer" allows a device that requires a low voltage power supply to operate from a higher voltage. The transformer takes in the high voltage at a low current and puts out a low voltage at a high current.

Examples:

Your Mailbu-brand landscape lights run on 12VAC, but you plug them into the 110 VAC line.
Your doorbell doesn't need batteries. It runs on 110 VAC, converted to 12VAC.

This page from the All Electronics catalog (#103, Winter 2003) shows some step-down transformers.

In many cases, step-down transformers take the form of wall warts.

Isolation transformers

An "isolation transformer" does not raise or lower a voltage; whatever voltage comes in is what goes out. An isolation transformer prevents current from flowing directly from one side to the other. This usually serves as a safety device to prevent electrocution.

Variable auto-transformers

A "variable auto-transformer" (variac) can act like a step-up transformer or step-down transformer. It has a big knob on top that allows you to dial in whatever output voltage you want.

This page from the All Electronics catalog (#103, Winter 2003) shows some variacs.

WARNING: A variable auto-transformer does not provide isolation from line current. For that you need an isolation transformer.

Inverters

An "inverter" takes a DC power source and boosts it up to a higher voltage. The most common type of inverter takes power from an automobile and cranks out 110 VAC to run appliances and power tools. Inverters are also used to operate fluorescent lamps from battery power.

Technically, an inverter isn't a transformer; it contains a transformer (and lots of other stuff).

Signal Transformers

"Signal transformers" also take one thing in and transform it to another thing out. But in this case, the power levels are low, and the transformed thing carries some type of information signal.

In most cases, these transformers are thought of as impedance matching.

Where can I buy one?

Transformers are electronic construction parts. Don't expect to find then at Sears or ordinary stereo shops.

Look in stores that cater to electronic experimenters, such as:

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