In most electronic power supplies, the transformer reduces the voltage. A transformer is a device that uses the principal of electromagnetic induction to transfer voltage and current from one circuit to another. The transformer uses a primary coil that’s connected to line voltage and a secondary coil that provides the output voltage.

The amount of the voltage reduction depends on the ratio of the number of turns in the primary coil versus the number of turns in the secondary coil. For example, if the secondary coil has half as many turns as the primary coil, the primary coil voltage will be cut in half at the secondary coil. In other words, if 120 VAC is applied to the primary coil, 60 VAC will be available at the secondary coil.

Common secondary voltages for transformers used in low-voltage power supplies range from 6 to 24 VAC. Note that because some voltage will be lost in the rectifier and filtering stages, you’ll want to choose a secondary coil voltage that’s a few volts higher than the final DC voltage your circuit actually needs.

Note, however, that the actual DC voltage level used for most circuits isn’t all that critical. So if you’re designing a power supply for a circuit that calls for 6 VDC and you use a transformer that provides 6 VAC in its secondary coil, the output from the power supply after it's rectified to DC voltage will be closer to 5 VDC. Most likely, 5 VDC will be close enough, and the circuit will work just fine.

Note that many transformers have more than one tap in the secondary coil. A tap is simply a wire connected somewhere in the middle of a coil, effectively dividing a single coil into two smaller coils. Multiple taps let you access several different voltages in the secondary coil. The most common arrangement is a center-tapped transformer, which provides two voltages.

In a center-tapped transformer, the voltage measured across the two outer taps is double the voltage measured from the center tap to either one of the two outer taps. Thus, if the voltage across the two outer taps is 24 VAC, the voltage across the center tap and either of the outer taps is 12 VAC.

It’s important to note that when a transformer reduces voltage, it increases current. Thus, if a transformer cuts the voltage in half, the current will double. As a result, the overall power in the system (defined as the voltage multiplied by the current) remains the same.

If the current didn’t increase as the voltage decreased, the transformer would violate a basic law of physics — the one about conservation of energy, which says that energy can’t just disappear. That’s a good thing. You don’t want to be violating the laws of physics unless you know what you’re doing or you’re in a science fiction movie, in which case you can violate the laws of physics at will.

A transformer is strictly an alternating current device. That means:

• Transformers work only when alternating current is applied to the primary coil. If you apply direct current to the primary coil, no voltage will appear across the secondary coil. (Actually, there will be a brief spike of voltage across the secondary coil the moment voltage is applied to the primary coil, but in most circuits this fleeting voltage is insignificant.)

• A step-down transformer reduces the voltage from the primary to the secondary coils but doesn't convert alternating current to direct current. The voltage at the secondary coil is always AC.

• A transformer isolates the circuit attached from the secondary coil from the circuit connected to the primary coil. Thus, you can use a transformer to isolate your project from line voltage.