A transformer changes electricity from high to low voltage using two properties of electricity. In an electric circuit, there is magnetism around it. Second, whenever a magnetic field changes (by moving or by changing strength) a voltage is made. Voltage is the measure of the strength or amount of electrons flowing through a wire. If there's another wire close to an electric current that is changing strength, the current of electricity will also flow into that other wire as the magnetism changes. A transformer takes in electricity at a higher voltage and lets it run through lots of coils wound around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Also around the core is an output wire with fewer coils. The magnetism changing back and forth makes a current in the wire. Having fewer coils means less voltage. So the voltage is "stepped-down."
In HVAC systems lower voltage means smaller wires. So thermostat wires can be real small like telephone wire with the use of a transformer. The voltage is stepped down from either 220 vac for electric heat and air or 120 to 24 for gas heat and ac. If either the high side coil or the low side coil within the transformer is OPEN ( no continuity) then the transformer is bad. If the wires are shorted together on either side then the transformer is instantly ruined. Many transformers are ruined when they get replaced and the shorted wires are not fixed. That is why it is recommended a fuse be mounted in the low side to prevent ruining the transformer in case the weed eater or the dog gets to those small tender thermostat wires. VA means volt amps. A standard home central unit uses a 40 VA 24 volt transformer. FYI: The VA is the muscle of the transformer. If you have a long thermostat wire run to the roof and are using a commercial system to cool an office or a store you would need a 75 va or a 100 va transformer to get the 24 volts from the controls to the unit.
HEADS UP: To keep costs down manufacturers provide a multi tap transformer as pictured above. Be sure to go by the directions on the side of the transformer and remember to tape off any unused wires to prevent shorts and malfunctions (do not tape em together either). On the high side there will be a common wire and 3 others with input voltage ratings of 240, 208, and 120 volts ac (VAC) so if you have gas heater you would use common and 120 and tape up the 208 and tape up the 240 . Never connect more than 2 wires. Pay attention ! If you connect 240 to the 120 wire then Boom the transformer is toast. The yellow wires are always the low side (24 volts).This many folk have visited as of Oct 29, 2009 Thanks