You have just found that the transformer on a piece of equipment is not producing any voltage on the secondary side, also known as the load side. You check the primary side and you find the correct voltage supply. So you install a new transformer and you’re done, right?
WRONG! You will probably be replacing the new transformer immediately!
A transformer typically does not fail on its own; some other situation or component has caused it to fail. You need to find that problem before changing the transformer.
The majority of transformer failures can be attributed to excessive overloads due to a short circuit on the secondary, or load, side of the transformer. When this load increases, it produces excessive heat beyond the capability of the transformer. This heat buildup breaks down the insulation between the windings, and the transformer eventually burns out, or as we say in the business, “smokes.”
Another reason the transformer may fail is contamination. The transformer can be in a wet or corrosive environment, which may also lead to a breakdown in the insulation of the windings.
Your first step before replacing the transformer is to check the circuitry for any signs of a short circuit to ground and use your ohmmeter to inspect all magnetic coils on contactors, relays, and solenoid valves for any breakdown in resistance. If there is an obvious shorted wire or coil, first correct the problem and then install the new transformer.
If you are not sure what caused the transformer to fail, you will need to temporarily fuse the new transformer. An in-line fuse holder will need to be installed in-line with the hot wire coming off the secondary side of the transformer. Use only fast-blow fuses. You will use a process of elimination to find the problem. Disconnect each component powered by the transformer. Reapply power to the transformer and confirm you have power out of your secondary side. Then start to reconnect each component until the fuse blows. The last part you connect is generally the cause of the transformer failure, and that component will also need to be replaced before replacing the transformer. Remember, it is cheaper to replace a few fuses than a few transformers!
To properly fuse the transformer for the load capability, take the VA rating of the transformer and divide it by the side you are trying to protect.
Primary side 60 VA ÷ 120 volts = .50 = ½-amp fuse
Secondary 60 VA ÷ 24 volts = 2.5 = 2½-amp fuse