If the transformer exterior is bulged or shows what appear to be burn marks, don’t test the transformer. Instead, replace it.
The schematic for the circuit will be available in the product information or on the website of the circuit manufacturer. [1] X Research source
The voltage being supplied to the primary should be labeled both on the transformer and the schematic. The voltage being generated by the secondary should be labeled in the same fashion as the primary.
Generally, you can find the transformer conversion and output filtering information on the schematic. Look for whether the transformer is AC or DC wherever the voltage is listed on the label. [4] X Expert Source Jesse KuhlmanMaster Electrician Expert Interview. 29 April 2020.
Generally, you’ll need to attach the leads of your DMM to the input lines to verify that the primary of the transformer is not shorted. The same process will be used to check the transformer secondary.
Separate the transformer from the input circuit. Test the input with your DMM. If the input power climbs to the expected value, the primary of the transformer is bad. If the input power does not climb to the expected value, then the problem lies not with the transformer, but with the input circuitry. [6] X Research source The input and output on the transformer may be labeled with “input” and “output,” or the input might be a black and white pigtail. [7] X Expert Source Jesse KuhlmanMaster Electrician Expert Interview. 29 April 2020. If the transformer has terminals, the input will usually be L, which stands for “line,” or hot power, and N, which stands for neutral, or the neutral power going into that wire. The output will be the low voltage side. [8] X Expert Source Jesse KuhlmanMaster Electrician Expert Interview. 29 April 2020.
If the expected voltage is not present on the secondary, either the transformer or a filtering or shaping component is bad. Test the filtering and shaping components separately. If the testing of the filtering and shaping components show no problems, then the transformer is bad. [9] X Research source
An overloaded transformer will often make snapping and crackling noises. If you hear sounds like these, cut power to the transformer to prevent burnout. [11] X Research source
Blackness, melting, and deforming in fuses are good indications that the fuse has been damaged. Simple remove and replace these. In some cases, it might be hard to tell if a fuse is in good condition. Attach your DMM to the fuse with one lead on each fuse end. If current runs through the fuse, it’s good. [12] X Research source
Test this by hooking up the secondary to its circuit and using your DMM to test the secondary lines. If the reading is above the amperage rating for the transformer, the circuit is drawing too much power. Many common transformers have 3 amp fuses. The amperage rating for your transformer fuse may be labeled on the transformer, but will also be available in the circuit schematic. [13] X Research source
