Help testing a step down transformer

Thread Starter


Joined Mar 29, 2018
I'm troubleshooting a circuit that has a step down AC transformer and I'm confused about the voltages I'm reading in the secondaries.

The specs are:
INPUT: 110V AC 50Hz/60Hz
OUTPUT: Yellow 15.5V / Black 9.5V

When I measure the outputs with the secondaries disconnected from the circuit I get around 16V in the yellow and 10V in the black, which is good. But when I measure with them connected to the circuit I get 4.4V in the yellow and 7.6V in the black.

Aren't the outputs supposed remain the same no matter if the secondaries are connected to the circuit or not?

Thanks for the help.


Joined Mar 14, 2008
There is always some drop under load due to the transformer's winding resistances, but that should be no more than about 10% under load.
Sounds like your circuit is overloading the transformer and will likely burn it out if left connected.


Joined Jan 23, 2018
Certainly it does appear that the load current is excessive. So the first thing to do, presuming that the transformer feeds rectifiers, is, with the power off, do a resistance check on the rectifier diodes. Beyond that the next thing would be to examine the circuit that you are checking. If you have a complaint about a specific symptom that will usually point in the direction of what to check next.