Question about transformers

Thread Starter


Joined Jan 18, 2016
I'm working on a control board of commercial refrigerator, the transformer's primary side was open - how I checked it was by measuring the ohms across the winding, it shows an 'overload', and I have confirmed that it is in fact the transformer as I used another voltage source to power up the board. My first question is should there always be an ohms reading on the primary and secondary side of the transformer? Can I safely assume that its faulty when there is no reading?
My second question is when trying to replace a transformer what parameters should I be aware of?
I have attached a picture of the faulty transformer - I couldn't figure out the ratings of this, in which case how do I make sure the replacement transformer will be adequate? The board needs 12V DC and 5V DC. So I am assuming that a transformer which has got a little bit over 12V on the secondary side should suffice.

Thanks in advance



Joined Mar 30, 2018
Yes, there should always be a resistance reading on both the primary and secondary windings (and neither open circuit).

Where a winding has gone open circuit, it will almost certainly be due to an overload condition. Normally there will be an embedded single shot thermal fuse within the primary winding; but if not then the winding itself will have gone open circuit due to excessive heat.

In terms of replacing the transformer, the input and output voltage ratings should be as the original, and the rating (in VA) be at least equal to that being replaced.
With reference to the above safety features of the transformer, you would be well advised to select a replacement transformer that includes thermal protection in the event of an overload condition.

In terms of replacing a transformer that has no VA rating marked, one of the same physical size (or larger) should be OK.

If the transformer has only one secondary winding to give the 12Vdc and 5Vdc, then the secondary AC winding voltage is likely to be either 9Vac if the 12Vdc is obtained by using a bridge rectifier and capacitor – or if the 12Vdc is via a regulator then the secondary winding is likely to be 12Vac (to give an increased voltage input to the regulator).


Joined Sep 17, 2013
That pcb looks ancient, so it's possible the tranny died of old age. However, it's more likely it was overloaded by something downstream drawing excessive current, so you should check there with a meter before risking the replacement tranny suffering the same fate. The usual culprits would be electrolytic capacitors and semiconductor devices.