I'm trying to troubleshoot a German ceramics kiln controller from about 1989.
There is a 100ma glass fuse in front of this PCB transformer that blows after a second or two when power is applied.
Schaffer B 222-1-01276
I de-soldered some components "downstream" to isolate its output, and the fuse now survives. That could point to anything downstream of course, but then I measured the transformer output.
I have not been able to find a spec sheet or on this transformer, but from the markings, I naively expected to measure about 9v ac across both outputs. What I see instead is about 13v on both, and also between them.
This does not make any sense to me. Can output voltage rise that much when there is no load? I'm starting to doubt what I thought I knew about transformers, but then my experience is limited.
To be more specific, there are four inputs, 1,2,3,4 where 2 and 3 are bridged, and 255v is applied between 1 and 4. (yes, the mains supply transformer is just outside in the street)
Between the four outputs 1,2,3,4 I expected 9v from 1-2 and 3-4, but measure 13v. I also see 13v on 1-4, and some lesser voltages on 2-3, 1-3, 2-4.
With the power off I have verified there is no contact between 2 and 3.
I'm grateful for any pointers to put me on the right track here.
PS.
This controller board is not supplied directly from the mains but a massive 400VA isolation transformer. Not sure how exactly that could affect things
Best Answer
13V sounds reasonable for open cicuit. The transformer output will be 9V RMS at 220V input and a resistive load with 800mA RMS on both secondary coils. I don't know the configuration, maybe the secondary coils are in series. Then you would have 18V at 800mA. I guess there's a short somewhere on the secondary side. You can measure the resistance of the secondary input when the transformer isn't connected. Maybe a rectifier diode is gone. Definitely the resistance of the circuit should be way more than few Ohms.