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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
Shaffer b 222-1-01276 transformer

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

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    \$\begingroup\$ What happens to the transformer voltage if you put a load on it? The secondary appears to be rated for 800mA, so you might try something like 500mA. My guess is that the transformer is fine. \$\endgroup\$ Jul 21, 2019 at 18:31
  • \$\begingroup\$ I tried a 220ohm resistor now. It got uncomfortably hot in under 5 seconds, and I still measured 13v across it. Same for both outputs. \$\endgroup\$ Jul 21, 2019 at 19:22
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    \$\begingroup\$ 13VAC from an unloaded 9V transformer sounds pretty reasonable to me (especially with +10% primary voltage). Without any load, reading a voltage between the two secondaries (say, pins 1-4) also sounds plausible to me, what happens to that reading if you load that down? \$\endgroup\$
    – marcelm
    Jul 21, 2019 at 19:41
  • \$\begingroup\$ Now I tried soldering a 470 ohm across both 1-2 and 3-4. Still 13VAC across both. 1-4 I also get 13 and 2-3 I get 8,5. At 13v that should be about 350mA, and plenty to pull the voltage down I suppose? I also verified I have 125VAC across both primary windings. \$\endgroup\$ Jul 21, 2019 at 20:08
  • \$\begingroup\$ 125 is more than 110. I would expect 9 Vac only at 110 Vac input and the full 2x800 mA load. \$\endgroup\$
    – winny
    Jul 21, 2019 at 21:08

2 Answers 2

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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.

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  • \$\begingroup\$ The rectifiers and diodes are gone yes. I removed them to isolate the transformer output :-) I measure about 1.5 ohm across both secondaries and no connection between them. \$\endgroup\$ Jul 22, 2019 at 6:27
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For the record:
First my apologies to the respondents here for me apparently not knowing any math or how to read my own multimeter! A 220 ohm resistor at 13v gives ~60 mA, not 600!

When I made two 16 ohm resistors using some nichrome wire the voltage dropped to 11v.

Considering that 255v is about 15% overvoltage that would suggest 10,5v, so all is good then :-)

The larger problem turned out to be some 16v 4.7uF tantalum caps that shorted out.

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