From: der Mouse <mouse at
[replying to two messages in the same thread together]
using a 40W light bulb in series with the primary, as a
current limiter, which is why the primary voltage was 25V instead of
~120V. The bulb was glowing (not quite full power, but close).
Well, even with a
40W bulb in series, I would expect a transformer
with no load on the secondaries to give almost full output if there
were not shorted turns.
Well, yes - since it would be drawing (to a first approximation) zero
current. That the bulb glows at all with all the secondaries open is
to me a strong indication that something is shorted.
That is not necessarily true. The transformer is an inductor and
would normally draw quite a bit of current. It would be 90 degrees
out of phase with the voltage across it but It is still current. It is
current that makes lamps glow as well.
To tell if the transformer had shorts, you'd need to measure the
relative phase angle of the voltage on the lamp and the transformer.
If it was 90 degrees, there was no short, just the open in the
For a quick check, measure the voltage across the transformer and
then across the light. If these totaled together was greater than
the measured line voltage, I'd say the transformer was most likely
OK. If the sum was equal to the line voltage, it is surely shorted.
Some 'oscope measurements on a dual trace would be more meaningful.
Just the fact that the light lit is not enough to say that the
transformer is bad or good.
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