William Sommerwerck wrote:

You are ignoring core saturation. That limits the maximum voltage
on the secondary.

But wouldn't that require enough current draw to saturate? Or are you
arguing that the transformer is so cheap that it normally operates close to
saturation, anyway?


Most cheap consumer items that I've looked at are so close to
saturation that their transformers start to saturate & overheat at less
than 20% over the rated voltage. I have seen some with less than a 10%
margin. Even worse, the cheap *******s didn't use a thermal fuse in the
core or metal bells for fire protection, and set the plastic cases on
fire. I had to locate & replace hundreds of substandard transformers a
service center installed in some old Jerrold/ General Instruments
JRX3/JSX3 Cable TV boxes. Luckily, the rest of their work was just as
shoddy, and all but a half dozen were back in the shop before they
caught fire out of over 8000 that were in service.

It isn't hard to test for saturation. A variac, a low voltage lamp
that is close to the primary current, and a scope across the secondary.
Slowly bring the input voltage up, until the output starts to flat top,
and read the input voltage. The lamp will light, or burn out if you turn
it too high, to protect both transformers. It was part of the process
to qualify a new transformer for our products. It also helped catch a
bad vendor who changed the design, do save them a little money after the
design was accepted.


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