Kinda OT but that what's true HW hacker do in a pinch.

From: Tothwolf <>
Date: Sun Jan 20 01:32:11 2002

On Sun, 20 Jan 2002, Tony Duell wrote:

> And sometimes the primary is designed to be the thermal fuse. If it
> overheats, the insulation melts, you get shorted turns, and it either
> melts the wire or blows the current-operated fuse. Apparently the
> former (wire that's designed to melt after the insulation has melted)
> actually meets most safety approvals, and it's the intended failure
> mode of most cheap wall-warts over here. Is it any wonder I can't
> stand the darn things?

The 9V AC wall-warts that are sold with USRobotics/3Com modems are made
this way. I had one spontaneously fail, and could find no problems with
the modem it had powered. The wall-wart had an open primary, so I
disassembled it. Inside, I found the core's plastic bobbin was melted.
While removing the primary windings, I found damage to the enamel wire
that indicated that the wire had been wound too tightly when the core was
manufactured. It appears that a few layers of the primary winding were
shorted out from the factory. What seems to have happened, is that the
remaining primary windings were over voltage, and eventually the heat that
it produced damaged both the plastic bobbin and the wire's enamel

USRobotics/3Com still would not send a replacement unit free of change,
even tho this one was clearly factory defective. They want $15 plus
shipping for these pieces of junk.

I've been considering rewinding this particular transformer, just for
practice (I haven't rebuilt one in awhile). I've not yet decided if it's
worth possibly wasting some enamel wire on. I think I'd want to replace
the plastic bobbin if I do decide to rewind this transformer.

Received on Sun Jan 20 2002 - 01:32:11 GMT

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