Arfa Daily wrote:
"Jim Land" wrote in message
. 3.44...

Matthew Kirkcaldie wrote in
:


Having found an open primary in the small transformer ....


That's strange. Transformer primaries don't open very often. I would be
inclined to dig deeper to find out why there's an open circuit (internal
fuse open? Bad connection? Melted wire?) and whether there is any fault
downstream of the transformer (shorted component?).


In my professional repair life, I replace trannies in hifi units all the
time, that have primaries that have gone open for no apparent reason. Yes,
sometimes there is a thermal fuse in the primary, and yes, sometimes it's
connected across a couple of external pins so can be temporarily bridged for
test purposes, but I find it rare to then come across a fault downstream,
that has caused a genuine overheat that has led to the failure.

Arfa

very small 50/60Hz transformers can be a right pain, as the wire gauge
is so damn fine. Its extremely hard NOT to slightly damage, say, 44AWG
magnet wire when winding such a transformer.

Low power transformers must have extremely high magnetising inductance
AND low volume, so require silly numbers of turns of astonishingly
(vanishingly more like) fine wire.

One I used in an OEM UPS' had, IIRC, 11,000 primary turns, of (again,
IIRC) 44AWG. Now 41AWG is about 0.09mm outer diameter; 11k turns is thus
around 70mm^2, ie 8.4 x 8.4mm, which is about the cross-sectional area
ISTR for the primary winding.

And we had quite a bit of trouble with those - although they were made
in Mexico, for a very low price - all of which was related to the
termination.

Cheers
Terry