On Aug 11, 6:03*pm, "Phil Allison" wrote:
"spamtrap1888"



I'm wondering though if a secondary is supposed to deliver say 20
volts with a 120V primary and it's hit with 240 then that secondary
will produce 40 volts out.

** Totally wrong !!!!!

Try it yourself and see what REALLY happens.

I'm confused. I would have thought that, at least for the first
instant, the turns ratio means there would be 40 VAC on the output,
until the primary's turns start shorting together as the enamel
combusts because of heating caused by the excessive input current.

** Nope.

Magnetic saturation is an instantaneous phenomenon.

The cores of most small transformers operate well into saturation, even at
rated voltage *- so doubling the incoming primary voltage causes complete
core saturation and the primary side current flow is limited only by the
resistance. The RMS current goes up by 5 or 10 times the normal level.

Crucially, the secondary voltage rises only slightly due to a large voltage
drop being caused by the primary's resistance.

A correctly rated fuse will blow immediately and the event is all over.

With the PSU in question, the AC fuse was missing - *but very likely had
been replaced with a larger one at some stage.


Thanks, Phil.