On 11/25/2010 12:01 PM Phil Allison spake thus:

"Steve Sousa" wrote:

"Phil Allison" wrote:

** Assuming the transformers are the same size and do the same job, the
one with the 4000 ohm primary is stuffed.

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?

** No.

The ratio between rated primary Z and winding resistance is never less than
5:1 and is typically 10:1 or more.

You may have a point there.

OK, let's climb down into N. Cook's pit of bodgery and post mortem
analysis and look at this.

It all depends on the expected failure mode of a transformer, correct?
Two main possibilities he open windings, or shorted windings (or I
suppose a combination). Obviously not an open winding, so probably
shorted ones.

Shorted windings could be within the primary or secondary, or between.
Let's look at the simpler case, shorted within the primary, which should
be 4K ohms.

It seems most likely that any short would be between adjacent layers of
windings. Not likely that there'd be a short from close to one end to
close to the other end. Which should limit the effect of the short. So
if the 4K ohm primary is now 900 ohms, there's something else going on.

But what? Could there be a short from the winding to the iron core?

Has the O.P. checked for a short between primary and secondary? How
about shorts to the core/frame of the xfmr?


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