5 Sep
2014
5 Sep
'14
1:28 p.m.
On Sep 5, 2014, at 3:37 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
2 nF is what I get, too. That wouldn?t be stray capacitance in the transformer. But it
could be an EMI filter capacitor, if the PSU has such things. A DC test would sort that
out.
paul
...
But it +has+ been a +while+ since I worked with a PSU that was driven by
such a large EI core xfmr. So I was asking if the ~85uA (that's micro-amps)
of leakage I measured would be typical for iron of that mass (18lbs or so)
and age. Virtually all (if not _all_) transformers exhibit some leakage -
it's just the nature of the beast. Stray capacitances in the xfmr provide a
coupling path for AC voltage, in addition to the other leakage paths which
can develop (due to imperfect insulation, etc).
Sure. Which is why a DC inslation test (and note that some 'meggers' use
AC) is worth doing. This will distinguish between an inslation problem
(which will pass DC) and stra cpacitance (which won't).
Now lte'#s see...
85uA at 11)V, say is a resistance/impedance of about 1.3MOhms. At 60Hz,
that would seem to correspond to the reactance of a 2 nF capacitor (check
this, I may have made a mistake in the calculation). That sounds high for
a 'stray' to me.