If not, then I
would disconenct the transformer primary and secondary
leads. Use a 'megger' (HV isulation tester) (...)
I have experience with these instruments, I've used them to test products
at a previous workplace..
I find it a useful instrument to have. I do own a genuine hand-cranked
magneto 'Megger', but the instuement I normally use for this has a
battery-powered HV gnerator that can be set to 100V, 250V, 500V or 1000V.
Anything that goes o nthe maisn over here had better stand 1000V, or I
want ot know the reason why :-)
Incidentally, while insulation breakdowns are not at all common, I've had
many more 'new' devices with what I consider to be excessive leackage
than 'classic' ones.
Hopefully it will not be thr transformer.
I've had a surpsing number of
switches go leaky over the years.
There is presently no on-board switch, it's hardwired 'on'. Switching is
being done at the Variac.
So the only things on the mains side are the power cable, the transformer
and a fuse (?).
Firstly, 'Digital circuits are built from analugue parts'.
Err.. yes.. I'm not sure what your point is? :-)
Basically,m if you undersatand eaanluge electronics then you can
understnad digital. In fact my expeirence is that the hardest part of a
digital ssytem to design proeprly are the interconnecxtions, which should
be designed as (analgoue) transmission lines
Seoncdly, and more inportantly, the power supply
is as 'aanlogue' as you
can get. Forget that it powers a computer, treat it like the PSU in an
audio amplifier. There is no real difference.
Again, yes.. did something I said give you the impression that I thought
otherwise? If you read my earlier posts, I specifically mentioned that I
had the PSU stuff taken care of months ago, as it's the sort of work I can
do in my sleep..
OK, I think I mis-interpretted soem of your posts.
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.
-tony