[Does the term
'megger' mean anything across the Pond? Over here it's a
common term (actually a trade name that's become generic) for a
high-voltage insulation tester]
Yes, we have meggers over here.
And you call them that? I am sure you have high voltage insualtion
testers, but I wasn't sure if you call them soemthing else.
The term 'Megger' is of course a contraction of Megaohmmeter, a device
that measures resistances of a million ohms or more. The important thign
is that they apply a fiarly high voltage across the 'resistor' under test
(which may well be the leakage from a transformer widing to its core) and
thus will cause any marginal insulation to spark over and show a low
reisstance.
I beelive that the normally-quoted requirement is that insulation should
stand at least twice the voltage it's goign to run at, so for UK mains
test at 500V or so. I normally test at 1kV, because my instrument can do
it and anything that can't stand 1kV to groudn probably shouldn't go on
the mains.
Origianlly a 'Megger' ahd a hand cranked generator to provide the high
voltage. It ws a trade name (I forget who). Therr was a competing
instrument called a 'Hum Metrohm' which used a battery-powered vibrator
and step-up tranformer (the former producing the 'hum') to give the high
votlage.
The think I normally use now is Austrian (I forget the manufacturer) and
uses a transistor oscillator and transformer (so it's a bit more modern,
but still not all that new :-)). The nice thign about it is that it can
be set to provide 100V, 250V, 500V, or 1000V. So I can use the first to
test a little 12V motor I've just reound and the last to check the
insulation on a mains transformer.
For the real enthusiasts there's something caled a 'Bridge Megger' This
has 2 modes of operation. One is a normal megger, the other is a
wheatstone bridge usign the generator as the supply (there is a built-in
4 decade resistance box). These normally work at a couple of hundred
volts (so not really suitable for checking stuff to go on the mains) and
were used to detect fualts in telehpone lines and the like. Not all that
useful for classic computer work (the high vbotlage prevents their use on
IC-based circuitry!), it's the sort of thing you buy if you see one cheap
and if, like me, you collect obscure measuring instruments.
At least
I've cleaned out the mouse nests. That's a start, I guess.
A cat is quite useful to prevent re-infestation :-)
And they also make for excellent hacking company. :)
Quite why soemthign that insists on jumping on your workbench and
scattering bits everywhere, and which is one of the best static
electricity generators known should be a hacking companion is illogical,
but I agree with oyu. I love Muon and Tigger...
-tony
