Testing HV Diodes

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Matthew Smith by way of Terry Fritz <twftesla-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>

Hi All

Seeing the posting on cheap DMMS made me think of a tip that I've been 
meaning to pass on for a while.

For those using MO diodes or possibly strings of 1N4007's, etc., you may 
find that testing using a diode tester or resistance range of a DMM 
shows no conduction in either direction - this, I guess, would be down 
to the measuring voltage being too low to switch the diode on (in the 
appropriate direction, of course!)

I now test all such diodes with a vintage 500V Megger tester - not the 
kiddy for measuring forward voltage, but fine to check basic function or 
polarity.

For those who have never seen one of these wonderful beasties, they are 
mega-Ohmeters used for checking insulation, earth leakage, etc.  They 
used a hand-cranked permanent magnet alternator (output is rectified) to 
provide the test voltage (300, 500, 1000, depending on model); modern 
equivalents use a battery and electronic means to generate the required 
voltage.  Meggers were produced by Evershed and Vignoles in the UK; that 
company has since been absorbed by Avo.

If anyone should come across an old (working) Megger for a reasonable 
sum, I would recommend them as very useful pieces of equipment.  The 
larger alternators may even be up to running a hand-cranked pocket TC!

Cheers

M

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