Transformer Question

[Rene Schmitz]

Hi Harry, and DIYers

At 20:48 27.02.00 -0500, you wrote:
>Hi Rene (et al)
>
>There are two possible problems with the ohmmeter...
>
>1) The readings are usually really small... takes a good meter to read
>them...
>If you have "relative" measurement capability (Fluke 8050 etc...) then its
>easier...

I have usually no problem with common power supply transformers, the most
have copper resistance of 1-100 Ohms on the secondary.

>2) If you are reading some really big Iron  (many turns, lot of core... like
>esp. TUBE power supplies....) and you use a meter with a substantial battery
>(like an old Simpson
>260....) when you "remove" test lead you can get knocked right on your @ss!!!
>Think
> "how many volts" is applied times the number of turns and.... bingo.

Yep, that may happen, but I never had trouble with my DVM, the measurement 
voltage is pretty low. (And usually I avoid touching the blank end of the
probes when measuring resistance. Aligator clips.)

>I had to do it three times... once for a surprise, twice "was that a charged
>cap??", three...
>Oh now I get it... ignition coil effect !!!

A simple step up switcher... ;-)

Another method to figure out the center tap on a transformer is to use the
"component tester" of a scope. Tom G. posted something like that arround a
while ago. My scope has it built in. It shows an ellipse on the screen, you
have to find the two wires that give the same figure.  

Bye
 René

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