Re: Scoping coils
Subject: Re: Scoping coils
From: mrbarton-at-ix-dot-netcom-dot-com (Mark Barton)
Date: Wed, 13 Dec 1995 23:09:52 -0800
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I'd just like to add an amen to everything Malcolm has said regarding
secondary coil parameter measuring technique. Here are some of my
procedures that have worked quite well.
The closest I have come to getting an accurate reading of secondary
unloaded Q goes like this. Ground the base of the coil and clear away
EVERYTHING around it for a radius of at least 6 feet. Make an
excitation coil by scramble winding about 30 turns of hookup wire
around a gallon paint can. Slide the wire off the can and wire-tie or
tape it tightly together. You should now have what looks like an old
color TV de-gaussing coil. Connect this coil to your signal generator
and place it near the base of your secondary. I have found that a
distance of 3 feet away from the base works well.
Now for the scope. Usually, just a piece of wire for an antenna-type
pickup works well, but since we are trying to be as uncoupled from the
coil as possible with our drive source, we have less than the usual
amount of signal and 60Hz and radio stations become a problem. I
constructed a little 3-pole passive high-pass filter from 3 R's and 3
C's, followed by a 1-pole low-pass. This completely eliminates the
interference problem and you can watch clean waveforms on your scope
with the pickup antenna several feet away from the coil. Also, suspend
the coax going from the antenna to the scope horizontally in a
direction pointing radially away from the coil (really, no kidding).
Using this technique, I have measured Q's upwards of 200. In fact the
Q's are so high, that short-term signal generator drift becomes a
problem during the measurement. Even the swaying of nearby trees and
any body movements (even 6 feet away) were clearly visible making it
necessary to stand dead still and wait for the winds to die down before
we could make a good measurement! Of course a frequency counter is a
must, and several measurements of f0 and the 3db-down points should be
taken and statistical methods applied (throw out the lowest and highest
BTW, the secondary used in this experiment was a 2 foot diameter
polyethylene barrel wound with 22awg teflon stranded wire 3 feet high.
It resonates at 80KHz with a small toroid. This is the secondary for
the FET driven system.
All this may sound like a lot of work, but it's really not and it will
yield very accurate readings. If you still get a Q of 32 from your
secondary after all this, throw it away.