Re: Coupling Questions & coil measurements

• To: tesla-at-pupman-dot-com
• Subject: Re: Coupling Questions & coil measurements
• From: Ed Phillips <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Wed, 11 Aug 1999 06:40:17 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

"I have always wondered what the k of my system is. However I have only
very
basic equipment to test it with - an analog meter, a home-made RF probe
and
a few bits with which I could make a GDO. Any hints on an easy way I
could
measure k with these parts?

Ross, as for your question about 'double humping', this occurs when
coupling is more than 'critical', ie 1."

	An easy and accurate enough way to measure k is to pass a known 60
cycle current (say about 0.8 amp by placing a 100 watt light bulb in
series), and measuring the 60 cycle voltage across the secondary.  The
mutual reactance is computed by"

	Xm = Vs/Ip
	Lm = Xm/(2*pi*60)
	k = Lm/sqrt(Lp*Ls)

All inducatance values have to be in the same units - henries,
millihenries, microhenries.

	Critical coupling occurs when the product of the coupling factor and
the circuit Q (primary or secondary, if both are equal) is equal to 1.
Since the Q of either primary or secondary is ill defined during the
time the gap is firing or/and there is a discharge from the secondary
terminal, this is not a really meaningful measure.  The "magic k" values
are the coupling factor k (as defined above), NOT k*Q, and are almost
totally independent of Q for any Q's which would be usable.

Ed

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