Re: Coupling Questions & coil measurements
"I have always wondered what the k of my system is. However I have only
basic equipment to test it with - an analog meter, a home-made RF probe
a few bits with which I could make a GDO. Any hints on an easy way I
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,
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.