Ignition coil measurements

To: tesla-at-pupman-dot-com
Subject: Ignition coil measurements
From: Scott Stephens <Scott2-at-mediaone-dot-net> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Mon, 16 Nov 1998 20:50:29 -0700
Approved: terryf-at-verinet-dot-com
I measured an ignition coil and found the following:

Core in:
primary:
L=6mH R=1.6 ohms Q=20 (tested with .1uF -at- 3.8 KHz, sharp response)
step up ratio=86 (measured with 600 Hz signal)

2ndary:
L=67H R=8K ohms Q=3 (tested with .1uF cap -at- 65 Hz 3dB BW; sharp lower break
point, upper response rolls off more gradualy)

K= 0.81  which I derived from (correctly? seems .3 too high for solenoid)
Step-Up Ratio Measured (86) / SquareRoot [L-2ndry (67H)/L-pri (.006H)]

Self tuned peak frequency response at 3KHz
Mu-eff core = 5 (including extreme air-gap of solenoid)

Core out:
primary:
L=3mH R=1.6 ohms Q=17

2ndary L= 13H R=8K ohm Q=8.8 (c=.001uF -at- 1.6 KHz, 3dB BW)
Peaks frequency response at 5.8 KHz, no capacitors (except meas. circ. stray)

step-up ratio= 40 (measured with 600 Hz signal)
K= 0.6
Step-Up Ratio Measured (40) / SquareRoot [L-2ndry (13H)/L-pri (.003H)]

I have another ignition coil, that I more quickly characterized, without
tearing it out of its can:

Pri: L=1mH, R=1.4 ohms; 2ndry: L=102 H, R=8.8K ohms; step-up ratio-81, K=0.25

My measurements were made with an audio generator, a scope & DVM. I would
hope they are within 25%. The values of "K" or coupling coefficient between
primary and secondary are much higher than Terry reported in a post a few
weeks back. I derived my value of K from dividing the measured square-root
of the impedance transformation (or the step-up/down ratio) by the value
calculated from the ratio of the primary and secondary inductance. Maybe my
step-up ratio measurements were affected by interwinding stray capacitance.
My measurement seem too high for K, and Terry's, around .005 IIRC, IMHO too
low.

Maybe the most significant is Q. The secondary loss is awfull. At a coils
best frequency, 6KHz, X-L(13H) is 500Kohm; now 500Kohm/8000ohm
(DC-resistance) = 61. So Q (unless my capacitor was crap, very unlikely)
should be 61, not 3!!!

Is the skin effect is kicking the hell out of the coils Q? Too bad. I tried
using a 3/8" powdered iron stick in place of the steel core, but both cores
resulted in a 3dB core loss. They do wonders for the inductance & coupling
though. If you can cool a steel or ferrite core better than those insulated
windings, it would be worth keeping, provided it doesn't saturate & suffer
hysterisis and other losses at much higher power levels.

So how many watts can it dissipate? I'll guess around 100 Watts in air for
30 second runs, and 1000 Watts in oil. Now if I fig'r right, the envelope
period = 2*Q/2*PI*F, so at 6 KHz, 2*(9)/2*PI*6KHz = 500 micro-seconds. So
you can let a car coil 'ring up' for half a millisecond at 6 KHz, before you
reach a point of diminishing returns, you can integrate 3 cycles of 6 KHz
energy. You want tight coupling to transfer energy quickly, before the coil
burns it.

This is significant for me, as I would like to pump it with a dual-scr 120
VAC inverter, and let the coil ring up 30 KV or so across a 2nF glass plate
capacitor, at 3KHz, about the size of a 4" cube. 100mJ for the TC, 100mJ of
heat dissipated by the ignition coil, up to 1000 bangs/second for 100 watts
average.

My second design would use a 1200V MOT & it's 1uF 2KV cap to break-over 3 or
4 series SCR's into a 900uH ignition coil primary. Higher energy, but uses
bulky MOT. Can SCR's be abused in such a way (operating like a spark gap)?
Higher output voltage (+100KV!) too.