[Prev][Next][Index][Thread]
Re: Ignition coil measurements
Hi Scott,
Interesting measurements:
> Original Poster: Scott Stephens <Scott2-at-mediaone-dot-net>
>
> 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.
Skin effect would be pretty well non-existent with the small
wiregauge in the secondary at that frequency. I would imagine that
the distributed capacitance is the biggest culprit.
> 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.
At those low frequencies it shouldn't be too hard to find SCRs with
suitable di/dt and dV/dt ratings. My Westcode devices would fit the
bill perfectly.
Malcolm