[Prev][Next][Index][Thread]
Corrected Ignition coil measurements
Since James pointed out my Q measurements were at different frequencies, I
did them again. This time I used a 470pf ceramic cap to resonate the
ignition coils secondary. I use a Fluke 77 DMM to measure frequency &
capacitance. It said my 470pf ceramic disk was .83nF. Realizing crapy
ceramic disk caps are often +/- 20%, I measured a few sub-nF polyester and
silver mica caps, marked 5% and 2.5%. My meter doubled the capacitance of
near all. But when I measured a 6800pf cap at .0071uF, and put the ceramic
cap in parallel, it measured .0076uF. Test equipment, like politicians &
salesmen, lie to you if you trust them.
The first method, used on my previouse post, I used to derive the inductance
of the coils was by measuring the voltage drop across the coil, in relation
to a comparable series resistance, at an audio frequency. The reactance
varies with frequency, due to the high interwinding capacitance.
This time I estimate inductance from resonant frequency and (hopefully)
known capacitance. But the Q is not high, so the F=1/2 PI LC isn't very
accurate, but a longer formula is needed, that takes the forced/loaded
responce rather than the natural/unloaded. Criticaly damped circuits, IIRC,
are typicaly 30% higher in resonanct frequency than the simple formula
predicts. Anyways, the data:
This car coil, unpotted, is about 3" high and 2" dia, and has an 11/16" dia
core hole. The core is a square array of metal strips, which is easily
removed.
No core:
L sec = 10H, C=.5nf, Fres=2.4Khz, Fh=2.6, Fl=2.2, BW=.4Khz so Q= 2.4/.4 = 6
Core in: (which reduced amplitude 3db after retuning)
L sec = 50H, C=.5nf, Fres=1.03Khz, Fh=1.32Khz, Fl=.9Khz, BW=.4Khz, so Q=1/.4
= 2.5
Not too much different than my first estimates :)
I wouldn't trust my cheap scope not to load the circuit down with any less
resonating capacitance. And I'm too busy to build an active buffer circuit.