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IamSmooth

Fri Sept 29 2006, 05:44PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

If I wrap a ten turns of wire around the high voltage secondary of an ignition coil (say it is at 10kv) or a spark-plug lead on a car will I get a high-voltage output that I can measure? Or, will the coupling be poor and I'll get close to nothing?

Hazmatt_(The Underdog)

Fri Sept 29 2006, 05:46PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You probably won't get much because the field is inside the can due to the way its constructed.

IamSmooth

Fri Sept 29 2006, 06:51PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I may not have been clear. The 10 turns of wire are looped around the high voltage lead coming off of the hot terminal. THis is similar to high voltage test clamps that are used by automechanics to probe the tension on the spark plug line without cutting the connection.

I want to know if the B-field around the single, "hot wire" is enough to induce a voltage within my test-coil connected to a meter?

Sulaiman

Fri Sept 29 2006, 07:00PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

the pickup from an ignition lead is normally by capacitance,
a few twists of wire around the eht cable is used.

There will be a B-field, milli-amps flow in the eht lead
but you don't pick up a B-field due to current in a wire by coiling around the wire.
A wire in parallel or more usually wire wrapped around a ferrite or steel-laminate core
that the eht lead passes through.

So the answer is
NO. a wire wrapped around the eht lead will not pick up the B-field.

vasil

Fri Sept 29 2006, 09:49PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

If the ignition coil is open core type, you can wind the turns (use insulated wire) on the metalic case. It is possible so to build a single transistor driver.....and also possible to measure a voltage.

http://www.geocities.com/teslina/igigbtpage.html

IamSmooth

Sat Sept 30 2006, 12:13AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Just for the record, I have an ignition probe for making HV measurement; however, I would like to experiment by making a crude one of my own.

I would like to wrap some foil around a HV line (this will be the capacitor) and connect a resistor in series to ground. I would then measure the voltage drop over the resistor using a scope.

My question is, since I haven't done this and the voltages will be in the KV range, what kind of voltage can I expect on the resistor so I don't have to worry about damaging my oscilloscope? Theoretically, at t >0+ I should see the entire voltage drop across the resistor.

I'm going to go to one of my engineering books and look up step responses for a RC circuit. Does this sound right? The step response for the voltage across the resistor of a RC circuit is:

Vr = (Vo)e-t/RC

What kind of capacitance should I expect from 1" of metal foil around a HV line (or a standard clamp that will surround the line)?

EDIT: I just put a 1.5" cylinder of foil around an insulated cable and got 0.02 nF.

teravolt

Sat Sept 30 2006, 02:13AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

If you want to mesure the pulse accuratly your rc should be 10 times faster than the pulse. Start with 10k or 100k .25w and let us know. You may have to know what the pulse duration and amplitude realy is. You could use a resistive divider directlly on the output to find this.

IamSmooth

Sat Sept 30 2006, 05:00PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I used my automotive ignition probe for a reference as well as my standart 1:1000 resistive divider:

By creating a 0.02nf Coaxial capacitor I was able to nicely pick up the voltage. I used a 1:1000 resisitive divider so I could see it on my scope. The signal on my Mcguiver probe had a lot of ringing. I guess the true probes add dampening to attenuate this property.

Sulaiman

Sat Sept 30 2006, 06:46PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For my ignition coil (and other) high voltage probe
I have a 'proper' Tektronix 40kV probe
and I also use a string of 33MOhm resistors as a divider
with similar results/waveforms.
I don't see the need for a capacitive probe, but
If you use a capacitive pick-up then it's probably better to use a capacitive divider
to keep the correct waveshape (i.e. not the DIFFERENTIAL of voltage with time)

There IS an incredible ammount of 'ringing' on the eht of an ignition coil.

IamSmooth

Sat Sept 30 2006, 08:33PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Thanks Sulaiman. I am just experimenting.

Speaking of experiments, I notice that as I move my probe away the voltage I pick up drops. Theoretically speaking, is the drop I see with increased distance do to a resistance element that has to be added to the transmission model or is the capacitance between the HV line, the air and my pickup plate frought with voltage leakage?

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