high voltage induction (capacitive coupling)
IamSmooth, Fri Sept 29 2006, 05:44PMIf 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?
Re: high voltage induction (capacitive coupling)
Hazmatt_(The Underdog), Fri Sept 29 2006, 05:46PM
You probably won't get much because the field is inside the can due to the way its constructed.
Hazmatt_(The Underdog), Fri Sept 29 2006, 05:46PM
You probably won't get much because the field is inside the can due to the way its constructed.
Re: high voltage induction (capacitive coupling)
IamSmooth, Fri Sept 29 2006, 06:51PM
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?
IamSmooth, Fri Sept 29 2006, 06:51PM
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?
Re: high voltage induction (capacitive coupling)
Sulaiman, Fri Sept 29 2006, 07:00PM
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.
Sulaiman, Fri Sept 29 2006, 07:00PM
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.
Re: high voltage induction (capacitive coupling)
vasil, Fri Sept 29 2006, 09:49PM
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
vasil, Fri Sept 29 2006, 09:49PM
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
Re: high voltage induction (capacitive coupling)
IamSmooth, Sat Sept 30 2006, 12:13AM
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.
IamSmooth, Sat Sept 30 2006, 12:13AM
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.
Re: high voltage induction (capacitive coupling)
teravolt, Sat Sept 30 2006, 02:13AM
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.
teravolt, Sat Sept 30 2006, 02:13AM
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.
Re: high voltage induction (capacitive coupling)
IamSmooth, Sat Sept 30 2006, 05:00PM
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.
IamSmooth, Sat Sept 30 2006, 05:00PM
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.
Re: high voltage induction (capacitive coupling)
Sulaiman, Sat Sept 30 2006, 06:46PM
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.
Sulaiman, Sat Sept 30 2006, 06:46PM
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.
Re: high voltage induction (capacitive coupling)
IamSmooth, Sat Sept 30 2006, 08:33PM
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?
IamSmooth, Sat Sept 30 2006, 08:33PM
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?
Re: high voltage induction (capacitive coupling)
Simon, Sun Oct 01 2006, 01:30AM
Simon, Sun Oct 01 2006, 01:30AM
wrote ...
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?
You're talking about the capacitive coupling, right? When you move the probe away, the capacitance between the probe and the source goes down. Remember that capacitive reactance goes up as capacitance decreases. Your measuring device is some sort of load so the theoretical model is just your usual voltage divider: as the impedance of one part goes up, the voltage across the others goes down.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?
Re: high voltage induction (capacitive coupling)
Sulaiman, Sun Oct 01 2006, 03:47AM
Test Screwdrivers of the type that need batteries and use a led, not the neon type,
make excellent field-strength meters!
My Ignition Coil invertor wire will light the led in my test pen from about 1m
My TC on low power sinewave testing can also light it from about the same distance,
surprisingly sensitive.
Sulaiman, Sun Oct 01 2006, 03:47AM
Test Screwdrivers of the type that need batteries and use a led, not the neon type,
make excellent field-strength meters!
My Ignition Coil invertor wire will light the led in my test pen from about 1m
My TC on low power sinewave testing can also light it from about the same distance,
surprisingly sensitive.
Re: high voltage induction (capacitive coupling)
IamSmooth, Sun Oct 01 2006, 04:50AM
IamSmooth, Sun Oct 01 2006, 04:50AM
Simon wrote ...
However, my measurement is across the resistor. The capacitor is made up of a plate that I move away from the HV line. Connected to the place is a resistor. My measurment is across the resistor. So the question is, how come the further I move away from the HV line does the voltage go down? Is it due to the field dropping as 1/r^2, a resistive element added in series to the existing capacitor/resistor, or something else? wrote ...
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?
Remember that capacitive reactance goes up as capacitance decreases. Your measuring device is some sort of load so the theoretical model is just your usual voltage divider: as the impedance of one part goes up, the voltage across the others goes down.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?
Re: high voltage induction (capacitive coupling)
Simon, Sun Oct 01 2006, 11:15PM
It's still what I was talking about. You have a capacitor in series with a resistor. That's a voltage divider, albeit a frequency dependent one. Just like any other voltage divider, when the impedance of one element goes up, the voltage across the other goes down. Just as I said.
Simon, Sun Oct 01 2006, 11:15PM
It's still what I was talking about. You have a capacitor in series with a resistor. That's a voltage divider, albeit a frequency dependent one. Just like any other voltage divider, when the impedance of one element goes up, the voltage across the other goes down. Just as I said.
Re: high voltage induction (capacitive coupling)
IamSmooth, Wed Oct 04 2006, 07:09PM
I've been walking around my basement with a makeshift, voltage-sensing device based on the capacitative coupling. I connected a piece of foil to a digital voltmetter and set it to AC. When I approach a live wire I can register up to 1-1.5v. If I ground one end of the probe I can pick up almost 5v. A dead wire picks up just about nothing.
IamSmooth, Wed Oct 04 2006, 07:09PM
I've been walking around my basement with a makeshift, voltage-sensing device based on the capacitative coupling. I connected a piece of foil to a digital voltmetter and set it to AC. When I approach a live wire I can register up to 1-1.5v. If I ground one end of the probe I can pick up almost 5v. A dead wire picks up just about nothing.
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