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Trying to measure spark gaps mA current?

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Ken Williams

Wed Jun 05 2013, 04:33AM

Registered Member #8247 Joined: Thu Nov 22 2012, 03:14AM
Location:
Posts: 18

Hello, I have a spark gap in the attached image. I would like to measure the current for it. I guess the voltage is probably 10-30KV and the frequency might be 120Hz. I expect the current to be in the low mA's such as 4 mA.

My question is can I use one of these precision clamp on meters to measure this current? For example something like this:

They say they have a resolution in the uA's. Can I just clamp that over one of the leads to get a current reading or does the high voltage stop my idea cold?

Thanks for any help.

The Lightning Stalker

Wed Jun 05 2013, 07:56AM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

Yes, that should work well at those frequencies.

Avalanche

Wed Jun 05 2013, 11:57AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I don't think you'll see anything with a standard clamp meter. Even though the repetition frequency of the spark is low at 120Hz, the risetime of the spark will be very high and the duration of it will be very low, so you'll need something with bandwidth into the Mhz range to get an accurate measurement.

A Rogowski coil with integrator is what you need to view high bandwidth current waveforms on a scope - but they are not cheap to buy. However I'd say they are definitely something which you could home brew. Here's a idea of what's available, we have one at work that goes upto 17Mhz and it cost about Â£700 I think

The biggest problem with a home brew setup would be getting the accuracy/scaling right, but I'd say it's definitely worth a try!

Got me thinking now...Another idea would be to put a low value shunt resistor in series with the spark gap, and then use precision op-amp circuits to capture the peak, average or even RMS values of the voltage across the shunt. Then convert these readings into currents.

HV Enthusiast

Wed Jun 05 2013, 01:47PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Yes, for a spark gap current, you really will need a high bandwidth current probe. The spark rise / fall times and durations are just too fast / short to be seen with a standard clamp-on current probe.

Also, if you are discharging a lot of capacitance in the spark gap, then you'd need a pretty large clamp on probe. Even the small and expensive high bandwidth Tektronic current probes would likely be damaged with the high peak currents you would see with a capactiator discharge application.

Patrick

Wed Jun 05 2013, 04:45PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

since the current is the same everywhere in a series circuit, you would obvioulsey measure the current on the low side with a resistor then you could use a instrumentation amplifier with high bandwidth and optical or capacitive isolation to reject noise and false coupled measurements. you could build a half way decent device for 40 to 70 US$. magnetic steel just wont work fast enough.

Ken Williams

Wed Jun 05 2013, 06:34PM

Registered Member #8247 Joined: Thu Nov 22 2012, 03:14AM
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Posts: 18

I know I can do the shunt resistor thing but I'm lost with that half the time. I was hoping a precision clamp meter might still do it. I've attached a few screenshots of how and what ends up going into the high voltage transformer. About 1000V @ 100Hz ends up going into it. The transformer is suppose to be a 120Kv transformer but thats when a full 2000V is going into it. So its probably more like 35Kv if I had to guess.

The two scope screenshots essentially show the same thing, this is a measurement across the .33uF cap or Gas Discharge Tube that shows this. Based on the screenshots does anyone think a more simple solution for measuring the current either going into or out of the high voltage transformer can be accomplished with a clamp on meter still?

Thank for any continued help.

Ken Williams

Wed Jun 05 2013, 06:45PM

Registered Member #8247 Joined: Thu Nov 22 2012, 03:14AM
Location:
Posts: 18

The Lightning Stalker wrote ...

Yes, that should work well at those frequencies.

What about at those voltages? That clamp on meter seems to be limited to 600V, does anyone know if that limit applies when measuring current too? I would not even be able to measure the current going into the high voltage transformer, never mind what comes out.

Steve Conner

Wed Jun 05 2013, 09:56PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What is your goal? There are a bazillion different ways of measuring current. RMS, average, rectified average and so on. The choice of transducer depends on what you're measuring. Tell us about what you are trying to do.

Ken Williams

Wed Jun 05 2013, 10:33PM

Registered Member #8247 Joined: Thu Nov 22 2012, 03:14AM
Location:
Posts: 18

The electricity going into the high voltage transformer is stepped up from ~1000V @ ?mA to probably 35Kv @ ?mA. I need to know the ?. I simply need accuracy which is difficult when working with high voltages. So I would like to measure the 1000V sawtooth pattern's (as shown in the above screenshot) current going into the transformer, and the current coming out after it is stepped down (because the voltage is stepped up).

I'm an amateur which is why I'm lost half the time. If there is anything else you'd like to know please ask. I just have difficultly measuring current at these voltage levels hence my interest in the precision clamp meters for measuring leakage. I thought it might work for something like this and was looking for confirmation.

klugesmith

Thu Jun 06 2013, 12:46AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

It sounds like you want to measure the DC (average) current in the spark gap circuit.

[edit] Welcome to 4hv, Ken. Now I see your schematic.
Nice circuit you got there. Has a lot in common with spark gap Tesla coils.
You can figure the average current in T2 primary by multiplying the charge swing on C3 (in coulombs) by the charge/discharge frequency. Since there's DC current in T2 primary winding, we can't rule out DC current in its secondary winding. [\edit]

Measuring average current is easier than trying to get high bandwidth pictures of spark currents, which are very brief , very high, and could be oscillatory.

The clamp-on meter you cited doesn't read DC current. It will attempt to give the RMS value of the AC component (within some frequency range) of spiky spark current. Is that what you want? Let's hope it's internally protected against current spikes orders of magnitude higher than the measurement range.

Putting an ordinary multimeter in series with the circuit is likely to fry the meter,
as I can attest.

A filter could fix that if you know what you are doing.

Can your spark gap work with one electrode grounded?
If so, then I would ground it through a current sensing resistor (10 or 100 ohms?) in parallel with a capacitor, such that R * C > 100 ms. Then measure DC voltage across the R. If voltage spikes are still present on that node, due to the capacitor's ESR and ESL, you could add a second filter stage using higher R values (to both ends of the new capacitor?) and, perhaps, lower C values.

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