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4hv.org :: Forums :: High Voltage
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HV Probe - Convert DC only to AC/DC

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Zenador
Fri Oct 09 2009, 03:41PM Print
Zenador Registered Member #1733 Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Hey all,

I picked up 2 HV probes, one like this - Link2 - and another one with a 30kV meter built in.

The probe only model has a 740MΩ internal resistor. My sacrificial-lamb DMM has a 1MΩ internal impedance. To achieve the desired 1000:1 division, the DMM cross-terminal impedance should be 7.47474747MΩ. No problem here - but this is DC only resistive divider.

Now this probe does not have any capacitors to pass the AC component. I believe I have the space inside the probe to fit some physically small caps. On to my questions...

1. Does cap voltage rating matter in this HV application? I assume it does. I'm looking to measure 60kV+
2. Does capacitance matter? I assume it does. Measuring tens of MHz would be desirable.
3. If the answer is "yes" to Q1 and/or Q2 above, can someone point me to the math to figure this out?

And out of curiosity...
4. Provided the above is possible, could one use this probe on an oscilloscope, by connecting the probe tip to the common clamp on the probe, and use the probe tip to measure a tank circuit in a TC?

Thanks in advance...

Zen
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Proud Mary
Fri Oct 09 2009, 06:04PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
You'll never get good results like this, because your main dropper resistor is probably shunted by about 5 or 6pF. You will get gain peaks and other anomalies.

Also, it's not easy to imagine a 120kV capacitor of any sort (twice your maximum voltage for safety's sake) inside the handle of a probe.

You'd be much better off doing things properly using a capacitance dropper circuit such as 5pf in series with 5000pF, (of appropriate voltage ratings) which will give a 1000:1 division ration.

Lastly, "tens of MHz" at 60KV will be no respecter of the insulation on the handle of your probe. The RF may look at the plastic, and think to itself "Ah ha, I thought I was stuck, but now I see this capacitor leading straight to that man-shaped antenna. Whoopeeee"

RF burns can be deep and you actually see and smell the smoke of cauterized flesh.
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Zenador
Fri Oct 09 2009, 08:09PM
Zenador Registered Member #1733 Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Thanks Harry...

Ok, let's start with the most harmful part - holding the probe. I'm willing to forgo the probe (I do have another one) and build a "bench" unit, where connections would be made by alligator clips or grabber clips. That should remove the likelihood of RF burns.

Harry wrote ...

You'd be much better off doing things properly using a capacitance dropper circuit such as 5pf in series with 5000pF, (of appropriate voltage ratings) which will give a 1000:1 division ration.

From your statement, I read that only the capacitors are required on the AC divider. 95% of the dividers on the net are for DC (resistive only) or both AC/DC (capacitive/resistive.) A straight AC only divider would suit me fine as well. If I'm drifting off mark, please let me know.

So assuming a 60kVAC target (120kVAC rated for safety ;) ) can the 120kV rating be derived from, say, twenty 6kV caps in series. Or would one be better to use fewer/higher rated caps?

Thanks,

Z
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Steve Conner
Fri Oct 09 2009, 10:18PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Making a divider to measure high voltage, high frequency AC accurately is surprisingly difficult. Making a high voltage scope probe with a response flat from DC to some high frequency is even harder.

Having said that, this is definitely a STFA topic. It's been discussed on the board many times before, and a few members have built devices like this. Outside of 4hv, Sam Goldwasser's Repair FAQ discusses it. Link2 (Notice who wrote the section on frequency response)
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Proud Mary
Fri Oct 09 2009, 11:56PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Zenador wrote ...

.
[/quote1255117223]

From your statement, I read that only the capacitors are required on the AC divider. 95% of the dividers on the net are for DC (resistive only) or both AC/DC (capacitive/resistive.) A straight AC only divider would suit me fine as well. If I'm drifting off mark, please let me know.

So assuming a 60kVAC target (120kVAC rated for safety ;) ) can the 120kV rating be derived from, say, twenty 6kV caps in series. Or would one be better to use fewer/higher rated caps?


Zen, I should stress that I have not myself constructed this divider, but I have a small knowledge of HV RF -

your alligator clip will appear to such a voltage as the ideal jumping off place for an arc to wherever its lazy nature strikes it as the easiest way to earth via Coronaville.

At several hundreds of MHz your insulators will look like capacitors, and the RF current will go right through them.

Better brains than mine would be needed to solve your problem, and a big budget too.

What I do know is that it is easier to build a DC only, or an AC only, divider than try to build a divider for all seasons.

Very high DC voltages are quite often measured with fields mills positioned strategically near one of the conductors, and the conductor voltage is then derived from the charge on the field mill.
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klugesmith
Sat Oct 10 2009, 04:39AM
klugesmith Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Though it would be too high-tech for the typical do-it-yourselfer,
high voltages can be measured by optical Pockels effect in a sensor crystal.
A minute on Google turned up commercial products including Link2

This laboratory device is accessed by optical fiber, and its response is flat to 400 MHz.
Link2

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Sulaiman
Sat Oct 10 2009, 06:52AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I think there is verry little chance of successfuly modifying such a probe.
This type of probe is for approximate measurement of crt anode voltages and has little other use. (May be ok at 50/60 Hz)

I can't imagine where you would encounter tens of MHz at eht !
(ok, commercial radio transmitters on the antenna maybe)

To put it in perspective, at 30 MHz a probe capacitance of 5 pF would be 1 kOhm,
10 kVrms would give a probe current of 10 Amps rms!

A rethink maybe ?

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Zenador
Sat Oct 10 2009, 12:53PM
Zenador Registered Member #1733 Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Sulaiman wrote ...

A rethink maybe ?

I guess so. I was looking at Dr. Gigavolts divider, which he used to measure the tank circuit on his mini SGTC and captured the ring down on his scope. I wanted to remove the large resistor network and use the single 740MΩ resistor. As I don't have a background in EE, and many have said that this is more than difficult to accomplish, I think I'll drop this one.

Thanks anyways guys..
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Sat Oct 10 2009, 01:34PM
Registered Member #2372 Joined:
Location:
Posts: 62
The HV probes I use are from this place, Link2, and they are a few grand. They have a 250Mohm resistor and some capacitors I think to tune the circuit. It only goes to around 80 MHz I think and it is only accurate to like 10% at high frequency. For measuring fast things usually derivative probes are used like Edot and Bdot probes, but they are usually pretty hard to calibrate and get working properly.
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Proud Mary
Sat Oct 10 2009, 01:50PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Zen, even for DC work I would say 740M was on the low side for measuring 60kV, as it will draw 81uA from the circuit under test, which will pull down the voltage of small C&W or other high Z source (giving a false low reading)and dissipate about 5W in the process.

Personally, I would choose a 3G/3M combination for measuring 60kV, which has the handy 1000:1 ratio but won't bleed your circuit dry of meaty goodness during measurement.

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