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What HV probe to measure monster cap?

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GammaRay

Fri Jan 31 2014, 12:22AM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104

I have a 41kV @ 7.5uf (145 lbs) cap manufactured by GE. I want to purchase a HV probe capable of measuring this large cap. I've done research but still have to admit I really don't know how to spec a HV probe regarding the current it will have to endure measuring such a big cap. I've searched Ebay and HV supplies, but still unsure which probe will get the job done (without exploding). Can someone point me in the right direction or recommend which probe I should use? Thank you in advance.

GrantX

Fri Jan 31 2014, 12:47AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Do you mean a HV voltage probe to measure the charge on the capacitor or something to measure the amplitude of it's discharge current?

A voltage probe just needs to be sufficiently rated for the peak DC voltage you are charging the cap to. They have a very high input resistance, so they shouldn't have to handle any large currents ever. I use this probe for most HV measurements, it's been reliable so far and is also very affordable. You shouldn't need to worry about explosions unless you overvolt it like crazy.

For discharge current you would probably want to look at a broadband current transformer and oscilloscope, although I'm not sure how to size the CT correctly.

Dr. Dark Current

Fri Jan 31 2014, 10:09AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Maybe Rogowski coil for the current measurement. As for the voltage, I would just use a string of sufficiently rated resistors, potted eg. in parrafin wax.

Mads Barnkob

Fri Jan 31 2014, 12:18PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I use a normal analogue meter for the voltage measurements and a Pearson current monitor model 101 for discharge current measurements. Charging the 2400uF to 1800VDC takes a while, so a analogue meter for the voltage is enough for me. My cap:

Pearson current monitors can some times be found at reasonable prices on ebay, but most of the time they are expensive. You might get lucky with "make offer" on some auctions.

GammaRay

Fri Jan 31 2014, 01:48PM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104

My interest is measuring voltage, however, I was concerned that there might be a spec that I should pay attention to that concerns the amount of current/amps/joules the HV probe could withstand. If I understand the above posts correctly, as long as I don't exceed the voltage spec, I need not be too concerned about the amount of current/amps/joules that pulse through the probe?? If that is correct, looks like the TENMA - 72-3040 - PROBE, HIGH VOLTAGE, 28KV/40KV would do nicely for me.

Finn Hammer

Fri Jan 31 2014, 02:30PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

GammaRay wrote ...

My interest is measuring voltage, however, I was concerned that there might be a spec that I should pay attention to that concerns the amount of current/amps/joules the HV probe could withstand. If I understand the above posts correctly, as long as I don't exceed the voltage spec, I need not be too concerned about the amount of current/amps/joules that pulse through the probe?? If that is correct, looks like the TENMA - 72-3040 - PROBE, HIGH VOLTAGE, 28KV/40KV would do nicely for me.

Gammaray?

Why do I get the impression, that you are unfamiliar with the use of ohmÂ´s law?

How are you going to charge that capacitor up to 40kV, and not least, how do you intend to discharge it again?

Cheers, Finn Hammer

Proud Mary

Fri Jan 31 2014, 02:34PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The TENMA - 72-3040 - PROBE, HIGH VOLTAGE, 28KV/40KV has input resistance of 1GÎ©, so when you use it to measure a 40kV supply it will draw 40Î¼A and burn up 1.7W.

This would be a serious drain, and pull down the voltage on flyweight HV supplies - for example only, a Wimshurst machine - but won't make much of a dent in your run-of-the-mill small HV supplies.

I have a couple of generic probes of this kind, and they are invaluable. Always make sure the Earth wire is firmly secured to Earth so you have less chance of disagreeable surprises.

GrantX

Sat Feb 01 2014, 02:12AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

As Proud Mary has pointed out, the HV probe has one billion ohms of resistance from the HV terminal to earth, so there isn't going to be much current flowing through the probe under normal conditions. The probe is simply a large resistive voltage divider that has been tuned to be accurate enough for DC and 50/60Hz measurements. Think of the behaviour of a normal digital multimeter: you can measure up to 1000V DC without worrying about how much current the multimeter is drawing.

GammaRay

Sat Feb 01 2014, 10:15PM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104

The last handheld HV Fluke probe used to measure this cap disintegrated on contact with the cap, exploding in my hand, spraying shrapnel across the room. Thus, my overabundance of caution on selecting a proper replacement. Thanks for the constructive input. This forum answered my questions and its appreciated.

Newton Brawn

Mon Mar 03 2014, 02:01PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

just reading the above comments came in my mind that GammaRay just wants to measure the capacitance of the capacitor....

However readind the last post,

"handheld HV Fluke probe used to measure this cap disintegrated on contact with the cap, exploding in my hand, spraying shrapnel across the room"

my best advice is stay away of any electrical device.

Newton

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