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quicksilver

Fri Jun 20 2008, 02:04PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I've seen quite a few different meters & measuring devices & they all seem to cap out at a few Kv. There must be some fairly common tool to measure wide variance HV in circumstances where the range could be broad.... Searching brings up voltages from over the 480 range to 2Kv....but that not what I would consider really high voltage.

Are there "VOM meters" or similar that reach far beyond 1Kv? For that matter HOW is really high voltage measured in situations wherein there are few "starting points" to extrapolate the likely results? Is there a device that can measure really high voltage from scratch????

NOTE: In a search of commercial metering devices (about 1600 devices) from 8 manufacturers didn't turn up one item that measured over 3Kv. I KNOW they must exist as I've seen desk-top or rack mounted items that would measure that level but I can't find a single manufacturer!

Bjørn

Fri Jun 20 2008, 02:47PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Google "high voltage probe".

cbfull

Fri Jun 20 2008, 04:28PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

You should find what you are looking for by what quicksilver said. I bought a few range extenders for DMM's off of ebay a while back, and it's nothing more than a HV resistor (HV resistors are typically longer to prevent an arc bypass) and a clip. They can pretty much be designed to have any range you need, but if you go too high you get corona problems which can destroy the multimeter.

The ones I currently have extend the range to be exactly 10 times the actual reading. So in essence, if my DMM says 500VAC, and I have the resistor in place, I know the voltage is actually 5,000VAC.

quicksilver

Sat Jun 21 2008, 02:04PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Gottcha! Now I also see why I missed it. Thanks.

Shaun

Sat Jun 21 2008, 06:26PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

You can actually make a functional version of these yourself with a few high-value resistors. If you know the internal resistance of your meter when in voltage mode, you can add series resistors to make your meter part of a voltage divider.

For example, say the internal resistance of your meter is X ohms. If you were to put another X ohm resistor in series with one of the probes, you would read exactly half the actual voltage. If you were to add 9X ohms in series, you would read one tenth the actual voltage. Google "resistive voltage divider" for more on this topic.

As for finding the meter's internal resistance, you could measure it with another meter, or you could look it up in the meter's manual. The actual internal resistance is almost always either 1Mohm or 10Mohm.

As a side note, those little quarter-watt resistors may not hold up indefinitely with such a high voltage across them. For quick checks they'll be fine, but if you are constantly measuring the voltage, for example watching capacitors charge, you may consider higher voltage rated resistors.

Or just use a commercial high voltage probe

teslacoolguy

Sun Jun 22 2008, 05:44AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Get a high voltage probe on ebay. They can go up to 40kvdc and are considerably accurate for the price.

quicksilver

Sun Jun 22 2008, 05:03PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

This a "MUST HAVE" tool for HV!

On a related note:
I also found a page for determining the needed resistance w/ an LED for dropping (or raising) current:

The formula elements could be found here:

This would allow the use of a "LED current limiting box" for things like a MOT! If current could be altered in a MOT cheaply, quick, and easy, the experiments dealing with currant's effects on an arc would be almost endless....! With a measuring tool, configuration aspects could be tested prior to construction.

quicksilver

Tue Jun 24 2008, 11:39PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I just found an OLD (American made RCA brand; old logo) HV Probe in near mint condition for $20 in a tiny electronics store in Tucson that sold a variety of used stuff. I immediately went home and tested it. What I found was interesting. It appears to have 11M switchable to 5.5M ohm resistance and seems "heavy for it's size" and appearance. It has a "arc shield" & a grounding wire. The shielding was interesting. There as another that actually took some fluid inside it.....But this one was mint condition. (I only wish I had a functional camera to post a pic). I haven't determined the wattage yet.

I believe that a 10M and a 5M ohm resistor and put together a little circuit you could drop the energy down to the level wherein a VOM or DMM could measure the voltage. It also has unique protective elements that may be copied by the hobbyist (I think it could be copied..... But I saw a Fluke that was VERY expensive)

I'm sure it could be made if you had the correct resistors and powerfully non-conductive materials to work with.

Bjørn

Tue Jun 24 2008, 11:57PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Remember that the voltage rating is only one side of the story. The safety class is another. If you connect a HV probe to a source that is capable of delivering high current the probe must be desinged so that it fails safely.

Home made probes would generally not be safe if connected to a high voltage, high current source.

quicksilver

Fri Jun 27 2008, 03:15PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

You are so right! The more I read about HV probes the more I see complications. I would imagine that for flyback (LOPT) transformers and perhaps even as much as <.5 A they may be useful but I still am very glad I found a commercial one.

Most people have seen the Utube video of the helicopter-testing of HV power lines in the USA. There was a notation that the probe/materials/testing gear that the guy uses are not actually designed for that level of testing and he is doing so in spite of the manufacturer's warning that it's not up to that job. If someone was a pretty creative fellow that might be a unique invention. Even Fluke's add-on probe is quite a bit of money!

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