Tesing a 50M ohm 65KV resistor

Try a power supply, 10 volts maybe, in series with the resistor and a DVM set to its voltage range. Most DVMs have a 10M input impedance on DC ranges, easy to verify.

The Rx and the DVM form a voltage divider. You can easily measure gigohms this way.

John

You might need to clean the resistor with some Isopropyl alcohol and let it dry well, you could scrub it with a swab that is soaked in 99% alcohol.

shaun

Oops my goof. 5Kv / 50Mohms = 100 uAmps

shaun

Still small though and I think meters don't measure that small.

Here's a Keithley electrometer measuring a 1 Gohm resistor. Pawing the resistor with greasy fingerprints didn't make any difference.

ftp://jjlarkin.lmi.net/Keithley_1gig.JPG

Incidentally, this meter will measure up to 1e14 ohms, and you can get used ones for a few hundred dollars, a good deal if you intend to do a lot of hi-Z work.

Or make your own gohm test box.

But as I noted, a cheap DVM and a 9-volt battery is all you need to measure gigohms.

John

Do you know how these meters actually work? I can't think of a need to get one, but they look like they'd be fun to play with for a week or so.

The lowest range on my Metex M-3800 is 200 uA. It's not a high end model, but also not one of these pocket units.

Bernhard

So I have this monster 50M ohm 65KV resistor I got off ebay that I am trying to test. My fluke multimeter has max rating of 50M ohm and when I test the resistor it shows an open circuit. Do I need a meter with a higher rating or is the resistor bad? I conneted it in series with a 1ma panel meter and hooked to a 5KV rectified power supply variaced. I figure it should be enough to test the resistor? The resistor should rescale the panel meter so that 1ma = 50KV so 1KV should move the needle one increment but the meter acts strange. The needle goes backwards when I apply a little voltage then back to zero. It does this each time I move the variac knob. Does that mean the resistor is bad or is my power supply of too low power. It came out of a bug zapper so it's pretty small.

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You have an AC panel ammeter with a DC supply hooked to it.

Every time you increase the voltage the AC component of the voltage registers on the meter backwards due to it's cheap internal construction.

mike