Measuring high resistance

Use a DVM on the 200 mV range as a picoammeter. 200 mV/ 20 M ohms = 10 nA full scale. With 4-1/2 digits, that's 1 pA resolution. Some meters have much higher resistance on the lowest voltage scale.

Cheers

Phil Hobbs

Sounds like an old run of the mill VTVM might do it. They're all over eBay.

My 1980s-era Fluke 8020B multimeter has a 0-199.9 nS range that is useful for this sort of thing. 1G ohm is 1nS, of course, so it's not much good above a few G ohms.

But often you really **need** a higher test voltage of hundreds of volts as in a "Megger" (they used to have hand-crank-driven generators) and the suggestions given here thus farly won't cut it.

I also have a modern Chinese version of the Megger than generates a high voltage internally using a switching supply (actually, I might have a bunch of them, have to check that..)

LOL, you can still buy the hand-crank kind:

Best regards, Spehro Pefhany

Right, "resistance" can be voltage dependent, especially in withstanding voltage testing.

Yeah, not many people realize that they can buy a digital picoammeter from Harbor Freight for $10.

Most are 10M input, I think. Just use another DVM as an ohmmeter to measure Zin of this one, or make a voltage divider with a 10M resistor and the DVM, and let it measure itself.

Measuring fA isn't hard, either.

On a sunny day (Fri, 14 Nov 2014 10:37:55 -0800) it happened John Larkin wrote in :

Isolation test, as others pointed out, are usually done at some 4 kV. Some dielectrics just break down, that test is just for that purpose. And they are done with AC. No leakage at a few volts means nothing. We used to test transformers with 4 kV AC (IIRC) between primary -and secundary.

My 5 $ ebay multi meter has a 2000 MOhm range, but would not want to use that to declare a transformer 'safe'. Liability... dangerous.

So, use a real megger.

Higher requirements such as 4 kVrms test voltage would be required for equipment intended to be installed into the main distribution panel/service entry (or whatever they call it in different countries).

On a sunny day (Fri, 14 Nov 2014 21:43:21 +0200) it happened snipped-for-privacy@downunder.com wrote in :

Yep, that is what we were making, transformers for the power stations, distribution. Some so big you needed a ladder to climb on it, some filled with that dangerous oil too. But even the smaller current transformers (almost the size of a big radio) were tested with 4 kV. Transformers and controlled rectifiers for the electric tram, for the railways too. We also had a nice HV test cage to 100 kV. Lots of thyristor controlled stuff, magnetic amplifiers too.

I should have said.... test is at 100VDC and I need to get the measurement into a micro... That has built in 10 bit DAC... Unless there's a cheap dvm with a comms interface. I have access to GPIB stupp but that's not a practical option. I wrote GPIB stuff back in the 80s... never again :(

OP says he wants to apply 100 volts.

First you need a source that can produce 100VDC, then you put that in series with the device under test (and 100M resistor) and measure the current that flows (or the voltage drop across the 100M (or part of it))

On a sunny day (Fri, 14 Nov 2014 16:57:47 -0800) it happened John Larkin wrote in :

So, use 100V.

BTW I was thinking yesterday to measure very low currents and have a low impedance, just use a NPN and drive the base, series resistor to limit current. Beta of 300 or so will help + | R | uA meter |----------------> ADC |/

On a sunny day (Fri, 14 Nov 2014 21:43:21 +0200) it happened snipped-for-privacy@downunder.com wrote in :

Actually I think you are right. I should not be posting while fault finding and watching Angelina Jolie movies at the same time without sleep.

That might work, except when there is a short...... poof ;)

That sorts the 'short' issue,by limiting curent into the BC147... but wouldn't I need a Vbe of >100V rating ?

On a sunny day (Sat, 15 Nov 2014 10:01:29 +0000) it happened TTman wrote in :

No, the 1MOhm limits the base current.

Put that circuit in the same file as radar recognition of aircraft insignia...

On a sunny day (Sat, 15 Nov 2014 05:53:49 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote: mama!

Beta is nonlinear on current, and falls off at low currents.

Better might be a high value resistor (1G or so) and a mosfet followr or cmos opamp thing.

This is one thing I built; measurement floor is around 10 fA, but it's really slow down there.

Or get one of these on ebay; great to have around: