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Measure +100Mohms, is it possible to increase....

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Patrick

Tue Dec 07 2010, 06:20AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Measure +100Mohms, is it possible to increase....the normal range from 40M to measure very high ohm resistorslike 200M? i vaguely remember a thead where this may have been mentioned, but for the life of me i cant find it. is this possible or am i mistaken?

radiotech

Tue Dec 07 2010, 06:38AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Sure it is, just get used to measuring nS (nano Siemens) on a
meter like the Fluke 87 or its brethren.

Or stick a 9 volt battery in series with the resistor you need to
measure after you do the math for your particular meter,

Adam Munich

Tue Dec 07 2010, 06:38AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

You could try sending 10kV through the resistor and then measure the current that comes out. Then use ohm's law. 10kV though a 100M resistor would give you 0.0001A.

Other than that I can't think of a ay to measure such a high resistance.

Patrick

Tue Dec 07 2010, 06:47AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

radiotech wrote ...

Or stick a 9 volt battery in series with the resistor you need to
measure after you do the math for your particular meter,

Once again Radiotech saves me thats what i was trying to remember!

Proud Mary

Tue Dec 07 2010, 07:15AM

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

Here is a good article by a senior Keithley Instruments engineer on the problems of high resistance measurement and how to overcome them:

Cigoy, D. Improving High Resistance Measurements with Voltmeter and Ammeter Instruments

radhoo

Tue Dec 07 2010, 03:28PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

I believe it is possible to use a known impedance multimeter, a known regulated DC supply (10V) and the big ohmic resistor.

Connect the DMM in series with the resistor to be measured, and read the voltage (you might need to set the scale to milivolts).

150x150

Assuming R1 is to be determined, and you know R2 - the resistance=impedance of your multimeter in DC, Vin is the known DC supply voltage, and Vout is what you read on your Multimeter, R1 would be:

R1= ((Vin - Vout) / Vout) * R2

This will work for sure, I did some measurements using this already.

klugesmith

Tue Dec 07 2010, 06:30PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

As others have said, you either need more voltage or sensitivity to lower currents.

Ordinary digital multimeters measure resistance using "force current, measure voltage" mode, so extending the range is nontrivial.
You could measure a 39M resistor, barely within the range of your DMM, then put the unknown resistor in parallel and measure the combined resistance.

Classic "meggers" have a hand-cranked dynamo to generate test voltages of 500 volts or more.
Then even multi-giga-ohm leakage paths conduct enough current to be accurately measured with robust electromechanical meters.

But for most of a century it's been possible to generate the HV with batteries & switching converters.

quicksilver

Tue Dec 07 2010, 07:13PM

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

I've owned one of the Meggars and IF you can find them used; they drop in price substantially. The hand-crank is not made for any long term use (not that robustly made). But they ARE available at substantially less if you don't buy them new. Often contractor's supply places (if they have used materials) will have a few.
I also have a source for very high resistance HV ceramic resistors at a good price. If you have any interest, PM me. I believe 100Mohm are available (around $30). They're not small (12' by 2", heavy screw connections).

Sulaiman

Tue Dec 07 2010, 07:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

How accurately do you need to measure the resistance?

What kind of meters do you have and what's their accuracy?

Meanwhile, you could get a string of resistors to add up to 100.0 MOhm,
the precision of each resistor as good as or better than the required accuracy.
e.g. a pack of ten x 10 M 1% = $1

This will enable you to use inaccurate/uncalibrated instruments to check your resistor using whichever method you choose.

I'd go for 'apply a high voltage and measure the current' approach.

P.S. I'm surprised how many uses 'pracision' resistor strings find in my shed, worth keeping.

radiotech

Tue Dec 07 2010, 10:03PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

30 Volt "safety megger" by Evershed and Vignoles.

Max scale 2 megohms. What is a C.C. Iniatiator?

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