Measuring Large resistances

Kreyen · 2009-11-12T20:12:29+00:00

I am having problems trying to measure resistance values above100MegaOhms. My Laboratory Ohmeter gives unstable values. Is ther anyway apart from the usual dc bridges of getting accurateresistance measurements.Thanks guys.Kreyen

Nov 12, 2009 25 Replies

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Spehro Pefhany 16 years ago

Hmm.. I think you're right on that point. I do have one that does, but probably most do not. Benchtop instruments like my Agilent 34401A meters do have an "infinite impedance" setting.

1M? What a POS. What do you expect for dollar store prices. Even the Chinese can't make a worthwhile meter for what Harbor Fright wants to pay.

Carbon? AFAIK something like Ruthenium oxide is more common in chip resistors. Ohmcraft makes their using a kind of printing method which is automated, so it should be consistent. Here is a white paper on their process:

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Spehro Pefhany 16 years ago

Fluke 87-5, for example. DC voltage accuracy is 0.05% + 1 count on the

6V range.

But only 1% in the 60nS range (infinity to 16.667M ohm), unfortunately. A 100M resistor would have a conductivity of 10.00nS so the resolution is pretty much there, but not the accuracy.

Probably lots of other 4-1/2 digit handheld meters too.

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Kreyen 16 years ago

Hi guys Thanks for all the enlightening on the subject. The model of the meter I use at the Lab is the Agilent 34401A which Spehro incidently mentions has an infinite resistence setting. I couldn't find that ... which means I'll have to browse through the thick manual. It doesn't seem to deliver more then then a 10V output though. I suspect it might be measuring the limitations in its accuracy readings or the thermal noise in the resistors.

Kreyen

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Robert Baer 16 years ago

Thanks.

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JosephKK 16 years ago

Oh drear, it has a 10M ohm input when measuring voltage; measuring resistance is entirely different.

The measurement problems may be exactly that, measurement method. For

100 Mohm and up i suggest putting the DUT in a closed conductive box with holes for the test leads. And use minimum length test leads, not touching anything if possible.

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