[sdiy] Large resistor value (LDR) / DIY ohmmeter question

[Ian Fritz]

It's surprisingly easy to measure large resistances.  You find an opamp 
with leakage in the low pA range, hook up your resistor in the I/V 
converter configuration and hook the input to first the positive rail then 
the negative rail.  The leakage currents will contribut the same amount in 
both cases, so take half the difference to get the current through the 
resistor.  You can easy get up to ~10^10 Ohms this way.  Just plug it into 
a whiteboard and away you go.

Ian



At 04:06 PM 4/22/2012, aankrom wrote:
>On Sun, 22 Apr 2012 11:22:38 +1000, Paul Perry wrote:
>>You certainly won't be able to use a LM471 because the input current
>>leakages will swamp the tiny current flowing through the high value
>>resistor you are testing.
>>But I doubt it is feasible to test such high value resistors without
>>using very special layout & materials anyway. An 'ordinary' ohm meter
>>circuit won't cut it.
>>
>>paul perry Melbourne Australia
>>
>>----- Original Message ----- From: "aankrom" Subject: [sdiy] Large
>>resistor value (LDR) / DIY ohmmeter question
>>
>>
>>>I am building an analog ohmmeter from a schematic out of Encyclopedia
>>>of Electronic Circuits. It uses a reference resistor to compare to the
>>>resistor under test which is in the feedback loop of an op-amp. The
>>>design goes up to 10MOhm, but I want to be able to measure resistance up
>>>to 400MOhm. My question is, is there a point at which a high resistance
>>>will surpass the open-loop gain of the op-amp, in this case an LM741.
>>>I'm guessing I should use an op-amp with higher open-loop gain which
>>>would probably be something like an FET op-amp like a TL071 or maybe an
>>>LF351. Or am I thinking about it wrong?
>>>
>>>My main use for the meter is to watch an LDR return to its dark
>>>resistance after an LED lowers it. I even have a mod. that I added that
>>>will put the meter in the 10kOhm range when the LED is on and then
>>>switch to a higher range (20MOhm-100MOhm) when the LED is off. I'm not
>>>after precision readings here, just the rate of change. I have a
>>>cool-looking old 1mA meter. I just hope it has a fast enough action to
>>>track quick changes.
>>>
>>>I make LED/LDR optoisolators for various DIY audio gear and I need to
>>>separate the fast from the slow because I have plans for each. My DMM's
>>>won't fit the bill because they don't track changing resistance in the
>>>high range very well. LDR's dark resistance shouldn't be much more than
>>>100MOhm, but I know it's higher than 20MOhm (the max. on my meter). I
>>>suppose I could put a resistor in parallel with the LDR and I could
>I suppose since I'm mostly interested in watching the increase in dark 
>resistance and not an accurate measure of the actual resistance, I may not 
>need a circuit that will handle much more than 20MOhm. On my DMM the 
>resistance changes faster than the gate time. I have my homemade LED/LDR 
>optocouplers separated into "fastish" and "slowish". Just wondering if 
>there are some "middle-ish" ones. Some will be used for filters and others 
>for compressors.
>
>AA
>
>
>>>still track it's rate of change in a lower Ohm setting (like 10MOhm).
>>>When the meter stops moving, I can roughly calculate the dark resistance
>>>based on the resistor in parallel.
>>>
>>>I doubt I'll need a 400MOhm range, but I thought I might test some of
>>>the high-voltage parts in my oscilloscope. I actually have some 100Mohm,
>>>200MOhm & 400MOhm resistors as well.
>>>
>>>So I'm still curious if I should use a different op-amp...
>>>
>>>AA
>>>_______________________________________________
>>>Synth-diy mailing list
>>>Synth-diy at dropmix.xs4all.nl
>>>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>>
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>>
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