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

[aankrom]

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
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>
>
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