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

[Paul Perry]

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
> 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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