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

[Ove Ridé]

Maybe you should characterize them in something close to the actual
circuit you have in mind. For example, for a filter, send a
sufficiently high frequency sine wave to the audio input and a very
short pulse to the control input, and monitor the output.

On 23 April 2012 00:06, aankrom <aankrom at bluemarble.net> 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
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>>> Synth-diy at dropmix.xs4all.nl
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>>
>>
>>
>>
>>
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-- 
/Ove

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