High Resistance Measurement

Discussion in 'The Projects Forum' started by seannz, Jul 2, 2012.

  1. seannz

    Thread Starter New Member

    Apr 10, 2009
    5
    0
    Hi,
    I am looking at building a circuit to measure some high resistance values. The resistances I want to measure range from 200kOhms to around 10,000MOhms.

    The first thing that comes to mind is a voltage divider. I am thinking about having a few different resistors to measure the different voltages at the divider. Then rearranging the voltage divider equation for the high resistance I could find its value.

    As I am intending to use as low voltage as possible (maybe 5V?), at the high resistance values, will I be able to measure any voltage? I could possibly step up the voltage, however I have limited space for this circuit.

    I haven't tested this practically yet, just running through some ideas. Is there any other possible circuits anyone knows of to measure resistances on the order of 10,000 MOhms?

    Cheers,
    Sean
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    Scrap that idea. It wouldn't work.
    To measure 10,000MΩ you will need about 10kV just to get 1μA.
     
  3. #12

    Expert

    Nov 30, 2010
    16,345
    6,831
    My Fluke 27 meter displays .01 nanosiemens. 10,000M is .1 nanosiemens. I (you) could do this with my Fluke 27 if they still made them. Perhaps you could buy a good quality meter.

    If you put a 10 megohm DVM in series with the 10,000M resistor and applied 10.01 volts, and put the meter on the millivolt DC scale, you would read 10.0 millivolt.
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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  5. seannz

    Thread Starter New Member

    Apr 10, 2009
    5
    0
    Hey guys,
    Thanks for the replies and offers.
    Unfortunately I am looking to send the resistance value (or equivalent voltage) to a microcontroller. So ideally I will have a small subcircuit connected to it. Does anyone know how the fluke 27 meters or otherwise work?

    So would this be any different than having a 10 MOhm resistor in series with the 10,000 MOhm and connecting voltage to the DAC on the micro to read it? I was thinking the problem is going to be the low current, how would the meter be different?

    Cheers for the help,
    Sean
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    The problem is that every measuring device, whether it is a DVM or ADC, has an internal loading resistance. You have to know what is the value of the internal resistance.

    #12 used the example of a DVM with an internal resistance of 10MΩ.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    There are op-amps with extremely high impedance that could be useful if you need to build something. Last time I looked I was looking at the LMC6035 or LMC660. I never got around to using either of them.
     
  8. Pich

    Active Member

    Mar 11, 2008
    119
    4
    As suggested above on post 3 and 6. You could setup a LMC662 duel opamp as a inverting amp, with 10M resistor connected to the inverting input and output. The resistor being measured to 10v and inverting input. the second Op amp would then be setup as a inverting amp where it would bring the voltage back to positive referred to common and the gain can be adjusted to a proper voltage input to the uC.
    There are resistors available of 100M or even 1000M that can be used as the reference resistor to sub for the 10M but they can be expensive.
     
    Last edited: Jul 4, 2012
  9. Skeebopstop

    Active Member

    Jan 9, 2009
    358
    3
    Jfet input opamp with reaistor divider using high value resistors. Probably something around 5 megaohm on the bottom leg of voltage divider would give you a decent signal range
     
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