Current Source for Resistance Measurement

Discussion in 'Embedded Systems and Microcontrollers' started by aimiazri, Jan 12, 2010.

  1. aimiazri

    Thread Starter New Member

    Jan 7, 2010
    I m going to design a circuit to measure a resistor,R n display to LCD. this circuit will using a constant current. by using voltage drop trough the R, I will programming the PIC to measure the resistance of the R using formula:

    R = Va-Vb

    however the circuit is using voltage source. And my problem is how to make the current flow trough the R always constant although the change of value of R. Because I have simulate a circuit that I get from internet, the current still changing when I change the value of R...WHY?:confused:
    Last edited: Jan 12, 2010
  2. russ_hensel

    AAC Fanatic!

    Jan 11, 2009
    There are lots of op amp constant current circuits. Often these are voltage controlled. It would be nice to have one with one terminal grounded, but many simple circuits are not. To use a non grounded circuit you may want to measure the voltage across the resistor with another op amp ( or dedicated ic ) circuit: the differential amplifier. For different ranges of resistance consider using different constant currents. For high current constant currents you can use a low voltage voltage regulator, good for currents in the 100 ma range. ( an op amp can be boosted by a transistor to give similiar currents ).
  3. Markd77

    Senior Member

    Sep 7, 2009
  4. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    What range of resistance?
    What accuracy?

    IIRC National Semiconductor and Linear Technology make three terminal current
    source ICs. You would put them in series with the resistor you are testing.
    A programmable current sink is another option.

    For a lower accuracy you could use a current regulator diode or a JFET.

    (* jcl *)
  5. Dragonblight

    Active Member

    Aug 19, 2009
    This is half experimentation, half intelligable guesswork, and a touch of awesome, but looking at finger-sized contacts composed of a shined up metal, i'd say you're working with a range of 500k/finger to 200/finger. 500k being a perfectly dry finger with little pressure to 200 being someone who just dipped their fingers in a saline solution of maximum concentration. That's why they use specialized plastic-like pads or a contact gel for medical purposes, it keeps the resistance at a fairly regular level.
  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
    For an easy to apply IC you could consider the LM334 which may be also compensated for temperature. It seems good for what you are trying to do.
  7. rijinca

    New Member

    Feb 23, 2010
    resistance is in few megaohms range
    error can be +/- 5%