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Measuring the resistance of a hot resistor

Discussion in 'Electronic Basics' started by Jon Danniken, Feb 25, 2007.

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  1. Jon Danniken

    Jon Danniken Guest

    I am using a 100W, 2 ohm power resistor (old wound cement unit) in a circuit
    to do some basic measurements with. When cold, the resistor measures 2.1
    ohms on both of my DMMs.

    After use, when the resistor is hot, I attempted to measure the resistance,
    but I ended up with some crazy results, which, depending on which way I
    connect the leads, tell me it is either between 3 to 4 ohm, or 0 to 1 ohm.

    Since the lead placement affected the reading, I assumed that somehow this
    resistor was generating a voltage when hot, but I can detect no voltage with
    any of my meters.

    What is going on here, and why in the heck is lead placement affecting the
    results? Additionally, how can I accurately measure the resistance of this
    hot resistor?

    Thanks for any insight into this,

  2. Try doing the measurements, in circuit? By this, I mean measuring
    both the voltage across it, in parallel, when in-circuit and measuring
    the current through it (meter set to measure at least 7A, which is
    what 100W suggests) using a series arrangement. The two will tell you
    what you need to know: R=V/I.

  3. Jon Danniken

    Jon Danniken Guest

    The measurement I am taking requires a value for the resistor itself,
    independant of reactive elements in the circuit. As such, I need to measure
    the resistance out of circuit.

  4. Eeyore

    Eeyore Guest

    I suspect your circuit is powered up when you're making the hot measurement and
    not when you're making the cold measurement.

  5. The resistance will not vary much with temperature. For accurate
    measurements under load, measure the voltage and current at the same time.



  6. me

    me Guest

    (meter set to measure at least 7A, which is what 100W suggests)

    or 1 mA or 100 Amps or.....
  7. me

    me Guest

    Use a DC circuit to indirectly measure the reistance as described (roughly)
  8. Phil Allison

    Phil Allison Guest

    "Jon Danniken"
    ** It will have the same resistance cold or hot, +/- 5%.

    You have some silly bad contact issue with your probes.

    ........ Phil
  9. john jardine

    john jardine Guest

    Like a thermocouple, it's generating a few DC mV when hot. This upsets the
    ohmmeter readings.
  10. Phil Allison

    Phil Allison Guest

    "john jardine"

    ** The resistance wire in the power resistor cannot do that - but maybe
    there is a thermocouple created by the terminals or when the DMM's probes
    are used to make connection - as little as 0.5mV would explain the OP's

    DMMs are poor at reading low ohms.

    If the OP simply connects a resistor of known value in series with the power
    resistor and applies say 12 volts DC - the voltage will split in the
    same ratio as the two values.

    Should put his false anxiety to rest.

    ....... Phil
  11. Jon Danniken

    Jon Danniken Guest

    Aha, thanks, John. I measured the resistor again while it was hot and
    you're right, it develops 0.4mV across itself when it is hot.

  12. Jon Danniken

    Jon Danniken Guest

    Excellent, thanks for that, Me. I'll cook up a little DC circuit to toss it
    into for the measurement.


  13. Jon Danniken

    Jon Danniken Guest

    "Phil Allison"
    Hey Dr. Phil, maybe you should go piss off to a psych group and let the rest
    of us discuss electronics.

  14. Phil Allison

    Phil Allison Guest

    "Jon Danniken CUNTHEAD "

    ** Huh - what the **** is your problem ??

    Why so keen to prove to the whole world what a pig ignorant & asinine
    CUNTHEAD you are.

    When it was obvious all along anyhow.

    ....... Phil
  15. Ross Herbert

    Ross Herbert Guest

    You could also do a direct measurement with the resistor heated by a
    hot plate or a hot air gun...
  16. MassiveProng

    MassiveProng Guest

    Put a good ammeter in series with it, and place a volt meter across
    it. Read both at run time, extrapolate, and you'll get very accurate
  17. MassiveProng

    MassiveProng Guest

    No. With the ammeter right next to it, and the voltage reading has
    ZERO effect, you WILL get the exact resistance at the moment you take
    the readings. Your imagined parasitics won't exist nor matter.

    The value extrapolated WILL be exact.
  18. MassiveProng

    MassiveProng Guest

    Yes. A very precise solution. Rent a thermal imager, and get some
    real repeatable results!
  19. MassiveProng

    MassiveProng Guest

    Do you not think that a known voltage and a good current reading
    would not be more accurate than an unknown calibration level ohm
  20. Ross Herbert

    Ross Herbert Guest

    I am fully aware of the method of performing a DC current and voltage
    measurement and applying Ohm's Law. I was simply suggesting an
    alternative which would avoid the need to lash up a circuit which
    requires both an ammeter and a voltmeter simultaneously in circuit in
    order to achieve best accuracy. Two measurements with meters each
    having accuracy and tolerance inaccuracy will also magnify any
    calculation error.

    A single meter measuring resistance directly might be more accurate
    depending on the meter used.
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