Negative resistance?????

Discussion in 'General Electronics Chat' started by spinnaker, Jan 14, 2012.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    How is it possible to read Negative resistance? I can see if you have power connected to the circuit but I have none.

    I have a simple surface mounted connector. It s hard to see under the pins so I am guessing I have a short somewhere but why would I be reading negative resistance?

    The only other thing in the circuit are some pullup resistors with the opposite end disconnected.

    What is strange is if I reverse my leads of the ohm meter I read infinite ohms.

    Could a bit of flux down between the connectors cause a junction that forms a battery??
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Or, you could have a charged cap on the node you are trying to measure. Disconnect (isolate) all connections on one side of the resistance prior to your ohms measurement.
     
  3. Adjuster

    Late Member

    Dec 26, 2010
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    A negative reading with the meter one way round, and infinite the other way round, actually fits very well with a DC voltage being present.

    Apart from charged capacitors, backup batteries or some kind of AC pick-up being rectified by a semiconductor somewhere may be providing the DC.
     
  4. spinnaker

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    No caps in the circuit. Just the connector and a few pull-up resistors.

    As mentioned there are pull-up resistors but only the end on the resistor connected directly to the connector is connected.
     
  5. spinnaker

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    But there is no voltage source at all connected. That is the mystery. It is a very simple circuit. Just a connector and some resistors. It is an SD connecter on a break-out board.
     
  6. kubeek

    Expert

    Sep 20, 2005
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    Low battery in meter?
     
  7. Adjuster

    Late Member

    Dec 26, 2010
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    Try measuring across the mystery device with a voltmeter.
     
  8. Lundwall_Paul

    Active Member

    Oct 18, 2011
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    What does your meter read when the leads are shorted and is the reading stable?
     
  9. spinnaker

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    Now I feel stupid. :mad: I measured 3V. As soon as I saw that I knew something was up. It was coming from my MCU that was connected via a jumper that I know I disconnected. Must have been that ghost that connected it back up, that hangs around here and causes trouble. :)
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Poltergeists that move things, and micro Black Holes (that cause small objects to disappear as soon as the hit the floor) are very common in any electronics lab.:D
     
    KeepItSimpleStupid likes this.
  11. galley4u

    New Member

    Aug 1, 2018
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    I just ran into this today, and this discussion does not help explain it. I was testing an electric dryer that was not heating. Power was shut off at the breaker. I was using my ohmmeter to check for continuity/resistance on the element, the thermal cut-out and high limit thermostat. On the high-limit thermostat, I got a reading of -6. I reversed the leads and got the same thing. Other tests seemed normal.
     
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