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

    Well-Known 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

    AAC Fanatic!

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

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

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    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
     
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