# Negative resistance?????

#### spinnaker

Joined Oct 29, 2009
7,835
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??

#### joeyd999

Joined Jun 6, 2011
4,370
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??
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.

Joined Dec 26, 2010
2,148
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.

#### spinnaker

Joined Oct 29, 2009
7,835
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.

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.

#### spinnaker

Joined Oct 29, 2009
7,835
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.

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.

#### kubeek

Joined Sep 20, 2005
5,716
Low battery in meter?

Joined Dec 26, 2010
2,148
Try measuring across the mystery device with a voltmeter.

#### Lundwall_Paul

Joined Oct 18, 2011
226

#### spinnaker

Joined Oct 29, 2009
7,835
Try measuring across the mystery device with a voltmeter.

Now I feel stupid. 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.

#### crutschow

Joined Mar 14, 2008
24,701
Now I feel stupid. 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.
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.

#### galley4u

Joined Aug 1, 2018
1
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.