DMM6500 strange resistance measuring - Page 1
Forum section: electronics / testgear

author: JimKnopf
author_rank: Frequent Contributor
timestamp: October 11, 2020, 06:26:02 pm
content: Hello, i got a brand new Keithley DMM6500.Today i was inspecting a dead Nintendo Switch, reading some resistance values between ground an some capacitors.I tested 2-wire and 4-wire measurement using testleads with 4mm banana jacks were i connected 2 wires togeter to a banana jack.1. 2-wire Ohm mode, measuring the resistance on one side of an capacitor show me 47,2230 Ohm.2. 4-wire Ohm mode, measuring the resistance on the same point of this capacitor show me 16,7738 Ohm.3. 4-wire Ohm mode, measuring the resistance on a capacitor on top of the Nvidia Tegra show me a negativ Ohm value of -0,011893 kOhm.How is it possible to have a negative Ohm value?Any explaination for the difference on 2-wire/4-wire measuring?

author: ogden
author_rank: Super Contributor
timestamp: October 11, 2020, 07:16:45 pm
content: Did you measure while device is powered on? Only explanation for negtive resistance measurement - external voltage or current applied to terminals.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2020, 07:23:42 pm
content: For the 4 wire ohms measurements there are 2 possible methods that can react different to additional capacitance:One is the more normal mode and the other is with offset compensation, turning the current on and off.One may get negative readings if the 2 sense leads are swapped. Residual voltage is another possibility.

author: JimKnopf
author_rank: Frequent Contributor
timestamp: October 11, 2020, 07:27:42 pm
content: No, battery was disconnected, power button pressed for several seconds and i shorted the battery +/- connectirs with a tweezer for several seconds.I asume it was powered off.

author: bob91343
author_rank: Super Contributor
timestamp: October 11, 2020, 09:40:59 pm
content: The problem can often be traced to dissimilar metals. A thermal junction is created, causing strange meter readings. The reason is that the very small voltage the instrument is trying to measure is compromised by the temperature difference between the various interfaces.One solution is to use ac instead of dc. Or try reversing the leads and average the results.

author: KedasProbe
author_rank: Frequent Contributor
timestamp: October 13, 2020, 05:27:00 pm
content: I would guess you are measuring more than a resistor, where is the other end of that capacitor going to...

author: JimKnopf
author_rank: Frequent Contributor
timestamp: October 13, 2020, 06:56:15 pm
content: Quote from: KedasProbe on October 13, 2020, 05:27:00 pmThe first capacitor is part of a dc-dc converter. The second one is on top of the Nvidia Tegra.

author: ogden
author_rank: Super Contributor
timestamp: October 13, 2020, 08:35:00 pm
content: Quote from: JimKnopf on October 13, 2020, 06:56:15 pmPlease measure 1) shorted leads 2) repeat negative resistance measurement on top of the Nvidia Tegra 3) measure again with leads swapped 4) check that sense leads are connected correctly. If not sure - provide connection diagram or repeat both 2), 3) measurements with sense swapped