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Electronics => Beginners => Topic started by: vutt on March 07, 2021, 04:20:00 pm

Title: Vintage DMM picking up phantom voltage (video)
Post by: vutt on March 07, 2021, 04:20:00 pm

Trying to identify problem with old DMM. It keeps showing "phantom" voltage after disconnecting from power source. Goes slowly down, but it never goes below few hundred mV and it keeps jumping around.
Short 30s demonstration: https://youtu.be/DnozavvCiIU
However as you can see from clip if I just switch power supply off probes still connected voltage drops immediately as it should.
Both DMM and PSU are properly grounded. I'm using 230v->110v transformer, but it's proper 3 prong one with ground pin connection.

Any ideas where to start?

Title: Re: Vintage DMM picking up phantom voltage (video)
Post by: Gyro on March 07, 2021, 05:26:53 pm

Bench DMMs like that have very high input resistance on their lower voltage ranges (up to 20V on the best ones) - something in the order of 10G \$\Omega\$, compared to 10M \$\Omega\$ on a handheld one.

This is to provide very minimal circuit loading, but it does mean that the input floats when disconnected, governed by the input bias current of the meter. The input bias current is normally a few 10s of pA, which stops it settling to zero.

Relax and enjoy.


EDIT: The manual can be found here... https://www.opweb.de/english/company/Advantest_Corporation/R6551 (https://www.opweb.de/english/company/Advantest_Corporation/R6551)  The input resistance is actually specified as greater than 100M \$\Omega\$ on the DC mV ranges, it drops to 10M \$\Omega\$ on the higher ranges.

Title: Re: Vintage DMM picking up phantom voltage (video)
Post by: Per Hansson on March 07, 2021, 07:31:44 pm

Watch "EEVblog 1379 - What's all this NPLC Stuff Anyhow?" it is only released to Patreons so far but it should answer your questions when it gets released.
Before then you can play with the "rate/line" key on the instrument and also make sure to set it to the correct line frequency of 50 or 60Hz.
Basically the instrument works as intended, edit: attached excerpts from the manual linked above by Gyro.

Title: Re: Vintage DMM picking up phantom voltage (video)
Post by: vutt on March 07, 2021, 08:51:28 pm

Thanks guys. It was my first vintage bench DMM purchase so I thought something is wrong.
And I have that manual. So LF is set correctly. I just didn't connect the dots regarding input resistance ... ...well due to lack of knowledge.

Also thanks Per for Patreon tip. I have been lurking around Dave's YT channel for years. This would be as good as any excuse to support his effort with slightly more than just words 8)

Title: Re: Vintage DMM picking up phantom voltage (video)
Post by: Per Hansson on March 08, 2021, 04:20:02 pm

Now the video has been released, I am sure it will answer your questions better than I could :)

https://www.youtube.com/watch?v=yKb7xIsPjVM (https://www.youtube.com/watch?v=yKb7xIsPjVM)