Strange Multimeter Interference - Page 1
Forum section: electronics / beginners

author: ethanfg
author_rank: Newbie
timestamp: February 19, 2020, 06:35:17 am
content: Hi Everyone, hoping you can help me solve a strange issue I’m having with my new Siglent SDM3055.I have my computer plugged into a power bar, and from that computer is an audio interface that sends audio to a tube power amp running two speakers on my desk. When testing devices, I run them through an isolation transformer. The isolation transformer is plugged straight into the outlet and doesn’t go through the power bar. My multimeter is plugged into the power bar along with the audio amp. I use the audio amp to listen to music while testing devices. If I measure the high voltage (around 450 volts) from for example a tube guitar amp, it causes my audio interface to lose connection to my pc. If I make a great connection with the probe the first time, it only makes the audio interface be interrupted for a split second. If I make a bad connection (very lightly touching the probe, causing it to arc slightly) this makes the audio interface lose connection entirely until I stop. I can’t think of what would cause this, as the device under test is isolated from every other device.Do any of you have any ideas? Or am I doing something wrong?Thanks

author: tautech
author_rank: Super Contributor
timestamp: February 19, 2020, 07:01:07 am
content: Welcome to the forum.I'm baffled as to what could be causing this other than some mains neutral or grounding issues.Is the SDM plugged into the power bar also ?Does your isolation transformer have a feedthrough mains earth ? Check this with continuity with a DMM.Do you have the SDM connected to the PC with a USB or LAN connection ?

author: ethanfg
author_rank: Newbie
timestamp: February 19, 2020, 07:43:05 am
content: Thanks, don't know why I didn't join this forum sooner!Yes, the SDM is also plugged into the power bar. The isolation transformer does have a feed through earth connection. The SDM is not connected to my computer as well. The only thing linking all the devices together is the ground/power connections. It's strange that the only device affected is my audio interface, but it is even stranger as it is USB Bus powered.

author: tautech
author_rank: Super Contributor
timestamp: February 19, 2020, 08:46:44 am
content: Quote from: ethanfg on February 19, 2020, 07:43:05 amI think it's a ground issue so try disconnecting the mains earth feed through from your isolation transformer. Technically it's not required as proper isolation removes any reference to mains earth and the output becomes phase and phase WRT each other.It's OK for the transformer itself to be grounded and as it's metal best that it is so just break the mains earth output.

author: ethanfg
author_rank: Newbie
timestamp: February 19, 2020, 04:23:57 pm
content: What you’re saying makes sense, but I don’t think it can be that because when testing 2 prong devices, the issue persists. Both the hot and negative lines should be completely isolated in this case. This is proving to be a rather tricky issue. I’m going to try running things on separate circuits and see if that helps.

author: AVGresponding
author_rank: Super Contributor
timestamp: February 19, 2020, 05:28:18 pm
content: What's extra strange is that surely the test connections on the meter are floating, ie completely isolated from the ground connection?The only thing that comes to mind is maybe some bizarre capacitive coupling going on?EDIT: Also maybe check whether the isolation transformer output earth is centre tapped.