strange DMM measurement - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: February 06, 2017, 07:07:00 pm
content: Ok, I dont know what is this or what I'm doing...I was making some measurements on a professional audio recorder and I accidentally made contact with the probes of the DMM on the GND connector of the power supply and the metal case of the unit. The DMM was in AC Voltage reading and... it measured 8,5 V AC ??The thing is repeatable. But if I connect for example a USB cable, it goes to 0V.So the "thing" is only when the unit is powered from mains and without any cable connected.If battery powered or some cable connected like USB, it doesn't read any value.I don't know if it's related, but I think, I tried to slowly horizontally slide my finger on the rear metal slot and I "feel" something, like a little "current", of course only if mains powered, on battery, nothing.What is this? What measurement is this? Is this unit badly shielded?

author: Jeroen3
author_rank: Super Contributor
timestamp: February 06, 2017, 07:15:39 pm
content: Leakage current from the mains input filter. Since the enclosure is not earthed until the USB is plugged in, there will be some voltage due to a capacitor divider.8.5 V is bit low. It is usually half mains. Can you measure Vac to confirm?

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: February 06, 2017, 07:22:50 pm
content: I had a DVD player, that had a capacitor from the 240volts mains active, to the case.This would be fine, if the case was actually grounded.But, it wasn't, unless you plugged something into the unit that had an earth connection.You could measure, and feel, over 200volts AC on the case......I took it back, the guy at the store didn't seem to understand what the issue was... I got a refund.

author: Unknown
author_rank: Unknown
timestamp: February 06, 2017, 07:25:36 pm
content: I was not explaining well, I'm sorry...When talking of "mains" I meant using its transformer that is a 220V to 12V DC. So the unit is powered from this 12VDC.I was meaning to differentiate cable power from battery power.I don't understand what do you mean with "can you measure Vac to confirm"?

author: Unknown
author_rank: Unknown
timestamp: February 06, 2017, 08:39:56 pm
content: I found out the DMM gives the same AC value even only touching the external metal power connector of the transformer with only the black probe.If disconnected from the unit, the metal power connector (it is an Hirose adapter) of the transformer measures nothing.

author: james_s
author_rank: Super Contributor
timestamp: February 06, 2017, 08:45:52 pm
content: A DMM has a very high input impedance, it will pick up noise right out of the air if you connect it to an antenna. As long as the leakage current to the case is below a safe threshold it's not something to worry about. Even with a transformer isolated supply there will be some capacitive coupling between the windings.

author: Unknown
author_rank: Unknown
timestamp: February 06, 2017, 08:57:43 pm
content: Ok, the fact it's safe, as 8,5VAC doesn't harm.But anyway this should confirm there is the leakage on the case/GND.I'm thinking if this could give problems to audio, as mics could be power phantom @48V (and here shouldn't be problems) or connected shorting XLR pins 1(GND)to3(cold) for unbalanced mics battery powered. Could this cause some problems on these mics? I mean if GND isn't really a GND but it has some little voltage/current on it?

author: Jeroen3
author_rank: Super Contributor
timestamp: February 07, 2017, 06:26:12 am
content: This is only a problem if you have two mains powered sources.You can get what's called, ground loop hum.