How much 50Hz is too much? - Page 1
Forum section: electronics / projects

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: March 28, 2014, 11:52:15 am
content: Hi All,While working on some stuff last night I realised that if I touch the tip of my oscilloscope probe with my finger, I measure around 100v p-p. If I'm wearing my ESD wrist-strap (2M to Gnd)or if I touch the probe to the metal popper on the curly lead I get just over 200v p-p.Is this normal? Obviously I'd expect to pick up some 50Hz but this seems like I might have something funny going on. Maybe a bad earth in the oscilloscope or the socket feeding it?I'd be interested to hear people's opinions on this.Cheers.(230v 50Hz mains here)

author: belasajgo
author_rank: Contributor
timestamp: March 28, 2014, 12:10:28 pm
content: Are you sure you are measuring 100 V p-p and not mV?

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: March 28, 2014, 12:19:16 pm
content: Yes. Unless I've done something really obviously stupid. 50 V/div, x10 probe.

author: han
author_rank: Frequent Contributor
timestamp: March 28, 2014, 12:23:16 pm
content: Quote from: DenzilPenberthy on March 28, 2014, 11:52:15 amSound like grounding problem.The Oscilloscope ground is floating.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: March 28, 2014, 12:48:56 pm
content: Ugh, thought so. Time to start tearing my workspace apart

author: ConKbot
author_rank: Super Contributor
timestamp: March 28, 2014, 01:17:16 pm
content: Quick check would be to check for continuity with a DMM from your ground point of your ESD strap to the shell of one of the BNCs on your scope. Ideally it should be a dead short (measure on the other side of the strap resistor) If its anything more than a few ohms, then indeed, time to start checking the grounding of your workbench area. Hopefully its just something like a broken power lead, or a loose connection in a wall outlet and not bad wiring between an outlet and a breaker.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: March 28, 2014, 02:23:28 pm
content: Huh, well there you go. Broken spot weld in the earth busbar inside a multi-way plug strip.Also interesting that it was allegedly PAT tested in December by the same external contractors who put a pass sticker on a lab oven with a mains flex that was so perished it had exposed conductors and the insulation fell off in chunks when you flexed it. Nothing has been plugged into or removed from this plug strip since the PAT test. Maybe it broke after the PAT test... who can say...! Also interesting to note that the PAT testing company use hand written labels rather than ones which are printed by the PAT machine so ... hmm.... just saying

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2014, 03:48:10 pm
content: Not so "dura"plug I'd say?

author: SeanB
author_rank: Super Contributor
timestamp: March 28, 2014, 07:15:08 pm
content: PAT= Pass by And say it was Tested more likely.

author: Wim_L
author_rank: Regular Contributor
timestamp: March 28, 2014, 11:55:01 pm
content: Hah. I've seen new calibration labels taped to equipment that was defective enough that it would fail to power up.