86VAC Between Plugpack negative/zero and mains earth - Page 1
Forum section: electronics / beginners

author: Blastcap
author_rank: Regular Contributor
timestamp: June 28, 2016, 08:46:25 pm
content: I was poking around a circuit fed from a 12V plugpack, when i saw a spark (between plugpack negative and scope ground) as i was connecting my scope ground.Curious i measured 86,4V ACbetween mains earth and plugpack negative whenever the plugpack connected to my circuit or not, and leaving the scope ground unconnected from the pack i scoped out a neat 336V! AC (116V Peak-Peak).My meter is not TRMS but shouldn't it show a little more than 86V?I retried with another plugpack from another manufacture... same result.Both simple 2 pin unearthed packs. EU Mains 230V 50HzI found this thread:https://www.eevblog.com/forum/beginners/dodgy-switch-mode-power-supply/similar problem, but not a voltage bigger than mains voltage.Also anyone knows why my meter shows such a low AC voltage compared to my scope? Scope bad? meter bad? both mad?(Scope, Hantek DSO5072P - probe set to 10x, input 10x, DC coupled)(Meter, Fluke 17B+)

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 28, 2016, 09:33:05 pm
content: Your meter measures RMS voltage, not peak. That waveform shows 166V peak, but it's not a sine wave, so the typical Vpeak*0.707 equation does not apply. Looking at it I'd say that 86 Vrms seems reasonable.

author: Seekonk
author_rank: Super Contributor
timestamp: June 29, 2016, 08:42:54 am
content: Try measuring that with a 10K load and see what you get. The readings may be closer with a uniform load.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 29, 2016, 02:05:36 pm
content: Common mains leakage current. Capacitive coupled due to lack of earth. Try measuring the AC current, should be less than a mA.There is a few pF or nF capacitor from primary to secondary. Either intentionally or parasitic.Here is an example model:https://www.maximintegrated.com/en/app-notes/index.mvp/id/716(chapterTransformers)The waveform of this voltage can be severely distorted by the noise generated by smps itself.

author: johansen
author_rank: Super Contributor
timestamp: June 30, 2016, 12:33:39 am
content: that doesn't look like mains leakage through a capacitor but rather something else.If the voltage you're measuring is from current flowing through a capacitor, then it appears to me the capacitor stops being a capacitor, and starts being a resistor when the voltage across it exceeds about 100 volts.

author: Blastcap
author_rank: Regular Contributor
timestamp: June 30, 2016, 11:37:06 am
content: I added a 20K Resistor between negative and ground and sure enough, the voltage dropped to 2-3VAC.Current was around 80uA.Guess ill just keep a resistor grounded and plug it to all my small circuits, that should keep them from floating away.And i thought small floating supplys was a good thing.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 30, 2016, 12:27:57 pm
content: So it is leakage. Very low, which is good. Nobody can make a perfect transformer.The supply is floating, it can be anywhere within the isolation rating of the transformer allows. 86 Vac is perfectly valid.It normally settles at half mains. You were measuring it, the impedance of the meter has influence. This is probably why it measured 86 instead of ~110.Schrödingers leakage.There needs to be more fundamentals friday about this. There is already a "how not to blow up your oscilloscope". It's a common pain in the * for EMI noise and sparks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2016, 11:04:50 pm
content: Quote from: johansen on June 30, 2016, 12:33:39 amIt is leakage -- apparently they connected the Y-cap to the DC side, not AC. Hence it's zero except when the FWB is conducting, so it looks like crap.Tim