I recently bought a cheap class 2 switching mode power supply (the type normally bought as replacement for laptop brick-type power adapters) and noticed a tingling sensation as I ran my fingers over the metal casing.
So I decided to open it up and inspect it for obvious faults. Everything seemed in order, nothing touching anything it shouldn’t and relatively well-built considering the price I paid. The overall layout looked sensible. Fused input, MOV protected, decent connections and reasonable PCB tracks, big caps, chunky coils, a myriad of power MOSFETs/transistors and a couple of OPTO-isolators. Some surface mount technology on the underside of the board, completing the picture.
The only thing I can put this leakage current down to, following my cursory inspection, is that somewhere they have used a capacitor, or two, between the Live/Neutral and internal ground connections to filter out noise (switch-mode PSUs create a lot of it).
Since it is a class 2 PSU, it has no earth connection. Just a two-pin plug which has no effect on the phenomena whether you plug it one way, or another.
A quick Google only turned up this thread, on the IET board which briefly discusses the issue, but seems no more than a “shrug, oh well”.
Perhaps this is brought about by the fact that some existing standards seem to permit this kind of design. Depending on the standard, as much as 1.5mA RMS at 277 VAC at 60Hz (EN/IEC) is allowed. Some standards are tighter, allowing only 0.25 mA.
I don’t know about anyone else, but 1.5mA AC at mains voltage leaves me a little uncomfortable, not least because it’s not always dry un-broken skin (at several KΩ) that could become exposed to it, but bare feet, a sweaty hand and a fresh cut could present as little as a couple of hundred ohms resistance to ground via vital organs.
I decided to see how bad the problem was. First, I checked the resistance between each mains input pin and the PSU outputs. My digital multi-meter (Uni-T UT71D) was unable to register any reading – meaning that the isolation DC resistance had to be well over 40 MΩ. Good enough for me. So far, so good. Next up, I measured the voltage from the output ground to mains ground (via the earth tag on my ‘scope). this came in at a whopping 93 VAC RMS and 93.1 VAC+DC RMS. Ok, ok, no need to panic until we’ve checked the current on a near-dead short to earth – unfortunate choice of words not-withstanding. I ran through the current scales on my meter, amps, milliamps and finally micro-amps, where I eventually got a 0.35 µA reading.
Given the very low current reading I figured I could just about live with that, even though I don’t like it one bit, no sir! I’m left worrying about the consequences should those caps seriously degrade, or ever fail (and they do from time to time).
Also, I can’t quite reconcile the lab measurements to the fact that I can detect this leakage current with the tips of my fingers. Perhaps it’s those whopping 90 odd VAC – which, by the way, are enough to light-up the neon light of an electrical screwdriver (you know the old-fashioned kind that you have your finger on the end of and have a resistor in series between you and the neon).
My conclusion is that this is the way it is and you won’t know until after you have bought product x, since this seems to be common practice and acceptable under current standards. I’m still not happy and I may decide to add an earth wire to the casing. The leakage will still be there, but at least it’ll drain away to ground and if those caps ever fail, whoever is using this thing won’t have such a nasty surprise.
As an aside, I tested a few smaller power adapters (the ubiquitous wall wart) and found them much tamer, with voltages of only around 10VAC bleeding into the output.
Filed under: Electronics Tagged: | Current, Leakage, Mains, Power Supplies, Switching
