How to test if an SMPS power adapter has galvanic isolation? - Page 1
Forum section: electronics / general

author: sidharth
author_rank: Newbie
timestamp: August 17, 2016, 03:34:47 pm
content: Hello All,Here's the background, a bad experience. Some time ago I powered a PIC uC with the typical application circuit of the LNK302PN from Power Integrations as shown in the attached schematic. NO GALVANIC ISOLATION. I blew the laptop and the PICKit3 when attempting to program the uC. Lesson learned!Later... I was given a no name brand PIC micro based dev kit and it came with the (SMPS based, 230VAC in to 9VDC out) power adapter (wall wart / power brick / whatever people like to call it) also shown in the attached image. Based on my previous experience, I was real reluctant to plug the adapter to the dev kit while the laptop is charging, meaning I can only use the dev kit with either battery power or with that power adapter and the laptop not plugged in.My question is, how do I find out / test if that power adapter (or any other similar no name brand or datasheet available kinda power adapter) has galvanic isolation for sure?Thanks in advance,Sid

author: Jeroen3
author_rank: Super Contributor
timestamp: August 17, 2016, 03:58:59 pm
content: With amegger.Or the double isolation symbol.

author: langwadt
author_rank: Super Contributor
timestamp: August 17, 2016, 03:59:14 pm
content: ohm meter from outputs to inputs

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 17, 2016, 04:01:52 pm
content: Or better still, use a megger between inputs and outputs. Set it to 500VDC. It should kick up momentarily as the caps charge and then settle down to infinite resistance, or very close.

author: Kjelt
author_rank: Super Contributor
timestamp: August 17, 2016, 04:02:44 pm
content: Or a resistor to earth/ mains neutral and measure if current flows, be sure that the current is limited to a mA or so. But I think there will be safe testers for this, just like the screwdriver with resistor and neonlight to test for mains.

author: wraper
author_rank: Unknown
timestamp: August 17, 2016, 04:08:41 pm
content: Unless it is broken, there is galvanic isolation in any power brick. Even if it is cheapest dangerous Chinese crap.

author: fubar.gr
author_rank: Unknown
timestamp: August 17, 2016, 04:47:54 pm
content: All plugpacks are isolated.If it was like the schematic in your post, then you'd have a 50-50 chance of getting electrocuted, depending how you plugged it in the socket (assuming you have a non polarised Euro socket)

author: sidharth
author_rank: Newbie
timestamp: August 17, 2016, 05:12:45 pm
content: @wraper & fubar.gr, how can you guys say with any level of surety that these power adapters are indeed isolated?

author: langwadt
author_rank: Super Contributor
timestamp: August 17, 2016, 07:53:39 pm
content: Quote from: sidharth on August 17, 2016, 05:12:45 pmif the one you pictured wasn't isolated, depending on how the adapter is plugged in, the exposed metal on the barrel connector would be at mains voltage

author: Jeroen3
author_rank: Super Contributor
timestamp: August 17, 2016, 08:29:30 pm
content: The parasitic capacitance or leakage current can damage your electronics. Especially on hot-plug conditions.

author: David_AVD
author_rank: Super Contributor
timestamp: August 17, 2016, 08:53:26 pm
content: Quote from: wraper on August 17, 2016, 04:08:41 pmNot true actually. I have seen (and gotten bitten by) one that was a capacitive dropper type.Granted it was for Christmas lights, but it had a 2 pin connector on the "low voltage" (but referenced to mains) output.

author: sidharth
author_rank: Newbie
timestamp: August 18, 2016, 05:22:43 am
content: Ok guys, getting back to my question (how do I find out / test if that power adapter (or any other similar no name brand or datasheet available kinda power adapter) has galvanic isolation for sure?), what I gather thus far is:a) Use a Megger and take an educated guess.b) Don't trust just any adapter as there is a good chance its NOT galvanically isolated.Meaning thereis no methodto test for galvanic isolation, unless the power adapter is heavy / bulky like the ones of old where you know by its sheer weight (and size) that it does have a mains step-down transformer. Thus either I have to either take apart the silly thing and check and for myself if galvanic isolation is indeed present or just dispose of it and use an adapter that I know for sure IS galvanically isolated, then?

author: Kjelt
author_rank: Super Contributor
timestamp: August 18, 2016, 07:47:39 am
content: Yes there are many ways to test but they all include you building something to test it.If it is not galvanic isolated the - or + from the output are directly or through some resistance/inductance coupled to the mains input.So what you do is measure ohmage with a multimeter, if there is low resistance you know it is not isolated. If you measure (very) high resistance you go to the next steps.1) use an isolated variac where you create mains isolated to the input of the device so you are safe yourself for the remaining measurements (better safe than sorry).2) measure the four voltages between the output pins and the input pins, when they are isolated there should be no (large) voltages between the four possible combinations.3) if there is a larger voltage on one of the combinations check if it can source current, so connect something that draws a few mA with the measured voltage (big enough resistor and Amp meter in between) and see if the voltage persists and with what current (then you know it can deliver substantial current and is not just a ghost voltage that dissapears when you load it with a few uA's).If you can draw a few mA's from the output to one of the input pins you know for sure it is not isolated.BUT even if you buy any SMPS that is galvanic isolated it doesn't mean it is safe.A lot of tear downs show that the voltage kreepage space of a lot of smps power supplies is less than the needed isolation space and some don't have airgaps either.So to know you have a safe galvanic isolated power supply, yes you have to tear it apart and measure the isolation distances, the solder joints, etc.

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2016, 08:00:32 am
content: Quote from: David_AVD on August 17, 2016, 08:53:26 pmI noticed there was no picture of the DC connector.Was it insulated? If so, I don't see the problem, as capacitive supplies are fine, as long as the live parts are adequately insulated from the user.If not, then that's bad.

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 18, 2016, 08:20:19 am
content: Quote from: Hero999 on August 18, 2016, 08:00:32 amThat would mean mains rated wire to the LED strings, wouldn't it? Doubt you'll find christmas lights with that.

author: fubar.gr
author_rank: Unknown
timestamp: August 18, 2016, 08:48:32 am
content: Quote from: Ice-Tea on August 18, 2016, 08:20:19 amOlder style incadescent type christmas lights are still sold and they are mains powered. There's nothing wrong with mains as long as everything is properly insulated.

author: David_AVD
author_rank: Super Contributor
timestamp: August 18, 2016, 09:14:32 am
content: Quote from: Hero999 on August 18, 2016, 08:00:32 amThe 2 pin DC output connector was female and somewhat shrouded. I copped a boot when I stuck meter probes into it to measure the voltage.The lights wiring was very thin (copper and insulation) and highly doubtful to be mains rated. It broke easily exposing effectively live wires.But back to the OP, can you easily take the unit in question apart? A lot of them have one of more screws (sometimes under a label) holding the shell together.

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 18, 2016, 09:27:17 am
content: Quote from: fubar.gr on August 18, 2016, 08:48:32 amDefine "properly" isolated? If Yo Lung Ho looked at the string, went "Oh, only 12V differential, let's use the cheapest, shittiest wire there is" I'd rather stay the hell away from it.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: August 18, 2016, 09:35:22 am
content: Quote from: sidharth on August 18, 2016, 05:22:43 amNot sure why you say that.First disconnect from everything else, and check there is no continuity which would be an automatic fail.Then use a megger at best to gradually increase the voltage, maybe 50v, 250V, 500V. Less chance of damaging equipment. One lead on input and one lead on output. Use the neutral, ground or common terminal on each side.If you get over say 200MOhms or full reading then it is isolated.The voltage you test at does depend on the voltages it is used at, according to IEC.Of course full compliance testing would do more specific testing at categories and voltages.Here is a couple of links, that might help.http://www.transcat.com/media/pdf/BasicInsulationTesting.pdfhttp://www.ni.com/white-paper/2827/en/

author: fubar.gr
author_rank: Unknown
timestamp: August 18, 2016, 10:04:16 am
content: Quote from: Ice-Tea on August 18, 2016, 09:27:17 amSome common sense is required.I once bought a chinese mains powered light. When I got home and opened the package, I realized it had wires so thin that tugging them a little harder than usual would be enough to severe them. It went straight to the garbage bin.

author: fubar.gr
author_rank: Unknown
timestamp: August 18, 2016, 10:11:49 am
content: Quote from: HackedFridgeMagnet on August 18, 2016, 09:35:22 amI think the proper way to megger test a power supply is by shorting the input and output like in this diagram:

author: Shock
author_rank: Super Contributor
timestamp: August 19, 2016, 03:24:33 am
content: Here is an interesting video.At 45:24 he discovers the DC on this (replacement) SMPS was floating 100V above mains earth/ground. The zap he got from it fixed the original problem.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2016, 08:47:47 am
content: Didn't watch the video as I'm at work at the moment. The current leakage through the Y capacitor can often be enough to feel but it won't do any harm to a human. However, some sensitive electronic devices can be damaged so it's a good idea to have over-voltage protection diodes on a product designed to run off a power supply like that.Quote from: fubar.gr on August 18, 2016, 08:48:32 amThere are also plenty of LED Christmas lights which are powered directly from the mains. I suppose the LEDs must be wired back-to-back and are probably all connected in series. I will have to take a closer look at some one day.