ET5410a Electronic load power supply not isolated - Page 1
Forum section: electronics / repair

author: hp3310a
author_rank: Regular Contributor
timestamp: July 23, 2025, 10:51:32 am
content: I noticed a faint clicking sound on my bench with all devices turned off. After some digging I finally identified the culprit: It's the East Tester 5410a electronic load.So I opened it up and found out that if the power cord is plugged in such that neutral is switched (the power switch has only one pole) and the mains line is connected to the power supply directly, the power supply emits a very faint clicking sound and its led lights up very briefly every 18.5 seconds. This only happens, if the output of the supply (1x 12V, bottom connector in the foto) is connected to the rest of the unit. If the supply's output is not connected, there's no clicking or blinking.Since neutral is switched and OFF it means some kind current is flowing from line via the load's chassis and the power supply to earth. The 30mA GFCI didn't trigger (and it works, I specifically tested it), so the current is less.So this leaves me puzzled and a little uneasy, because1) The high voltage side of the power supply isn't really isolated from the rest, otherwise no current would be flowing2) The load's electronics are connected to the chassis and earth.Even if I exchange the power supply, number 2) wouldn't be fixed. What to do?

author: hp3310a
author_rank: Regular Contributor
timestamp: July 23, 2025, 05:34:35 pm
content: I took out the power supply and measured for continuity across the MOV and across the primary and secondary side of the main transformer. Both showed >60MΩ on my Brymen 789.The underside of the board looked like it was colored using a black Edding and I thought maybe the isolation is bad due to it. So I scratched to color off between high and low voltage side. The fault is still there, but the charging time between clicks/led blinks got 1 second longer. So I think I found the problem: Bad paint job.Can someone recommend a replacement supply that can be trusted?I don't know the wattage, East Tester doesn't say what the power consumption is for this device. I guess it is less than 2-3A at the 12V side, as the power supply has no heat sinks.The ET5411a schematics (https://www.eevblog.com/forum/testgear/east-tester-et5411a-(and-et5410a)-electronic-load-schematic-reversed/msg4882754/#msg4882754) say it's a 12W 12V 1A power supply, so a Mean Well EPS-15-12 should work as a replacement.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 24, 2025, 08:52:31 am
content: Yes, this fascinating story continues...The power supply took a bath in IPA over night, didn't change a thing.I'm certain not only I wanted to know: What kind of current is flowing? So here you go:70-80µA while turned off (with N connected to the switch and L to the supply of course).150-160µA when powered on.The replacement is already ordered, I'll repeat this measurement when I have it.

author: Seekonk
author_rank: Super Contributor
timestamp: July 24, 2025, 06:16:28 pm
content: It would help to say what country you are in or what voltage this is operated at. Can't tell from back od socket. A lot of stuff from China is dual voltage and they switch the neutral which is wrong. Bleed off resistors and connecting this to some device with unknown inputs can cause a small switch mode supply to run. It bumps because of the 1M> bump start resistor provides enough current for a start. Without going crazy trying to find a gremlin, why don't you just move the switch to the hot side. I would have done that regardless of an issue or not. Low side switching is dangerous.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 24, 2025, 07:39:17 pm
content: I'm on 230V (actually it's 236V here), 50Hz, central Europe.Made a quick and dirty schematic that might clear things up. The upper case is when the cord is plugged in "the right way" - by pure chance, turn the power plug 180 degress and it is N that is switched.Anyway, it doesn't matter really. There's about double the "OFF-current" flowing via chassis to the earth contact of the IEC connector when the thing is turned on. That should be 0.0A, in both cases.

author: Kean
author_rank: Unknown
timestamp: July 27, 2025, 09:19:23 am
content: Some leakage to earth for an SMPS is normal. It will be via the class Y capacitor across the isolation gap. If you look at the EPS-15 spec sheet, Meanwell just specify it as being less than 1mA at 240VAC.The correct fix is as mentioned by Seekonk and changing the input/switch wiring. Even better, see if you can source a replacement power switch that can handle both live and neutral - obviously with suitable voltage & current rating.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 27, 2025, 10:14:29 am
content: Quote from: Kean on July 27, 2025, 09:19:23 amApparently I don't get it. The switch directly follows the IEC plug before the power supply. There's no way to move it further upstream.

author: Kean
author_rank: Unknown
timestamp: July 27, 2025, 11:11:15 am
content: Looking at your photo in the first post, the black wire is going from IEC N to PSU L, and the red wire is going from IEC fused L to the switch, then back to PSU N. So, try swapping the two crimp terminals in the plug end going into to PSU.After doing that you will still see some leakage current when powered on, but hopefully none when powered off. And if your country uses non polarized mains plugs you will need change the mains power switch from single pole to double pole.My point was that switching to another SMPS like the Meanwell ESP-15 isn't necessarily going to fix the problem.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 27, 2025, 11:56:49 am
content: In much of Europe Schuko sockets are used, in Germany (and some other places?) the original, "unpolarized" version (CEE 7/3). With products shipping world-wide, I'd think the distinction between neutral and hot is futile (as e.g. in Germany it cannot be ensured; at least not for single phase supplies). I.o.w. the safety and functionality of the device must not depend on that distinction.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 27, 2025, 05:28:57 pm
content: Quote from: Kean on July 27, 2025, 11:11:15 amYou might have missed this a couple of posts up:Red goes from the IEC connector to the switch, from there to the PSU. Black goes from the connector to the PSU directly. Just as it is supposed to.Whether the switched line is hot or not depends on the orientation of the plug in the socket.

author: Kean
author_rank: Unknown
timestamp: July 28, 2025, 08:14:51 am
content: Quote from: hp3310a on July 27, 2025, 05:28:57 pmNope, I saw that... but neither the upper or lower schematic that you have drawn match what is in your photo.If you look at the N & L labels on the IEC connector block in the first photo, the IEC N connection has the black wire and goes to the PSU L terminal.The IEC L connection goes via the fuse then the red wire, switch, another red wire, and the PSU N terminal.Swapping those seems to be the first thing to correct, and should remove the reported issues.Quote from: hp3310a on July 27, 2025, 05:28:57 pmIsn't that what I said?As mentioned above, the N & L connections on the PSU are swapped.This will likely be the cause of the leakage current and the PSU bootstrap cycling you see, but I don't believe it to be a safety issue.I've not done any analysis, but I guess that the cycling could lead to early failure of the PSU.If you have a non-polarised mains plug and the potential for this this leakage to happen again concerns you, then you could change the mains power switch to double pole.