Do you think this PS is salvagable? - Page 1
Forum section: electronics / beginners

author: bogart219
author_rank: Contributor
timestamp: September 26, 2025, 10:42:58 pm
content: Hello community, need your input. I have one of those Chinese benchtop power supplies made by Longwei, 30 volt 10 amp. I've probably had this thing for 15 years. Never used it too much, just on occasions.Well lately in the last three months I've been using it a lot to charge up lithium batteries that I salvage. I never charge over 4.1 volts and 1.2 amps, but like to keep the amps under one. Well today I tried charging a 18 volt 4ah battery I salvaged (just the one cell that was low and like an idiot I was careless and shorted something out. All of a sudden the power supply shut down and just had faint #6 flashing on both screens. There was a 10 amp fuse under the power in ac cord that I replaced but it did no good. I took the case off and seen this mess on those mosfets. I assume that this has been an ongoing thing with those mosfets getting constantly over worked but never more than two amps. Can you use a power supply like this safely to charge batteries without damaging the supply? do you think If I replace those fets I'l get it working again? thanks for any info!

author: Stray Electron
author_rank: Super Contributor
timestamp: September 26, 2025, 11:21:00 pm
content: Short answer: No, it's not going to be repairable. There are too many non-standard parts and too many erratic designs in Chinese electronics and there are no schematics or other service information available so IMO throw it away and buy another one. You can TRY replacing the obviously failed parts but especially in power supplies, there is usually a chain reaction of other failed parts.

author: wraper
author_rank: Unknown
timestamp: September 26, 2025, 11:57:12 pm
content: Quote from: Stray Electron on September 26, 2025, 11:21:00 pmMost of SMPS are easily fixable, even when there was a chain reaction. That white stuff seems to be messily applied thermal paste, nothing to do with the issue.

author: thm_w
author_rank: Super Contributor
timestamp: September 27, 2025, 12:00:50 am
content: Your going to want to start at the output and work your way backwards looking for blown parts.If there is a FET, measure it with your multimeter on diode or resistance mode.In the future never charge a lithium battery on a normal lab supply without using a diode.

author: BradC
author_rank: Super Contributor
timestamp: September 27, 2025, 04:17:50 am
content: I've repaired a 60V 5A unit that I blew up while repairing/modifying. It came internally wired for 110V but with 230V stamped on it. I ended up replacing the bulk filter caps, the FETs and a couple of other bits, but it has been good ever since.Those units have the FET heatsinks live, and they use one thermistor to measure the temperature of both. It's quite likely sitting between the heatsinks under that mess of white compound.if it's trying to start (you are seeing pulses on the displays) the primary side is probably (at last mostly) ok. If the FETs were shorted it'd be just blowing fuses. There are a few circuit diagrams floating around and they're pretty much all quite similar. As thm_w says, work backwards from the output.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 27, 2025, 06:23:35 am
content: Some findings from LW 3010 users wich are pointing to 1. bricked rectifier at secondary stage or 2. a not specified faulty diode. A molten capacitor was also mentioned. The issues occured suddenly at charging batteries or after charging (w max current), the unit stopped delivering any voltage on terminals. Control supply seems to be independent from main output, so display was not affected.https://forum.allaboutcircuits.com/threads/no-output-from-bench-power-supply.129299/https://diysolarforum.com/threads/longwei-power-supply-problems-anyone-else-have-this.23660/(postings are from 2016 and 2020, so there is a good chance that they adressing the same PSU)

author: bogart219
author_rank: Contributor
timestamp: September 27, 2025, 03:16:44 pm
content: Hey guys thanks for all the input. Thats the thing, I opened this case up a couple years ago and don't remember that thermal paste being all over like that.I don't have that big diode on the output line.I get continuity between the positive and negative terminals. I didn't see anything that looked burnt up, no sign of anything wrong (visually) on the board.Besides that paste being all over those mosfets, everything looks normal1)Can I test those fets in circuit? Can you tell me the best way with the meter? Maybe those fets got hot but are still good. When I took the ten amp fuse out that right below the power cord, the end of the fuse came right off. I put in a 4 amp fuse in its place and it didn't blow, the same faint flashing lights.2) If I get this one working, or another one, for charging batteries would i put that diode on the positive, right?3) This is a 30 volt 10amp model. If I got another one, I was thinking of getting a 60 volt 5 amp model. What you guys think? I never run anything where I would need more than 5 amps.

author: wraper
author_rank: Unknown
timestamp: September 27, 2025, 07:00:21 pm
content: Quote from: bogart219 on September 27, 2025, 03:16:44 pmYou do not need to test them as it's a secondary side issue. Also fuse would instantly blow if they were faulty. Most likely a shorted diode that seems to be mounted on a third heatsink.

author: bogart219
author_rank: Contributor
timestamp: September 28, 2025, 10:57:19 am
content: I think Stray Electron might be right. I tested four small diodes with the diode setting on my meter and get a buzzing in both directions. There are some more that I didn't even bother to check. Time for a new one , 60 volt 5 amp How big a diode do I need and I hook it to the positve side right? (for charging batteries). Thanks!

author: bogart219
author_rank: Contributor
timestamp: September 28, 2025, 01:59:43 pm
content: Those were small orange glass fuses. Not sure if I was testing them right?

author: bogart219
author_rank: Contributor
timestamp: September 28, 2025, 03:49:13 pm
content: Edit, I meant diodes.

author: bogart219
author_rank: Contributor
timestamp: October 01, 2025, 11:29:26 am
content: There is a Switch mode power rectifier (MBR2020CT) on a heat sink next to the two power fets that appears to be shorted. I ordered a couple new ones. hopefully thats it.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 01, 2025, 07:06:13 pm
content: Just to clarify, with the AC input disconnected, on the power supply output (one of the connectors), there is a short-circuit (low-resistance, something below 100Ohms) between VOut+ and VOut-? I do see two large 330ohm power resistors on what I assume is the output, to provide a constant load for stability.Do you have an exact model number for this power supply? If not, what's the model of the Chinese benchtop power supply, that this power supply was pulled from? Additionally, on your images, can you label where AC in is, and what was connected to each of the connectors, and provide a better image of the rear of the PCB. I cannot tell if it's actually 2x small transformers + 1x large inductor (possibly for power factor correction; PFC), or 3x transformers.

author: wraper
author_rank: Unknown
timestamp: October 01, 2025, 09:29:26 pm
content: Quote from: bogart219 on October 01, 2025, 11:29:26 amThese fail alone unless SMPS is terribly designed with no proper overload protection in place.

author: BradC
author_rank: Super Contributor
timestamp: October 02, 2025, 01:42:14 am
content: Quote from: bogart219 on October 01, 2025, 11:29:26 amThat device has a PIV of 20V (RMS 14V). Isn't that a bit low for a full-wave rectifier on a 30V PSU?

author: wraper
author_rank: Unknown
timestamp: October 02, 2025, 03:11:09 am
content: Quote from: BradC on October 02, 2025, 01:42:14 amShould be center tapped rectifier,not full wave. And yeah, 20V is way too low, good catch. Actually, it should be MBR2045CT as an absolute minimum, normally MBR2060CT or higher.

author: BradC
author_rank: Super Contributor
timestamp: October 02, 2025, 03:18:25 am
content: Quote from: wraper on October 02, 2025, 03:11:09 amIt is center tapped. For the terminology I was (and have been for my 50 years) under the impression a center tap with 2 diodes was full wave (essentially 2 half waves anti-phase). A single winding with bridge rectifier was a full wave bridge and a single winding with one diode was a half-wave. Is that incorrect?

author: wraper
author_rank: Unknown
timestamp: October 02, 2025, 03:30:58 am
content: Quote from: BradC on October 02, 2025, 03:18:25 amWell, on second thought it's technically correct but ambiguous. As the bridge or center tapped type is omitted. IMHO one would normally assume you meant bridge rectifier with no context given.

author: bogart219
author_rank: Contributor
timestamp: October 02, 2025, 02:31:02 pm
content: Hey Loose, I've been sick with the flu thats why I didn't respond sooner. Thanks for your help. Here are some more pics that maybe will tell you something. Thanks again!

author: bogart219
author_rank: Contributor
timestamp: October 03, 2025, 03:04:28 pm
content: Well I replaced the power rectifier (MBR2020CT) with a new one and its still doesn't work. The display still flashes 666.6666 but now its NOT faint but as bright as it normally is, don't know what that means.back to the drawing board:(I plugged it in and used my thermal camera to look at it. The two mosfets appeared to be showing a lot of heat.

author: wraper
author_rank: Unknown
timestamp: October 03, 2025, 05:02:47 pm
content: What actual output voltage do you get? Depending on particular specimen of MBR2020, it may be conducting in reverse direction if output voltage is close to 30V. It's clearly a wrong part to be used here, don't know what Chinese were thinking putting it there.

author: bogart219
author_rank: Contributor
timestamp: October 03, 2025, 06:44:39 pm
content: hey Wrap, where exactly should I measure that at?

author: wraper
author_rank: Unknown
timestamp: October 03, 2025, 06:57:46 pm
content: It seems that rectified voltage goes straight to PSU output.

author: bogart219
author_rank: Contributor
timestamp: October 05, 2025, 01:37:09 pm
content: Quote from: wraper on October 03, 2025, 06:57:46 pmCome again?

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 05, 2025, 02:46:16 pm
content: He's sure, the rectified voltage behind converter is connected direct to the terminals without fuse or other components between.Interestingly, a couple of reports in web about this or similar units - near all fried at charging (Li)-Batteries - found a p-channel Mosfet STP75NF75 or STP55PF06 (both TO220) also destroyed. Located behind retifier/smoothing capacitor either in +Ub or negative line close to the out-terminals. Normally they shoud be mounted on a heatsink.Symptoms: no output.Unfortunateley no schematics for this model are available, so for understanding the operation of these fancy units two similar or nearest matching schematics were cited.One is a 20V/5A, Manson NRP 2050, the second 60V/5A Maisheng MS605.

author: MrAl
author_rank: Super Contributor
timestamp: October 05, 2025, 04:28:14 pm
content: Quote from: BradC on October 02, 2025, 03:18:25 amHi,To be certain you need the schematic or trace the wiring and any copper clad traces.Two diodes with center tap is 'usually' a full wave rectifier but not always.

author: MrAl
author_rank: Super Contributor
timestamp: October 05, 2025, 04:49:16 pm
content: Quote from: bogart219 on September 26, 2025, 10:42:58 pmHi,Is there any chance that some pigeons were flying overhead when you first took this thing apart?Have you been able to identify what that white gook is?The success of fixing one of these depends on a number of things. The most important is what exactly is wrong with it. If it is a blown output diode (due to accidental reverse polarity of the battery to be charged) then replacing that fixes it, for example. Another example if the device is old, is that electrolytic caps tend to go bad fast sometimes, and then replacing those fixes it. If the device has not been run for a long time that may be different though because most of the aging will come from the unit being run up and kept on as in normal use.The other things include:What test equipment you have on hand or can get.What parts you have on hand or can purchase.Can you get a schematic (can't stress the importance of this) or be able to create a good schematic by tracing the wiring and PC board(s).What you are willing to do to fix it.The last one there is interesting because to fix one of these the right way, you usually have to be able to power the control circuitry up separate from the drive output circuitry. The control circuit part is the chips and other components that controls the waveforms like the pulses. The drive output circuitry is like the output transistors and any drive circuitry that drives those transistors.The idea is to power these up separately so that you can fully power the control part while turning the DC bus for the output drive section up slowly, a little at a time, to check for problems like overcurrent before the transistors blow out again. This is the technique used in industry especially for new designs.You could consider that these are the more advanced types of devices that need special methos for testing unless the problem is very simple.So you have to think about all that.Now as far as charging Li-ion or similar batteries, yes you can charge them, and you can also do lead acid batteries. You do have to be very careful though or you could get some really bad results like fire or explosion. You have to know your power supply is reliable too as far as the settings go. If you set it to 4.2 volts max you have to KNOW it is 4.200 volts for example. That means testing with a good meter too.You also have to set the max current for whatever the battery specs say it can take.These are very important points because one mistake could burn the whole house down, no kidding, and if a fire starts it may be very hard to put out even impossible, so you have no recourse except to exit the dwelling. You don't want that to happen, so if you have any doubts now is the time to stop and just buy a good charger. A good charge is not that expensive.BTW, switching power supplies have come down dramatically over the past 10 or so years. You can get one that does 30v 5amp for $30 USD, and probably 30v 10 amps for a little more. I got a little one for that price and it's a lot more compact then the two older giant ones I had in the past.

author: bogart219
author_rank: Contributor
timestamp: October 05, 2025, 09:35:10 pm
content: Quote from: Uunoctium on October 05, 2025, 02:46:16 pmThanks for the info brother

author: bogart219
author_rank: Contributor
timestamp: October 05, 2025, 09:44:59 pm
content: Quote from: bogart219 on October 05, 2025, 09:35:10 pm

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 06, 2025, 04:12:08 am
content: In principle, I agree with @wraper, as there is no fourth heat sink visible. But what is connected to the contacts circled in red? If it is a switch (red in 2nd image ie MOSFET) that is defective, the entire setpoint and current limiting may not be seeing anything, i.e., they are blind. The opto-isolated feedback loop to the primary side does not send an inhibit signal to the controller, which now sets the duty cycle to full. No output at the terminals, but the display shows the devil's error message due to overload?Is there any voltage measurable at the secondary side smoothing capacitor behind the fast double diode? Probably in 60V region?Without a circuit diagram we are tapping in the dark...

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 06, 2025, 05:18:55 am
content: Is there an antiparallel reverse voltage protection diode? Perhaps on the small pcb with the output sockets? Should be measured without the mainboard connected

author: BradC
author_rank: Super Contributor
timestamp: October 06, 2025, 07:34:20 am
content: That board layout looks pretty similar to my MCH-605 and -1030. I've attached what might be a closer representation of the schematic.The connector next to the pair of shunts is for the ammeter.

author: wraper
author_rank: Unknown
timestamp: October 06, 2025, 10:57:20 am
content: Quote from: bogart219 on October 03, 2025, 03:04:28 pmOn second thought, considering those displays are driven by ICL7107 ADCs, this claim is very strange. First of all, the maximum value this IC can display is 1999, so four sixes in a row (6666) is not even physically possible.Secondly, you'd need to intentionally apply a very precise voltage to inputs of those for multiple 6s to be displayed at all.IIRCICL7106/7107 start with displaying 1999 when power is applied (EDIT: Intersil 7106/7 display 1666), then after about half a second start displaying measured value. Considering displays likely use just 3 digits, 1st digit is omitted and both are flashing999666.Beside that, the only plausible reason of displays flashing I can think of is power of ICL7107 disappearing. Basically, auxiliary low power SMPS is shutting down and starting again.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 06, 2025, 05:03:37 pm
content: From Intersil App-Note 052 (only via waybackmachine*).Page 5:QuoteWhen the 1st digit missing, the appearance of that failure is like:*https://web.archive.org/web/20151203110117/http://www.intersil.com/content/dam/intersil/documents/an05/an052.pdf

author: wraper
author_rank: Unknown
timestamp: October 06, 2025, 05:30:05 pm
content: ^Hmm, I've certainly seen various clones of ICL7106/7 (xx7106/7) displaying 1999 on startup. Now found some videos where Intersil ones display 1666 on startup. So it looks like OP just typed one extra 6 as this PSU uses original Intersil parts. Anyway, the rest of my post above stays valid.

author: bogart219
author_rank: Contributor
timestamp: October 07, 2025, 10:50:41 am
content: Yes, this is what mine is doing now. Mind you, before I replaced that transistor, the led display was real faint,now the brightness is normal. Here is the link, couldn't figure out how to embed this video clip.https://imgur.com/a/mtP1Esu

author: wraper
author_rank: Unknown
timestamp: October 07, 2025, 11:23:21 am
content: Yep. That's auxiliary SMPS restarting. Something should be overloading one of its outputs or primary side electrolytic capacitor has gone bad. Try disconnecting the fan. I recall you said you have a thermal camera. Look if some part on secondary side is not blinking on camera. Check these diodes first.

author: bogart219
author_rank: Contributor
timestamp: October 09, 2025, 10:04:19 am
content: This is the underside of the board, first pic, two large resisters attached to the two large capacitors, getting real hot. With no load?Second pic, a diode I already removed from the board because I thought it tested shorted. I removed it and it test good. something is causing it to heat up?

author: wraper
author_rank: Unknown
timestamp: October 09, 2025, 10:17:12 am
content: Those resistors most likely provide minimum load for main SMPS and have nothing to do with the issue. Diode most likely measured short because something on that power rail is shorted to GND as its anode is connected to GND through transformer winding.

author: Sorama
author_rank: Frequent Contributor
timestamp: October 09, 2025, 10:57:11 am
content: you may want to measure the ohmic resistance between the voltage rails and ground and look for the worst/minimal value.

author: bogart219
author_rank: Contributor
timestamp: October 09, 2025, 08:51:40 pm
content: I'll try to track down that short and report back. Thanks for your help guys!