Generic PSU 400W

[UserXP]

Hi, guys. I now you'll probably say something like "why on earth are you bothering with that cheap PSU", but I would like to try figure out what could be the issue here, and learn something in the process. Also, if I could get some more life from this PSU.

Here is a quite generic PSU rated to be 400W and a 20A on the 12V rail. The PSU still works, however, recently it started to make funny smells. As you can see in the photos, these three large resistors are probably what makes this smell due to overheating the PCB, but only after a prolonged operation.
The resistor located between 5V and 12V rails gets so hot after just first two minutes of power-on that it actually unsolders itself from the PCB. The PCB itself is burnt in the same area. On the botom, this resistor seems to go between ground and +12V (I really don't understand why, since connecting "-" and "+" will make any conductor overheat).

I was planning to put this PSU in a rather old computer running AMD Socket462, it is a fairly basic PC which will be used in the shop, nothing demanding. So I opened up this PSU to clean it from dust, which I did, and then I noticed these burn marks. The resistor gets so hot that it actually makes a burning sensation upon touching (I guess the PCB felt that also for who knows how many hours). And after a sufficient time of being that hot, the solder melts (I resoldered it).

So, I was wondering if you might have some idea what could be wrong here, maybe the problem is one of these caps that has starting to fail (although they do not look bulged).
Any help will be much appreciated, thanks in advance for any insight you might have regarding these.

[mariushm]

Re: Generic PSU 400W

The resistors are required to put a bit of load on the rails.

The power supply can't keep 3.3v , 5v and 12v somewhat steady if there's nothing connected to the cables.

This happens for example when the computer is turned off and the power supply is in stand-by... computer will start only when it gets the "power good" signal, which only happens when the outputs are steady, which only happens when there's some load on the outputs, but the computer doesn't pull any power from those cables.

So a trick is used, those resistors are put there to pull a tiny bit of power no matter what happens to make it easy for the power supply to stabilize the voltages.

The resistors will be naturally HOT but shouldn't be so hot to warm the PCB. They only dissipate about 0.2-2 watts of power.

For example, that resistor in the 3rd picture looks to me like a 100 ohm resistor (brown-black-brown is 100 ohm) ... that would mean it dissipates VxV/R = 0.25 watts of power.
If it's on 12v rail, it dissipates about 1.44 watts.

Desolder it and measure it and see if it's still 100 ohm.

The resistors may also be broken, shorted or nearly shorted, so they no longer have the resistance they had initially

On this particular power supply, it looks to me that they were really cheap with the solder, and those resistors may not be perfectly soldered to the pcb, so the poor contact heats up that point. I also see the cables around with so much insulation removed around the holes... ideally you should see a bit of solder flowing up from the other side on the wires.

It would be nice if you can post some pictures of the back side. As it is nice, guessing from how the main transformer looks on the 4th picture and the lack of components around, the power supply may be good for about 150 watts.

[mcdaydavies]

Re: Generic PSU 400W

The resister is putting a load on the 12v and 5v in order to stabilize the voltages, they will get hot, although they shouldn't be get to the point where they desolder themselves from the pcb. The capacitors are cheap and should be replaced, also there is plenty of points on the pcb to install more capacitors. The dust probably restricted the resistors from getting airflow, which caused them to desolder.

[PeteS in CA]

Re: Generic PSU 400W

Have to post-and-run, but IMO, if any of those resistors is dissipating more than half of their rated maximum power they will get really hot, enough to burn the PCB and possibly enough to de-solder themselves. Either use resistors that are double the value (and hope the voltages are still OK) or find higher wattage parts you can make fit. Be careful of the wires to the PCB; I think UL 1007 wire insulation is rated for 85*C.

[eccerr0r]

Re: Generic PSU 400W

That PSU sort of looks like the one I parted with 470uF primary input caps and probably should have been a 200W to 250W PSU not 400W

Basically all of the (older) PSUs that aren't rated 80+ have these resistors, even my Antecs. I don't know what designs that don't need them - they're to reduce overshoot when load drops off. One of the "double takes" I had to do while debugging my SP300 is that I knew these resistors were there and they were less than 100 ohms, but I measured only 0.5 to 2 ohms on the output, indicating a serious problem with the secondary.

As an aside, virtually every single one of my PSUs have heat marks somewhere on the board because most are not 80+'s... With 80+ designs, can't afford to waste power on these overshoot resistors (and startup resistors, in the case of UC3842 designs... I was staring at one of the 120/240VAC autosense input 40W UC3842 PSUs I have, that startup resistor can dissipate 1W at 240V, and that's already a 2% waste of power versus its output capacity... )

[c_hegge]

Re: Generic PSU 400W

Add the missing caps and coils on the secondary side, too. As it is, that PSU won't be able to deliver anything with the ripple in spec.

[Behemot]

Re: Generic PSU 400W

Do I smell diode treatment for +12 V?

Kill it with fire before it lays eggs!!

[UserXP]

Re: Generic PSU 400W

Hey, guys, thanks for all the replies! I will take some new puctures tomorow morning on daylight, as they will be better. I will take the PSU apart and take pictures of the other side of the PCB. That other side does not show any burn marks. The insulation of the wires located around that resistor is actually burnt and that's why it lost its original color. I guess it got pretty hot.
I might try with another resistor if I can manage to decipher its correct value by using these color stripes and find the correct replacement (I am not at all into electronics, but am good at soldering).
Also, my idea was to actually put some additional capacitors in those empty slots, but I do not know what ratings they should be. I guess they will probably be something like 10/16V, 1000uF - but that is just my wild guess.

The resistor gets so hot so fast that it actually looks like it simply shorts the "+" and "-", leading to direct overheating. I will post some more pictures tomorrow. The label on the PSU says BSP ATX Switching Power Supply Model LC-B400ATX.

[c_hegge]

Re: Generic PSU 400W

^
Oh Deer me! Looks like it was made by Deer!!

[tom66]

Re: Generic PSU 400W

Burn it! Burn it while it is switched on! It is the only way to rid it of its demons!

[Pentium4]

Re: Generic PSU 400W

Even though a Deer will burn all by itself?

[c_hegge]

Re: Generic PSU 400W

Kill it with fire before it lays eggs!!

Knowing Deer, the eggs will probably be something like the eggs from this fella.

[UserXP]

Re: Generic PSU 400W

OK, guys, here are some additional photos of this PSU. You can see its full label now. Also, you can see the whole PCB (I cleared the dust so it can be viewed better), and the problematic resistor also. I don't see any burn marks on the solder side, just on the other side. Today I also noticed that one +12V yellow wire which is closest to the burning resistor also got unsoldered, so I soldered it back again (it is the first pin down and right of the resistor, on the close-up picture).
I am a bit scared now to continue fixing this PSU because it obviously gets so hot in there that there's no way to know what else is going to overheat and unsolder. Unless there is a certain fix...

[Pentium4]

Re: Generic PSU 400W

I had the same exact L&C PSU except rebranded under "Nift"....The worst part of the thing is the amount of secondary filtering...where are all the caps?!?! Pretty sure it has a 30A schottky for 5V though, that part is worth keeping

[Behemot]

Re: Generic PSU 400W

No, this one is doomed from the very moment they even drawed it's schematics.

I suggest taking usable components from it and getting something more of a power supply than reduction from 230 V AC to low DC voltage to play with.

[mariushm]

Re: Generic PSU 400W

I know the model, it's like one of my old power supplies, only that one was LC-B450..

Yes, insufficient solder on the bottom where the cables are.
The amount of solder should be like on the orange and black cables, maybe even a bit more.

Transformer is actually ERL-33 , not ERL-35. If you desolder it completely, you'll probably find a label saying ERL-33 on its bottom. There's also the pcb holes for ERL-35 right near the heatsink.

It's good for about 150-200w like I said, could do a bit better with inductors and capacitors in the missing locations.

That resistor on the 4th picture.. almost looks like a bad solder join, but with lead free solder you never know...

[UserXP]

Re: Generic PSU 400W

Well, if the repairs bring more risks and investments than the actual gain, then usable parts will come in hand. The fan is surpisingly quiet and is still working well, even with its age. :-/

[Behemot]

Re: Generic PSU 400W

Better safe than sorry - disassemble it anyway and lubricate if needed. It's never nice when it seizes

[UserXP]

Re: Generic PSU 400W

Thanks, everyone. After all the above said, I am surprised it lasted these 6 years or so.
I can not measure the outputs because I do not have the appropriate equipement to do that, but even if it tops 250W, I will not use it. That would be just too weak and hazardous for motherboard and GPUs...

[Pentium4]

Re: Generic PSU 400W

Better safe than sorry - disassemble it anyway and lubricate if needed. It's never nice when it seizes

Yes. That's what killed the one I had, the fan seized up stiff as a board, and it wasn't quiet when it was working

Thanks, everyone. After all the above said, I am surprised it lasted these 6 years or so.
I can not measure the outputs because I do not have the appropriate equipement to do that, but even if it tops 250W, I will not use it. That would be just too weak and hazardous for motherboard and GPUs...

I doubt the ripple is in spec even at 75W!