Dell Power Supply NPS-250KB B

[mockingbird]

Re: Dell Power Supply NPS-250KB B

There's a DNA1002E on daughterboard all the way to the edge of of the secondary side. Is this what you're refering to? The little daughterboard near the primary caps what looks like a SOIC-8 SMT IC. There is a single 16V 1500uF Nichicon PW with the aforemtnioned 470uF Taicon in the 12V area, and two Rubycon 6.3V 2200uF caps right next to them. There is a single blue wire connected further down the PCB somewhat near 3 Rubycon caps. One 25V 470uF and two more 6.3V 2200uF caps...

As far as I can tell, those are the only three 16V caps in this PSU so I assumed the Taicon 16V 470uF and the Nichicon 16V 1500 compose the single +12V pi filter... Maybe the -12V rail only has that single 25V 470uF 8mm Rubycon YXG as a smoothing cap.

[Wester547]

Re: Dell Power Supply NPS-250KB B

I believe those PSUs use a Delta-made chip known as DNA1001DL or DNA1001D, which I believe combines a chip that powers +5VSB and PWM (like UC3843) into one. 470uF, 16V sounds very small for the +12V rail. It could have been on the -12V rail. I believe the +12V rail uses two 1500uF, 16V capacitors in those PSUs. I have not been able to locate the PI filter for the +12V rail for whats it's worth, but I do see a silver shunt on that rail, though I don't think resistors pass ripple and that it's probably for OCP (but I have seen a bronze shunt in some Hipro PSU's 12V rails in place of a PI filter as well, so...).

[mockingbird]

Re: Dell Power Supply NPS-250KB B

I have my NPS-250KB B (Date Code 0218) out of the chassis sitting on my lap right now. I inititally planned to re-cap all the Taicon in it eventhough it has not even been tested, but changed my mind and instead relegated it to the pile of 20-Pin PSUs I will probably never use.

All the secondary filtering caps are Rubycon YXG and Nichicon PW, save for one Taicon PW 16V 470uF cap in the 12V pi filter. The 5VSB pi filter is composed (If I understand correctly) of the Rubycon 10V 2200uF, the small winding, and a Nichicon 10V 470uF cap. Check if these are also Japanese on your particular unit.

Regarding the startup caps, there are only a total of 4 5mm electrolytics in the primary section. One of which is 50V 1uF, one 50V 100uF, one 25V 47uF, and one 50V 0.47uF. All are Taicon "VT" series. I can't say with utmost certainty that the second 5mm cap is 50V 100uF because eventhough I can make out the voltage on the cap just fine, the capacitance is covered with the selastic and the bottom part of the sleeve ripped off witht he selastic as I bended it to get a good look so I can only make out the 1 and 0 on the top. I *think* I can just barely see a seond zero on the top part of the sleeve though, so coupled with your suggestion I'm going to conclude it is 100uF.

Also, I desoldered the daughterboard in the primary section hoping to find a standby chip for the 5VSB. As far as I can remember, there was no standby chip on it.

[Wester547]

Re: Dell Power Supply NPS-250KB B

I gave that a try and it worked just fine. I guess that means it had a chance to power down normally and do whatever it is it needs to do to prepare for the next power up. At this point my only guess is that during a power failure, the standby +5V part of the power supply, which should always be on when AC is applied, is, for some reason, not turning back on when AC is reapplied. I believe it's the +5V standby that powers the PC ON/OFF power button, so without that the PC has no idea the power button was pressed.

The NPS-250KBs I've seen have 4 optocouplers. I believe one of them is a crowbar protection circuit of sorts that precludes the user from simply restarting the computer right away if something causes the power supply to shut down. Of course, this might only apply if +5VSB is still being supplied by the power supply (which it wouldn't be in the case of a power outage), but I wonder if that fourth optocoupler is configured such that it prevents you from turning on the PSU for a while in the case of a power loss... but replacing the startup capacitor - possibly an 100uF, 25V Ltec TK or maybe Taicon VT - and the +5VSB capacitors - either Rubycon YXG, Chemi-con KY, Nichicon (possibly Taicon as well) PW, or Ltec LZG, may help too - there should be a 2200uF, 10V one and a 470uF, 10V one and a PI filter coil in between. I don't think you'd have to replace them if the brands are Rubycon, Nichicon, or Chemi-con. I listed the above mentioned brands and series because they were typical choices for Newton/Delta at the time (if it has a 2002 date code - 2003 and later date codes seemed to have mostly Ltec capacitors).

As for +5VSB capacitors being the first to fail, it doesn't help that there's no air movement and it's usually a linear regulated flyback topology used (along with only a single diode) - very inefficient (not to mention some two transitor +5VSB circuits).

[JayArr_BC]

Re: Dell Power Supply NPS-250KB B

Correct, and it's the circuit that logs the most "on" time since it produces that 5VSB whenever plugged in so it has the electrolytic caps that fail first, Just replace all of the small caps around the small standby circuit and see if it solves.

[Pilm2013]

Re: Dell Power Supply NPS-250KB B

What happens if you remove AC power after powering it down normally, through Windows (unplugging it from the wall after that)? Does it turn on immediately after giving it AC power again (plugging it back in) and pressing the power button, after a few minutes, or does it exhibit the same behavior as it would in the duration of a power outage (not coming back on for a long time)?

I gave that a try and it worked just fine. I guess that means it had a chance to power down normally and do whatever it is it needs to do to prepare for the next power up. At this point my only guess is that during a power failure, the standby +5V part of the power supply, which should always be on when AC is applied, is, for some reason, not turning back on when AC is reapplied. I believe it's the +5V standby that powers the PC ON/OFF power button, so without that the PC has no idea the power button was pressed.

[Wester547]

Re: Dell Power Supply NPS-250KB B

What happens if you remove AC power after powering it down normally, through Windows (unplugging it from the wall after that)? Does it turn on immediately after giving it AC power again (plugging it back in) and pressing the power button, after a few minutes, or does it exhibit the same behavior as it would in the duration of a power outage (not coming back on for a long time)?

[ben7]

Re: Dell Power Supply NPS-250KB B

Might have to do with the PG (Power Good) sensing.

Not sure what else to look at...

[Pilm2013]

Re: Dell Power Supply NPS-250KB B

Thanks ben7. If I turn off my PC normally (through Windows), it always turns back on without a problem, however twice I've had the power fail momentarily and when I went to turn it back on it did nothing, even after sitting for 10 minutes or so. So I suspect when I do a normal power down the power supply has some sequence of events it follows, perhaps discharging or powering down in some fashion, that isn't getting done when I suddenly lose power. So I guess the issue here is it works fine in normal operation, but has trouble rebooting after a power failure, and my guess is there may be a design issue with the PSU that Dell didn't account for sudden loss of power hanging the thing up. The main reason I suspect some kind of improper discharging is because if I leave the thing sit overnight after losing power, it'll start right up the next morning and work as expected.

[ben7]

Re: Dell Power Supply NPS-250KB B

Is it also hard to turn on after you unplug it and let it cool down?

Most likely a small electrolytic capacitor has failed.

[Pilm2013]

For the above mentioned power supply, I've had issues where the power supply will not restart after a power outage. Both times I removed the power supply and examined it looking for any damaged parts like a blown fuse, or caps, etc, but found nothing. Then did some live probing, had AC where expected and rectified high voltage (doubled) DC in some places, but w/o a schematic that's about all I could do, nothing thorough. Anyway, despite having some AC and the high voltage DC, it had none of the lower voltages (+12, +/-5, etc). So I'm wondering if there's something that isn't getting RESET properly after an unexpected loss of power that's inhibiting the lower voltages. I see there's a PS_ON signal, I think it comes from the motherboard, to put the PS into Standby mode, wondering if that might not reset to 0V after power failure? Or maybe something in the protection circuit isn't resetting? Anyway, what I've found is if I leave the unit unplugged overnight, it'll restart in the morning, at least it has the 2 times this has happened. Kind of seems like something on the board is still energized and inhibiting turn on. I did by the way discharged the 2 input filter caps, but perhaps there's more stored energy elsewhere that's keeping the thing from immediately restarting after a power failure. Any ideas appreciated, thanks.