Delta DPS-250AB-24A Unstable voltages when loaded

[cpt.charlie]

Hi, i have a PSU Delta DPS-250AB-24A that belongs to a compact computer and due to it's pinout and size i cannot use a generic ATX PSU, the problem is that the computer won't start at all, just a brief flash of the power on led.

After removing the PSU if i short the green with black the psu starts normally and all voltages are fine, however if i connect a 3.3 ohm/10watt resistor to the 5V line, the voltage jumps to 5.64V, if i connect the same resistor at the 12V rail, the voltage drops to 10.5V, after removing the PSU case i found that secondary caps looked fine and ESR/capacitance were within spec, i haven't removed the primary filter cap, but looks OK, in fact almost all caps in secondary are good quallity caps (nichicon, rubycon nippon-chemi-con) solder joints seem OK and pcb is clean, can anyone suggest what could be the cause on this strange behavior?

Thanks in advance.

[momaka]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

I think this could also be a motherboard issue. The symptom you see PC lights flash and then turn OFF is typical for a motherboard that has a shorted MOSFET or ceramic cap.

Therefore, first test the computer with another power supply. I understand this power supply is a special form factor, but if it uses a standard ATX pinout, you can use any other PSU at least to test the computer. If it works, then you know the problem is with this power supply. And if not, then we know it's probably a fault on the motherboard.

Let us know what you find in your testing.

[Th3_uN1Qu3]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

What you are seeing is normal behavior for a group regulated power supply, you have to load ALL rails at once like a computer would.

I agree that there is likely something wrong with the mainboard.

[cpt.charlie]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

I have checked the computer with a stardard atx psu and despite it needs some recapping it works just fine, the problem seems to be located on the PSU that for some reason it's unable to regulate properly when loaded, tomorrow i'll try to check the output rectifiers outside of the circuit as i'm getting weird readings when checked in circuit (like conducting in both ways but not shorted, maybe leaky? but i don't think so because the 3 rectifiers show the same behavior).

momaka: you are right, the PSU has a special form factor and a slightly different pinout but it comes with an adapter to a standard pinout.

Th3_uN1Qu3: I thought that computer PSU's had a single feedback circuit connected to +5v on old PSU's and +12V on modern units.

[cpt.charlie]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

After checking the rectifiers out of the circuit they seem to be OK, no leaky diodes.

Brigde rectifier OK
Mosfets OK
Main capacitor OK
Output rectifiers OK

Looks like it's a problem with regulation circuit, any thoughts?
Power supply supervision IC is a DWA103N (maybe an equivalent of TPS3513)
Control IC is a UC3845B


Thanks in advance.

[stj]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

i would replace all the small caps with something like panasonic FR.

[cpt.charlie]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

The small ones on the control board? I couldn't check the ESR but capacitance was OK and they hadn't sings of damage.

3 X 10uF 25V Ltec branded
1 X 100uf 25V ltec branded

[stj]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

all small ones you can find - they have the lowest lifespan.
usually rated for 1000hours.

[Th3_uN1Qu3]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

Check power good signal (gray wire). If it comes up too late, mainboard will not start. It is a good idea to replace all small capacitors near supervisor chip.

[momaka]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

The small ones on the control board? I couldn't check the ESR but capacitance was OK and they hadn't sings of damage.

And quite often, that is the problem with them: they show okay capacitance, but ESR can be through the roof. No signs of failure, either. I learned this the hard way after banging my head a few times with power supplies that I couldn't figure out why they were not working.

So it's worth a shot replacing those caps. Especially around the supervisory IC, like uN1Qu3 suggested. This looks like a well-designed Delta Electronics PSU, so definitely worth trying to fix it.

[cpt.charlie]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

I take my hat off to you, that was exactly the cause of the problem, the 10uF caps had high ESR, now the PSU is working perfectly.

[PeteS in CA]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

Lelonian crap, what a non-shock.

[Wester547]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

Actually, those are Luminous Town Electric. TH is a LTEC series, as is LZG. A 10uF capacitor reading 0.94uF is certainly a goner. Good job on the repair.

[momaka]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

I take my hat off to you, that was exactly the cause of the problem, the 10uF caps had high ESR, now the PSU is working perfectly.

Good to hear that!

To be honest though, I too am a bit surprised that the PSU stopped working because of that cap. While 6 Ohms is considered high/bad ESR, I don't think it's that bad for a small 5x11 capacitor. I have many legit 16V, 100 uF, Rubycon YXA and Chemicon KMG that typically read 1-2 Ohms ESR on my meter.

But then again, it is possible that the PSU wasn't designed to take such ESR deviation for that cap spot.

A 10uF capacitor reading 0.94uF is certainly a goner. Good job on the repair.

I see 9400 nF displayed on the screen of the meter above. 1000 nF = 1 uF. So that cap is reading in spec on the capacitance at 9.4 uF... unless *I* missed something here?

[Wester547]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

Whoops, I missed a 0. You are right, they read 9.4uF. I thought that was strange that it read so low for capacitance but okay for ESR. Maybe it's just me, but those 10uF caps look like they're shorter than 11mm... possibly 9mm long? That LZG looks mislabeled too (110˚C rather than 105˚C, although 110˚C is technically correct for the core temperature as the maximum permitted ripple current at the maximum rated ambient temperature will cause a 5˚C rise in core temperature).

[CapLeaker]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

Good to hear that!

To be honest though, I too am a bit surprised that the PSU stopped working because of that cap. While 6 Ohms is considered high/bad ESR, I don't think it's that bad for a small 5x11 capacitor. I have many legit 16V, 100 uF, Rubycon YXA and Chemicon KMG that typically read 1-2 Ohms ESR on my meter.

But then again, it is possible that the PSU wasn't designed to take such ESR deviation for that cap spot.



I see 9400 nF displayed on the screen of the meter above. 1000 nF = 1 uF. So that cap is reading in spec on the capacitance at 9.4 uF... unless *I* missed something here?

x2
However, the ESR would be lower if the cap was measured, while it still was warm. We also don't know what the other caps measured.
In this case if he had warmed up the PSU first place with a hair dryer before using it, it should have worked also.

[cpt.charlie]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

The 10uF caps had an esr of about 6-7 ohm at ambient temperature.

[PeteS in CA]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

Actually, those are Luminous Town Electric. TH is a LTEC series, as is LZG. A 10uF capacitor reading 0.94uF is certainly a goner. Good job on the repair.

Mea goofa. The Ltec logo looks like the Lelon logo.

[momaka]

Re: Delta DPS-250AB-24A Unstable voltages when loaded

Mea goofa. The Ltec logo looks like the Lelon logo.

Yeah, I suspect that they are made by the same company, even though I think I've read otherwise.