Dell 550w Power Supply Shorted Outputs

[mmartell]

This is from a Precision 470, it's been laying around here since I replaced it with another used one. I have 165v at main filter cap and 5v standby is present. All other outputs show short to ground with power off.

I'm a newbie and still struggling to understand the bigger picture with smps design and structure. Near the outputs there is a heatsink with three schottky barrier diodes and an LM7912. The schottky's measure shorted. Are they a good place to start or is there a more likely culprit making them appear shorted ?

(pics coming, phone is dead.. so at this point this is a general theory question as I can't expect specifics)

[stj]

Re: Dell 550w Power Supply Shorted Outputs

before you do anything, know 2 things.

1: the capacitors could make it look shorted on a meter.
2: there are probably low (like 10-20ohms low) value resistors linking the outputs to ground.

if you check between the outputs and ground and see 0-ohms then i'd remove and test the diodes.
if they are o.k. i'd look for bad ceramic disk capacitors.

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

Have to dig closer. Since this is a non-standard non-atx supply the connectors are of course non-standard (thanks Dell!). I'm getting a mixed bag of readings some around 40ohm some about seven ohm and some zero but now I have to trace those to the board to find what the silkscreen might tell me.

[CapLeaker]

Re: Dell 550w Power Supply Shorted Outputs

Just measure all the diodes and fets for shorts, check the caps, put a light bulb in series with the input of the power supply and force the Power supply on. If your good and didn't miss anything it will work. If there is a short, the light bulb will light up and buy you some time to turn it off, before everything goes poof! Meh!

[stj]

Re: Dell 550w Power Supply Shorted Outputs

no it wont, if the output is shorted and the psu is cheap then the primary fets will explode - i'v seen it.

[budm]

Re: Dell 550w Power Supply Shorted Outputs

Yes it will light up the lamp brightly without damaging the power supply. Been there, done that for 40 Years.

[stj]

Re: Dell 550w Power Supply Shorted Outputs

too bad you didnt see the one i had last month - the lamp would have changed nothing.
it had no OCP circuit, a shorted output diode made the primary fet explode!

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

Ok stj in light of what you said earlier I went back and checked the connector pins against ground and some are in the 7 ohm range. Now I'm not sure what I've got and may again have thread titled erroneously.

I've jumped the pson pin to ground and still have about 160v across the main filter cap though I can't be sure I'm even doing that right as this is not a standard atx supply.

I think I need to start from the beginning here. And where would that be ?

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

Even I know that. PICTURES

[Khron]

Re: Dell 550w Power Supply Shorted Outputs

Does the pinout here happen to match this psu?

http://pinouts.ru/Power/dell_atxpower_pinout.shtml

Single primary cap, plus this being in a pro workstation, would be a safe bet that it has active PFC. Since you still have "only" 160v on that cap, it's quite clear that the PFC isn't starting. Seems that L4981A is the PFC controller.

If i'm not much mistaken, the PS_ON pin is the only one, apart from the 5Vsb that has any voltage on it.

Personally, i'm quite suspicious of all the small caps on the primary. In many cases, those turn out to be the culprits for non-starting SMPS's. What brands / series are they?

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

Yes the PFC isn't starting but like I said I may not be doing it right. Are you saying pson should have 5v on it and if so do you ground it through a resistor to start it ?

The pin configuration in your link is for 20pin. I have been focusing on the 24pin connector so not sure if that is correct or not.

[selldoor]

Re: Dell 550w Power Supply Shorted Outputs

Yes the PFC isn't starting but like I said I may not be doing it right. Are you saying pson should have 5v on it and if so do you ground it through a resistor to start it ?

The pin configuration in your link is for 20pin. I have been focusing on the 24pin connector so not sure if that is correct or not.

You need to try and find the pinout for both sockets 20 and 24 either searching for dell 470 or dell 550w.

Cant recall if you said - why are you doing this?- just for fun/practice

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

This was replaced in a pc I have by a working one and this was just lying around so nothing on the bench atm so wanting to repair it and possibly sell it.

I have the pson pin, just not sure if grounding it is enough to turn it on.

@Khron yes the controller is the L4981A. Most of the small caps in the primary are Teapo.

Damn I'll be pissed if I have shitty start up caps, that's the first thing I should have checked. I did apply heat to the startup section and still couldn't turn on the secondary.

[stj]

Re: Dell 550w Power Supply Shorted Outputs

small caps are an esential starting point, i replace them even if they work - just to make it last longer.

it's not worth opening something twice just so you could save maybe 20c.

[stj]

Re: Dell 550w Power Supply Shorted Outputs

btw, are the voltages marked on the board where the output wires are soldered?

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

btw, are the voltages marked on the board where the output wires are soldered?

Yes they are I just need to separate the mess of wires to see them.

[stj]

Re: Dell 550w Power Supply Shorted Outputs

that's one issue sorted then - make a note of the plug pinouts for future use.
maybe submit them to pinouts.ru

[selldoor]

Re: Dell 550w Power Supply Shorted Outputs

I have the pson pin, just not sure if grounding it is enough to turn it on.

When power is supplied to the psu the pson pin (at least on a normal atx psu)
has 5v. To turn on the psu this voltage has to be pulled low (less than 3v, I think) To start an atx it is usual to just place a paperclip in the socket connecting PSON AND GND (GREEN AND BLACK)
So yes just ground the pson to gnd. No resistor required,

you leave the jumper in position then test the other pins on the socket with meter on 20vdc neg lead to ground probe each pin with red probe.


L4981A datasheet

http://www.google.co.uk/url?url=http...KVd4Aq-2I16AEg

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

Done exactly that - no secondary voltages and no PFC boost.

[mmartell]

Re: Dell 550w Power Supply Shorted Outputs

Personally, i'm quite suspicious of all the small caps on the primary. In many cases, those turn out to be the culprits for non-starting SMPS's. What brands / series are they?

BINGO!

I seem to have a problem where I get an idea in my head of what the issue is and continue on a tangent wasting time. I need to keep an open mind.

Two caps in what I presume to be the start up section were bad. A 10uf 50v and a 220uf 35v, both Teapo. The larger one was not domed.

Item has been repaired and tested for power with a DVD burner and a meter on the ouputs. Thanks everyone.