Dell PS-5251-2DF2 missing -12V line

[everell]

Re: Dell PS-5251-2DF2 missing -12V line

Here is a data sheet for the A916. Since this is a trouble maker for several power supplies I fixed, I ordered a handful from Digikey.

[momaka]

Re: Dell PS-5251-2DF2 missing -12V line

And I am still curious, the coils from that toroid must present some low resistance, right ? Is it possible, that the same thing that made the mosfet to shortcircuit could somehow have damaged that ?

If the wire for the -12v was very thin, it's possible.
In any case, check the -12v rectifier. Based on everell's pictures, it looks like the -12v rectifier is right behind the big inductor, close to the small ceramic capacitor C206.

Also, all in the name of science and research, provided that I be careful, rewinding that with the same wire gauge and same number of turns of every coil, should probably work ?

Yes.
Although if the 5v and the 12v rails are working fine, then the wires on the toroid for the 5v and 12v rails are fine too and you don't need to rewind them.
If you can't find which other wire connects to the -12v, then just trace out the 5v and 12v rails and see which sets of wires they use. Then see which wires they don't use, and that should be the -12v rail. As for the 3.3v rail, it uses that other smaller toroid.

Over the past few years, I have fixed at least three of these Dell power supplies. In every case the A916 transistor was either shorted or leaky. I have never been able to figure out just how that circuit works, but it is tied to the -12 volt line, and thru other components also ties to the +12 volt line and the 3.3 volt regulator. So that transistor can cause some strange problems.

Sounds complex. Then again, this is a Lite-ON power supply .

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Hi LongBow,
A friend says they have a bunch of these, with various defects , in a backroom at his work. He promised to try to get some for me. But when they come, I would like to already have a good understanding of this one... Currently asking for some friends that deal with used computer gear, and the like, but they seem to just have those very small Dell supplies.

After getting one that works ok, will try to make a drawing with important voltages, to at least contribute something useful for the forum.

Thanks.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Hi Everell,

Thanks, will search for that or an equivalent ( our electronic stores seem to store just the more common parts ) .Will take some photos in Monday, and also try to draw part of the circuit.

And I am still curious, the coils from that toroid must present some low resistance, right ? Is it possible, that the same thing that made the mosfet to shortcircuit could somehow have damaged that ? Also, all in the name of science and research, provided that I be careful, rewinding that with the same wire gauge and same number of turns of every coil, should probably work ?

[Longbow]

Re: Dell PS-5251-2DF2 missing -12V line

Good find on that leaky transistor. I believe what we're looking at is a circuit that makes the +12v track with the -12v. Interesting that they had to do it with discreet parts in a messy circuit. Since all the other voltages are good, you know that the main power supply functions are all fine. If that PNP transistor shorts, then finding it is easy. If leaking, it might show some strange numbers when doing the normal diode tests with the part desoldered.

Since this circuit is a known trouble maker, please take a voltage reading on the IC terminal from a known working supply and put it in the stickies.

[everell]

Re: Dell PS-5251-2DF2 missing -12V line

Over the past few years, I have fixed at least three of these Dell power supplies. In every case the A916 transistor was either shorted or leaky. I have never been able to figure out just how that circuit works, but it is tied to the -12 volt line, and thru other components also ties to the +12 volt line and the 3.3 volt regulator. So that transistor can cause some strange problems. I would suggest that you remove and test the A916 transistor, or just replace it with a new one. Save a few headaches later.

That transistor is located on the small pc board soldered to the main pc board.

Also a couple more pictures.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Hi,

I will try to find a place with good ilumination to take the pictures. The camera I have is a little crappy in the macro photos.

I need to draw the my schematic in a presentable form, but the -12V line goes to the pin Everell mentioned in the supervisory board, and to a 7912 regulator. I removed that and it tested good ( feeding with a -16V supply, gave -9v in its output pin ) .

The input pin of the 7912 goes to a leg of the toroid. Problem is that no other pin of the toroid gives continuity to it. I have tried to identify the other coils, as they have different color / wire diameter, and they seem to be ok ( at least have continuity between at least two ). Just the -12V wire seem to not give continuity anywhere. I will remove it from the board to be right.

Also, removeing the 7912 from the board and measuring at its input pin, gives just 0.5V. I believe it should be something like -15 or -18V, right ?

Thanks.

[momaka]

Re: Dell PS-5251-2DF2 missing -12V line

Tomorrow I will probably remove the big toroidal inductor, and test continuity in it. I understand I need to read more about the theory of ATX psus, but would you mind explaining if that toroid is critical to working, or if it can be substituted with one take from other model of psu ?

Yes, that toroid IS absolutely critical to the function of this power supply. Do NOT remove it.
I don't encourage you to replace it with a different one, either, unless of course, the original one is burned.

While I haven't worked with this particular PSU, I would suggest you check the rectifiers for the -12v rail. To get to them, start at the input of the 7912 regulator and go back towards the toroidal coil, then make note of the wire that's connected to it. Next, use a multimeter on continuity check and probe the other wires on the toroidal coil until you find the other end of the wire (when you find the two wire sets for the -12v rail, your multimeter will show a very low resistance/short circuit). After that, you should be able to find the -12v rectifiers. Most likely they will be regular axial diodes. Checking them in circuit is a bit tricky, so I suggest to remove them and test them out of circuit with your multimeter on diode/continuity test function.

Feel free to post some pictures too, if you like.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Thanks, that photo is exactly my psu. Had a lot of dirt inside, and the fan was stuck also.

Tomorrow I will probably remove the big toroidal inductor, and test continuity in it. I understand I need to read more about the theory of ATX psus, but would you mind explaining if that toroid is critical to working, or if it can be substituted with one take from other model of psu ? If necessary, I can rebuild it, just would like to learn and understand the whys and hows.

And just for testing, would it be very bad if I somehow disconnected the -12v from it, and conected them with a piece of wire for testing the rest of the -12V ?

Also, would you mind elaborating a little about the importance of the supervisory circuit ? Or directing me to some reading about its functions...

BTW, I tried to update the user profile, but the board would say I was not allowed to access the administrative interface. For the sake of politeness, as I think I should have made my introduction, I´m currently from Brazil .

Thanks for your help.

[everell]

Re: Dell PS-5251-2DF2 missing -12V line

I worked on a dell same model a while back. Seems like I remember it having problems with the 3.3 volt line. Turned out to be a bad A916 transistor. I drew out the supervisory circuit to try to understand what was going on. I will include it and a picture of the power supply.

[rogfanther]

Hello,

Good morning to all. Been frequenting here mostly for motherboard recapping, since those cheap chinese ones started developing this "plague".


I´ve started messing around with power supplies, trying to learn how they work and how to fix them.

Got this Dell one, that would not start. The fuse was burnt, power MOSFET shorted, and two caps bulged ( Ltec ones, 470 x 10 and 100 x 35 ) . After replacing them ( the original MOSFET was rated at 900V 9A, I´m testing with a 500V - 14A one, while the compatible is on its way ) , most of the lines work ok , with exception of the -12V one.

5VSB gives 4.9-5.1V, Power Good ok, 5V, 3.3V and 12V all are at the right voltage or near enough ( like, 4.9 or 11.8 ) .

The board uses a 7912 regulator for the -12V ( removed it, tested good ) . Measuring at the board, there is 0.5V at its input pin. Following this pin circuit, both caps in it tested good . Will replace them as well, just in case. The circuit goes to one big toroidal inductor, with the other voltages as well, but I could not find the other pin of what I suppose is a -12V coil.

Trying to understand, this toroid mixes all the voltages to somehow equalize them, or remove intereference ?

Is it normal/possible for this to fail, or should I search for other failed componentes, even in far places of the board, that could be causing this ?

Or, maybe, could Dell have made a power supply that will not put out -12V when not connected to a motherboard ?

Thanks for the help.