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

[rogfanther]

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

No glue involved, just that little bit in the supervisory board, that has already been removed. And the toroid was bad, because there was not continuity between the -12 coil pins, and there was some continuity between one of these pins and the other coils.

I feel it is strange that the 3.3V was altered after testing it without its filtering circuit. Going to remove and replace the caps around that again

[momaka]

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

I doubt the caps would change anything.

It may also be possible that when the -12v coil overheated, it ruined the toroid. As far as I know, those toroids do change properties when they are overheated.
The only other thing I can think of is there is still a bad component somewhere.

If bad comes to worse, you could always try my earlier idea and simplify the whole circuit by disabling all of the complex circuits I mentioned. But that's probably best left as a last resort/fun experiment.

[rogfanther]

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

Hi,
I will confirm the schematic of the circuit, because it seems to have less components than that from everell. At least, it seems not to have the 2222 transistors... Will put the capacitors back today. One interesting thing : without the -12V, the fan doesn´t turn.

[Longbow]

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

Not used as a transformer.
It's coupled inductors. The coupling allows the field in each rail to oppose current change in all the rails.

Thanks. This type of filtering isn't found in home entertainment equipment (my area of experience). Apparently, current draw from the various output lines in a computer remains relatively constant, and thus the cross-coupling can be effective. Whereas in an audio amp or a TV set, current demands are constantly changing. This type of transformer reminds me of trifilar wound RF coupling devices used for broadbanding in transmitter circuits. The science behind it escapes me.

I might add that knowing what this transformer does, and that the windings are most likely in series with the output lines of the supply that it is even less likely to fail than a switching transformer. One reason I say this is because the voltage drop across any particular winding is nearly zero, and the voltage between any 2 adjacent windings is no more than 24 volts maximum. Of course, something next to this part could melt down and physically damage or burn the windings. All sorts of possibilities.

[momaka]

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

Apparently, current draw from the various output lines in a computer remains relatively constant, and thus the cross-coupling can be effective.

Quite the opposite, actually. The current draw from a computer varies quite a bit, especially on whichever rail the CPU is using for power. Back in the day it was the 5v rail (for AMD Athlon XP CPUs and Pentium 3) and now it's almost entirely the 12v rail. It's not unncommon at all to get output demand swings of 50W and more just from the CPU. With more power-hungry CPU and graphics card, you could get a lot more than that.

This type of transformer reminds me of trifilar wound RF coupling devices used for broadbanding in transmitter circuits. The science behind it escapes me.

See post #40 by PCBONEZ.

I might add that knowing what this transformer does, and that the windings are most likely in series with the output lines of the supply that it is even less likely to fail than a switching transformer. One reason I say this is because the voltage drop across any particular winding is nearly zero, and the voltage between any 2 adjacent windings is no more than 24 volts maximum. Of course, something next to this part could melt down and physically damage or burn the windings. All sorts of possibilities.

Actually, this inductor/toroid is a lot more likely to fail than the main transformer. Why? - I don't know, but I've seen many more burned toroids than transformers. In fact, I've never seen a main transformer go bad in a computer PSU, even after catastrophic failures.

I will confirm the schematic of the circuit, because it seems to have less components than that from everell.

Good idea. If you can draw out a diagram of the circuit, even better!

One interesting thing : without the -12V, the fan doesn´t turn.

Sounds like the CPU fan is ran from both the 12v and -12v rails. If I remember correctly, my Macron MPT-301 had a similar circuit like that - it was using 12v and -5v rails for the fan.

[Longbow]

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

Thanks for the insights, momaka. Now you've got me very interested in seeing a few computer psu schematics so I can see how these coils are connected.

[momaka]

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

Thanks for the insights, momaka. Now you've got me very interested in seeing a few computer psu schematics so I can see how these coils are connected.

If you look for JNC ATX on Google Images, you will find quite a few.

Here's some that I found (Note: All of these use the older half-bridge design and cheap group regulation. Particularly, because of the group regulation, you will see that the toroid we talked about is represented as coupled coils on the schematic.):

http://radiom.newmail.ru/atx/JNC_Y-B200-ATX.gif
These first two are pretty similar in terms of how the 5vsb is derived.
The next one is an even older design, with a 7805 regulator for the 5vsb.
http://radiom.newmail.ru/atx/JNC%20200-ATX.gif

I think the best way to understand how one of these works is to grab a cheap junk PSU, take it apart, and follow the traces with your hand.

[pfrcom]

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.

Is that located at Q251, next to (under) IC511 ?

[benyosua]

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

hello! Good afternoon. Thank you for allowing me to enter this wonderful forum. Can some body tell me the part number of IC which control the stand by transformer in a dell ps-5251-2ds atx power supply, please? I need to repair it, but this IC is complete damaged, I can not see the reference part number about it in order to by it. Thanks a lot.

[momaka]

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

HI benyosua, welcome to the forums!

I have the same PSU, I think. Will try to dig it up and open it tomorrow, then let you know. Hope the delay in response wasn't an issue.

[momaka]

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

Standby (5VSB) IC part number: TNY267pn