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ATX PSU 3.3V rail too low - dead dual diode?

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Pfriemler:

--- Quote from: wraper on March 25, 2023, 10:37:29 pm ---Check large electrolytic caps on the primary side (do not forget to discharge them first). 100Hz ripple on the output can mean they are dead. Also measure recified mains voltage too.

--- End quote ---

That's what I mentioned 6 hours earlier - and I was right, as we see now...  :D


--- Quote from: wraper on March 25, 2023, 04:52:43 pm ---
--- Quote from: Pfriemler on March 25, 2023, 04:01:44 pm ---4R7 as a permanent load on the 3.3 V rail? Never.

--- End quote ---
Why not? ...

--- End quote ---

700 mA rsp. 2,3 watts dumb load under all conditions? It's a waste of energy, if not a poor design. A whole PC's do not need more than 8 watts with an idle desktop. But you are right - nobody cared about efficency for a long time.

wraper:

--- Quote from: Pfriemler on March 26, 2023, 08:03:22 pm ---700 mA rsp. 2,3 watts dumb load under all conditions? It's a waste of energy, if not a poor design. A whole PC's do not need more than 8 watts with an idle desktop. But you are right - nobody cared about efficency for a long time.

--- End quote ---
If you'd opened some older ATX PSUs, it would be hard to find one without these resistors. Yes wasted power, but it's better than PSU outputting wrong voltage with no sufficient load present on all of the rails and destroying something.

RoGeorge:

--- Quote from: Pfriemler on March 26, 2023, 08:03:22 pm ---
--- Quote from: wraper on March 25, 2023, 04:52:43 pm ---
--- Quote from: Pfriemler on March 25, 2023, 04:01:44 pm ---4R7 as a permanent load on the 3.3 V rail? Never.

--- End quote ---
Why not? ...

--- End quote ---

700 mA rsp. 2,3 watts dumb load under all conditions? It's a waste of energy, if not a poor design. A whole PC's do not need more than 8 watts with an idle desktop. But you are right - nobody cared about efficency for a long time.

--- End quote ---

That resistor must be there for the magnetic amplifier to work properly.  That is a design to regulate very high currents with a simple magnetic core, no switching transistors.  It's a very clever way to control big currents with minimal effort:

Magnetic Amplifier Control for Simple, Low Cost, Secondary Regulation by Bob Mammao
https://www.ti.com/lit/ml/slup129/slup129.pdf

The 3.3V rail can drive 10-20A.  If the post-stabilisation were to be made with switching semiconductors, switching 10-20A with semiconductors would probably dissipate more than those 2.3W needed by the mag-amp's 4.7 ohm resistor.

Now if the whole PC needs only 8W, that's more like a Raspberry Pi, maybe a laptop.  A wall adapter would be enough.  High end desktops or servers can easily draw 200-500W at an average load, and about 50-100W when idle.

Pfriemler:
Lesson learned, thx to @RoGeorge and @wraper.

HwAoRrDk:
It's fixed! :D It was indeed just the primary side capacitors. Fitted new ones and now the output is perfect and noise free all round - with or without external load.

Thanks all for the help. :-+

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