Dell model L275E-01 pn: Wu142. Optiplex 755 small form factor.

[PeteS in CA]

Originally Posted by stj

it's getting pretty common in europe now to bypass hot thermisters with a fet/triac or relay just to save "something" or reduce heat output.
we are under the thumb of the scum that are obsessed with carbon!!
...

While I won't disagree with your assessment of the enviros, improving efficiency of PSUs in a big server-storage farm can improve the farm's electric bill, so to speak.

Shorting out the inrush thermistor also allows use of a higher value thermistor that cannot sustain the normal continuous current: the inrush is lower in magnitude; the thermistor gets shorted out before it desolders itself or blows up. I saw the latter usage back in 1980 in which a simple wirewound panel mount resistor was used; the resistor did not survive long if the triac failed to fire, .

[Rainbow]

The 180uF/420V cap is known to fail without any visible changes in L275E and also L235P (OptiPlex 780). Desolder it and you will see leak on the bottom.

Then usually IC501 (TNY280PN) shorts, burning R600 (1R fusible resistor) and sometimes also F001 fuse (this should be same for L235P and L275E).
Sometimes, PFC burns (instead of IC501 or in addition to) - Q100 (FCP16N60), D101 (STTH8S06D or BYC8-600) (this is from L235P)

[Duranitron]

Originally Posted by Rainbow

The 180uF/420V cap is known to fail without any visible changes in L275E and also L235P (OptiPlex 780). Desolder it and you will see leak on the bottom.

Then usually IC501 (TNY280PN) shorts, burning R600 (1R fusible resistor) and sometimes also F001 fuse (this should be same for L235P and L275E).
Sometimes, PFC burns (instead of IC501 or in addition to) - Q100 (FCP16N60), D101 (STTH8S06D or BYC8-600) (this is from L235P)

Gud day sir. may i ask if you have the schematics of L235 and L275e you can share it with us. thank you so much.

[Rainbow]

I don't have any.

[Per Hansson]

Originally Posted by Duranitron

thanks for the suggestion mates! i already repaired the psu by placing a thermistor sck054 and replacing the opto isolator 817B. now the psu is working perfectly and can be turn on and off. i think the relay is for in rush current protection an old 2005 design, now a days they omitted the relay and just place the thermistor. Thank you very much for the help and sharing your expertise.

Glad you got it working, but your understanding of the NTC and Relay relationship is still wrong.
I have drawn it here in PerCad v1.0:

L------NTC-------------
    \--Relay*---/      

                      
N----------------------

* = Relay with 500ms on-time delay

This is used for power efficiency, I think it was Corsair that wrote it's almost impossible to attain the highest levels without switching the NTC out of the circuit.
(Obviously a pretty expensive design, as quality high-power relays are not cheap).

[momaka]

Originally Posted by Per Hansson

I have drawn it here in PerCad v1.0...

LOL Per, you almost got me with that one. Took me a moment to realize what you did there.