450w computer smps problem troubleshhoting

I AM FACING PROBLEM WITH A 450W COMPUTER ATX SMPS THE PROBLEM IS WHEN I START THE SMPS WITH SHORTING ITS GREEN AND BLACK WIRE IT'S COOLING FAN ROTATES FOR WHILE AND STOPS IN THAT CASE HOW TO TROUBLESHOOT THE PROBLEM ? PLEASE TELL ME STEP BY STEP TROUBLESHOOTING. I HAVE CHECKED THAT THE 5V STANDBY IS OK. THANKS IN ADVANCE..:grin:

PSU is good ?

Assuming you try to start without any connected load ?

What you consider you check 5V ? What is with 12V and 3,3V ? How you check 5V if you cant turn on ?

Before new start of turning on you must switch I/O on back side of PSU (turn off wait few sec and turn on, then try green black wires).

yes friend i checked that switch and the smps shows 5v standby power when connected to power and when i short the two wires its fan rotate for just one rotation and stops i checked it with some load on 5volt and 12 volt rail like cd-rom ..

peter_england said:

yes friend i checked that switch and the smps shows 5v standby power when connected to power and when i short the two wires its fan rotate for just one rotation and stops i checked it with some load on 5volt and 12 volt rail like cd-rom ..

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Wow, wow, slowly,

We have PC PSU (SMPS) turned off.
You say you checked PSU switch and SMPS shows some 5V standby power. What standby power of 5V before starting PSU ?

Please disconnect everything from PSU, I mean everything, turn off PSU switch, wait few sec, then turn on, and then short black and green, look fan operation. Then use unimer as voltmeter check 5V, 12V, 3,3V.

its an atx smps and it gives 5 volt standby power when connecting to power lid and not shorting its green and black wire i have removed everything from it..

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you know its standby power of 5v for motherboard and you can check it with the dmm on pci slot pin.. i dont know the pin number exactly ..

There is no standby power in PC PSU. When you put switch on PC PSU case on OFF, there should not be any voltage from PSU connectors.

If you have 5V when you turn switch off your PSU is probably badly modified by some expert.

see my friend we talking about smps that not having any physical switch and only a socket for power lid ok ? when i connect that power lid to wall socket and switch on the power from wall socket the smps then shows me 5volt dc supply on the purple wire of 20 pin connector without shorting any wire ..

Ok, ok, you want to say you dont have any switch on back side of PC PSU ? You can call SMPS also PSU nothing special about that.

This is case with some older PC PSU, that they dont have back switch. In that case you must remove power cable from PSU and reconnect it, then try black/green wires.

yes now u get my point then pls go ahead and give some tips.. also can you give me your mail address or gmail chat ??

1. Disconnect power cable from PSU.
2. Open case.
3. Inspect all electrolytic capacitors for leaking and inflating.

Dont touching anything with hands, specially capacitors.

ok i will try and reply ..

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there is no any of cap is leaking by visually..

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there is no any of cap is leaking by visually..

Can you post some sharpen photo or several photos if not problem.

Yes wait for while ..

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Hope we have not forgotten that for PC PSU starting it is necessary connect PC ON (14) to ground and PG (8) to +5V.

peter_england said:

I AM FACING PROBLEM WITH A 450W COMPUTER ATX SMPS THE PROBLEM IS WHEN I START THE SMPS WITH SHORTING ITS GREEN AND BLACK WIRE IT'S COOLING FAN ROTATES FOR WHILE AND STOPS IN THAT CASE HOW TO TROUBLESHOOT THE PROBLEM ? PLEASE TELL ME STEP BY STEP TROUBLESHOOTING. I HAVE CHECKED THAT THE 5V STANDBY IS OK. THANKS IN ADVANCE..:grin:

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Hi peter,
It is normal to have +5v at the time of starting it is used as starting pulse to turn on the smps. let us go systematic can you please tell which IC is used in your smps is it TL494 or KA7500 or IC 2003 or any other ic please give details.
regards ani

Try to replace the electrolytic capacitor.it may be in weak condition. No need to replace the big capacitor(input side filter capacitor).

picgak said:

Hi peter,
It is normal to have +5v at the time of starting it is used as starting pulse to turn on the smps. .....

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Yes, but if you turn off main default switch on back side, there is no voltage from connector. But he says he dont have it. :smile:


I dont know, pictures are not sharpened. Capacitor on first picture can cause this, but they looks good on eye, you can change it to try, I cant see good from this pictures. This capacitors can be normal, not low ESR.

ATX power supply have a small so called standby power supply, independent from main power converter; this standby power supply assure 5Vsb to keep standby status of a PC and also to provide start-up voltage to main power converter (after start-up, IC control and drivers are supplied from 12V output); this small power supply start-up imediatly ATX power supply is connected to grid and can't be stopped/started from an external switch or by mainboard (don't have any start/stop mechanisme, to avoid any errors in suppling 5V standby to PC). When push power-on, is started main power converter and in very next time a so called "Power good" circuit check that all output voltages are in correct range; if all are ok, Power-good signal is switched to +5V and inform mainboard that all voltages are ok and mainboard control circuit may initiate procedure to start-up onboard power converters for CPU, memory and so on. Also, there are in ATX power supply an overcurrent protection circuit that fast shut-down main power converter when a short-circuit conditions (high current) occurs from an external or internal fault conditions. In your case, if cooler start-up for a very short time mean that main converter try to start but there are an internal fault like overcurrent (due of abnormal driver or power transistor) or Power-good fault conditions (over or under voltage). Check IC controller type and see datasheet for details about Power-good, overcurrent protection and all related functions.

ATX power supply have a small so called standby power supply, independent from main power converter; this standby power supply assure 5Vsb to keep standby status of a PC and also to provide start-up voltage to main power converter (after start-up, IC control and drivers are supplied from 12V output); this small power supply start-up imediatly ATX power supply is connected to grid and can't be stopped/started from an external switch or by mainboard (don't have any start/stop mechanisme, to avoid any errors in suppling 5V standby to PC).

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This is not true, power switch (switch on back side of PSU) turn off totaly PSU and all voltages on all connectors, if this is not the case buy new PSU or call Fox Mulder & Skully to investigate. Usually all good and better PSU have that switch, if there is no switch :smile: what to say.

It's not mandatory to have this switch; this switch cut power from grid and obviously all outputs go to zero. This swith is to isolate power supply from grid and it's not Power-on switch according ATX specifications.