Thanks for the tutorial. TOP223's application note pointed me to realize that the 'hidden' power rail is called the bias supply in flyback transformer's lingo, which is used to power the low voltage goodies that switches the MOSFET to pulse the primary winding so we don't have to burn off the say, excess 288Vdc, to derive 12Vdc out of a say 300Vdc rail.
Thank you very much for posting this video. It enabled me to fix a Bestec ATX0300D5WB that wouldn't start the computer until after it warmed up for a period of time. I measured the 5-V standby voltage and found that it started out low--around 2.5 V--and then very slowly increased until it reached about 4 V, when the computer started. I traced the 5-V standby wire back into the power supply to an electrolytic filter capacitor, which I replaced, although it tested OK. I then tested the supply on the bench, the standby voltage came right up, and the supply started immediately. I thought it was fixed, but when I reinstalled it in the computer, it had the same warmup delay problem. So I removed the power supply again, tested it on the bench , this time with a load resistor connected to the standby voltage, thinking that the delay might occur only under load. I found that the standby voltage did in fact droop under a load of 47 Ohms, but it was hard to believe that the motherboard would draw 100 mA from the standby voltage. At that point, I turned to your video, again traced the standby voltage line with the ohmmeter and found another large filter capacitor near the diodes that you mentioned. That capacitor, a 2200-uF, 10-V electrolytic had a bulging top. Replacing it fixed the delay problem. I tested the defective cap and it measured only about 100 uF. I then warmed it up with a heat gun and noticed that its capacitance increased with temperature. Can you explain why this happens? Thank you again.
@@LearnElectronicsRepair Hi Richard and thanks for all the infos you provide. My issue with the psu is when i start it without connecting it to the mainboard it starts, i measured all voltages and they are ok. Also the standby is 5v. Connecting it to the mainboard it wont start. The MB is functional. One more detail , when i connect another working psu to the MB some leds flash one time. When i connect this faulty one the leds on the MB flash constantly every 2 seconds. Should i open the psu and check for that hidden voltage? Any other info or advice? Thanks again.
Thanks for sharing. I have a PC that won't power on. The power button goes solid Amber and the manual says Boot failure: This is a system fault error condition, including the power supply. Only the +5VSB rail on the power supply is working correctly. I thought it was a PSU issue but am getting the same with a new PSU. Can the motherboard have issues in not being able to request the amount of power it needs?
Hi. Are you able to do a little video on how to add a -5v output rail to an ATX psu for use on AT computers? I would have swap out the connector for AT of course. Many thanks.
interesting. I have a psu that is powering the led on the m/b but won't turn on the fans and the drives etc. including the psu fans. I can start the psu through the connector, (green-black) and it then outputs correct voltages. From your video I now suspect a hidden power rail failure. I can see a bulging cap when I take the lid off, so I need to investigate a little more closely and hopefully a re-cap will fix it. I'll look for that little 35v cap whilst I'm at it. Thx
My psu has an interesting fault. It won't turn on while shorting the green wire. However if I short the standby 5v to ground it turns on. Any idea what's going on?
Hi. My cooler master silent pro gold 1200w supply gives 5+vsb only. No other voltage and its not powered on. How to solve it? I see capacitor with value of 16v 470 h28, is it the located near by +12v lineout. So is it culprit part?
Is it possible for this hidden voltage to be less then 16v? I have a non working psu (5v stb present, doesn’t start up) which has 13.5v on the second winding of the standby transformer, the caps seems good though
Can you help me, my mainboard has no 5V standby connector (HP), there are only two 12V connectors (CPU, Motherboard). I need to somehow make the 12V rail work for standby.
awsome video thank you, i have this issue on my seasonic, but the funny part is that when i power it i only get 5v sb. no 12v 5v or 3.3v but, when i slowly play with the power switch on the psu and let it make a arcing noise then the psu will all of a sudden give me the 12,5 and 3.3v. when i restart or power down and switch on again then its back to square one and only supplies the 5.5v sb. untill i play with the switch again
Really useful. Thank you. My psu is powering on but just for a few seconds and then going off. I’m getting 5v on the 5VSB and also on the PSON. When I connect PSON to ground the unit powers up but then goes off. Any ideas?
Can any significant amperage be drawn from this hidden rail? I was going to se 5Vsb to power raspberry pi that controls PC fans/pumps before and after startup/shutdown. Powersupply offers 2A, which is enough. But if it was 2A at 12V or 16V, a number of far better computers could be used (from phone/automotive world, like rk3588).
i have PSU atx with the same issue, 5 volt standby ok, but won't start, i checked the voltage it's about 11 volts, so i replaced the cap with a new one, and still got 11 volts, i measure the output Voltage of Transformer and gives 16 v AC, why did after rectifier become 11 volts?
This could be normal. Check the rails under normal load designed for this PSU. Since rails share the same transformer naturally one rail tends to get out quite a bit if not loaded.
Hello. I have some shitty unnamed PSU. I have done laboratory psu from it. I have accidentially putted 5v on its housing. Now its giving only 5V standby. What can cause this problem?
You need to diagnose the problem in the main switching supply. So you have 5V standby, you may need the 'hidden rail' which is probably about 16V-24V. I would start by finding the datasheet for the PWM controller - read the part numbers for any ICs in the PSU then look them up until you find it. From here you can start checking for problems, does the PWM controller have power, is it oscillating etc
hi, i've watching all your videos trying to fix my evga 700w power supply, its does have standby, but the voltage in green wire is 2.5v instead of 5v, when i try to power it on, it start for 1 sec, then power off, it does give voltage 3.3v 5v 12v for that second, then they start to drop. will check that capacitor, any other ideas?
See if you can trace where the green wire comes from. it should have a pull up resistor maybe a few K ohms or some similar arrangement to put 5V on that wire. Having said that the PSU does start so it probably is not causing the problem. It could be that you have some fault in the circuitry that monitors the voltage rails or current (usually a Quad Comparator like LM339) so the PSU thinks there is an overload when really there is not. For example a bad voltage reference or a low value resistor gone high could cause this sort of fault Another option would be a real overload on one of the voltage outputs
@@LearnElectronicsRepair thank you very much for your time to answer, i replace that capacitor with no luck, the same, replaced the tny279pn, same result, it does have this wt7527v PC POWER SUPPLY SUPERVISOR, not sure how to check this :(, its the closest to the green wire, also have CM6800tx
@@TheWeftmen From doing a little research this is likely to be a Zener diode or a TVS (Transient Voltage Supression) Diode. If it is a TVS it would read open circuit both ways on a multimeter. If it was a Zener then you could unsolder it and check it using a bench PSU and a series resistor for example 470R or 1K. +ve -----/\/\/\-------|
Sir thank you so much for your good idea and answer, I sent now a link T2D diode series, for example in this link T2D 33 diode, it is similar with T2D 87, thank you
Thanks for the tutorial. TOP223's application note pointed me to realize that the 'hidden' power rail is called the bias supply in flyback transformer's lingo, which is used to power the low voltage goodies that switches the MOSFET to pulse the primary winding so we don't have to burn off the say, excess 288Vdc, to derive 12Vdc out of a say 300Vdc rail.
Thank you very much for posting this video. It enabled me to fix a Bestec ATX0300D5WB that wouldn't start the computer until after it warmed up for a period of time. I measured the 5-V standby voltage and found that it started out low--around 2.5 V--and then very slowly increased until it reached about 4 V, when the computer started. I traced the 5-V standby wire back into the power supply to an electrolytic filter capacitor, which I replaced, although it tested OK. I then tested the supply on the bench, the standby voltage came right up, and the supply started immediately. I thought it was fixed, but when I reinstalled it in the computer, it had the same warmup delay problem. So I removed the power supply again, tested it on the bench , this time with a load resistor connected to the standby voltage, thinking that the delay might occur only under load. I found that the standby voltage did in fact droop under a load of 47 Ohms, but it was hard to believe that the motherboard would draw 100 mA from the standby voltage. At that point, I turned to your video, again traced the standby voltage line with the ohmmeter and found another large filter capacitor near the diodes that you mentioned. That capacitor, a 2200-uF, 10-V electrolytic had a bulging top. Replacing it fixed the delay problem. I tested the defective cap and it measured only about 100 uF. I then warmed it up with a heat gun and noticed that its capacitance increased with temperature. Can you explain why this happens? Thank you again.
Thank you for this valuable information, i just repaired a Thermaltake 730W smart se with this fault, thank you for sharing!
Just what I was looking for - cheers !
That's a valuable information thank you for sharing
Very welcome
@@LearnElectronicsRepair Hi Richard and thanks for all the infos you provide. My issue with the psu is when i start it without connecting it to the mainboard it starts, i measured all voltages and they are ok. Also the standby is 5v. Connecting it to the mainboard it wont start. The MB is functional. One more detail , when i connect another working psu to the MB some leds flash one time. When i connect this faulty one the leds on the MB flash constantly every 2 seconds. Should i open the psu and check for that hidden voltage? Any other info or advice? Thanks again.
Excellent information! Thank you.
Good to see it was helpful!
Thanks for sharing. I have a PC that won't power on. The power button goes solid Amber and the manual says
Boot failure: This is a system fault error condition, including the power supply. Only the +5VSB rail on the power supply is working correctly.
I thought it was a PSU issue but am getting the same with a new PSU. Can the motherboard have issues in not being able to request the amount of power it needs?
Very useful information. This helped me fix my power supply!
Hi. Are you able to do a little video on how to add a -5v output rail to an ATX psu for use on AT computers? I would have swap out the connector for AT of course. Many thanks.
interesting. I have a psu that is powering the led on the m/b but won't turn on the fans and the drives etc. including the psu fans. I can start the psu through the connector, (green-black) and it then outputs correct voltages. From your video I now suspect a hidden power rail failure. I can see a bulging cap when I take the lid off, so I need to investigate a little more closely and hopefully a re-cap will fix it. I'll look for that little 35v cap whilst I'm at it. Thx
Thanks for that interesting tip ! nice video.
My psu has an interesting fault. It won't turn on while shorting the green wire. However if I short the standby 5v to ground it turns on. Any idea what's going on?
Hi. My cooler master silent pro gold 1200w supply gives 5+vsb only. No other voltage and its not powered on. How to solve it? I see capacitor with value of 16v 470 h28, is it the located near by +12v lineout. So is it culprit part?
Is it possible for this hidden voltage to be less then 16v? I have a non working psu (5v stb present, doesn’t start up) which has 13.5v on the second winding of the standby transformer, the caps seems good though
Can you help me, my mainboard has no 5V standby connector (HP), there are only two 12V connectors (CPU, Motherboard). I need to somehow make the 12V rail work for standby.
Resp Sir! Thank U for Very Nice Class. Sir What Next Sir... -- ChinnaRao Paderu Andhrapradesh
8:45 - Does the output of that rail go directly to the VCC pin of the SMPS controller chip?
what about if 5vsb slightly higher? about 5.6v?
awsome video thank you, i have this issue on my seasonic, but the funny part is that when i power it i only get 5v sb. no 12v 5v or 3.3v but, when i slowly play with the power switch on the psu and let it make a arcing noise then the psu will all of a sudden give me the 12,5 and 3.3v. when i restart or power down and switch on again then its back to square one and only supplies the 5.5v sb. untill i play with the switch again
Its because of the mosfet that is in nstandby.or not enoguh amperage to start its design flaw in the process
brilliant!!!!
Really useful. Thank you. My psu is powering on but just for a few seconds and then going off. I’m getting 5v on the 5VSB and also on the PSON. When I connect PSON to ground the unit powers up but then goes off. Any ideas?
Mine does the same, I just use a hairdryer and heat it up ,then all good. I think it might be a bad cap
If adding heat makes it work then it's a bad cap.
Can any significant amperage be drawn from this hidden rail? I was going to se 5Vsb to power raspberry pi that controls PC fans/pumps before and after startup/shutdown. Powersupply offers 2A, which is enough. But if it was 2A at 12V or 16V, a number of far better computers could be used (from phone/automotive world, like rk3588).
That supply does not have much of a load on it. The only answer to your question is to try it and see what happens.
@@LearnElectronicsRepair That's the best answer I could have got! he he he! :D
i have PSU atx with the same issue, 5 volt standby ok, but won't start, i checked the voltage it's about 11 volts, so i replaced the cap with a new one, and still got 11 volts, i measure the output Voltage of Transformer and gives 16 v AC, why did after rectifier become 11 volts?
This could be normal. Check the rails under normal load designed for this PSU. Since rails share the same transformer naturally one rail tends to get out quite a bit if not loaded.
Input 16v ac and after single diode (half wave) is 16 x 0.7 = 11v dc.
Hello. I have some shitty unnamed PSU. I have done laboratory psu from it. I have accidentially putted 5v on its housing. Now its giving only 5V standby. What can cause this problem?
And when i am doing a shortcut on 5v standby and ground, it wants to start, giving normal 5v but after a milisecond it turns off
You need to diagnose the problem in the main switching supply. So you have 5V standby, you may need the 'hidden rail' which is probably about 16V-24V. I would start by finding the datasheet for the PWM controller - read the part numbers for any ICs in the PSU then look them up until you find it. From here you can start checking for problems, does the PWM controller have power, is it oscillating etc
hi, i've watching all your videos trying to fix my evga 700w power supply, its does have standby, but the voltage in green wire is 2.5v instead of 5v, when i try to power it on, it start for 1 sec, then power off, it does give voltage 3.3v 5v 12v for that second, then they start to drop. will check that capacitor, any other ideas?
See if you can trace where the green wire comes from. it should have a pull up resistor maybe a few K ohms or some similar arrangement to put 5V on that wire. Having said that the PSU does start so it probably is not causing the problem.
It could be that you have some fault in the circuitry that monitors the voltage rails or current (usually a Quad Comparator like LM339) so the PSU thinks there is an overload when really there is not. For example a bad voltage reference or a low value resistor gone high could cause this sort of fault
Another option would be a real overload on one of the voltage outputs
@@LearnElectronicsRepair thank you very much for your time to answer, i replace that capacitor with no luck, the same, replaced the tny279pn, same result, it does have this wt7527v PC POWER SUPPLY SUPERVISOR, not sure how to check this :(, its the closest to the green wire, also have CM6800tx
@@androsk Did you able to fix the psu. Mine has same problem. Start for second then off
@@MdShamim-cl5ig nope, I gave up
first to comment XD
thanks alot for sharing your knowledge
and glad to see your channel mr rechard
greets from libya
Hi Mohammed, good to see you liked it, welcome to the channel I hope you enjoy my other videos too :D
Sir, what is T2D 87 diode in standby transformer? How can I find this diode equivalent? This is normal diode or what is this I can not find it ?
This diode in generally stay near the standby transformer, this is normal diode or what I don't know
@@TheWeftmen From doing a little research this is likely to be a Zener diode or a TVS (Transient Voltage Supression) Diode. If it is a TVS it would read open circuit both ways on a multimeter. If it was a Zener then you could unsolder it and check it using a bench PSU and a series resistor for example 470R or 1K.
+ve -----/\/\/\-------|
Sir thank you so much for your good idea and answer, I sent now a link T2D diode series, for example in this link T2D 33 diode, it is similar with T2D 87, thank you
@@TheWeftmen maybe yours is an 8.7V zener. I know that is a rather strange value but.... hmmm maybe
@@LearnElectronicsRepair okay thank you
WOW
LER Takes a bow..... lol :D :D :D
Someone should design these power supplies so they have fewer components.