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Old 01-30-2021, 11:12 AM   #1
falcontech
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Default Dual output power supply 5 and 12 volts (no output)

Hello everyone
I have a problem with this dual output power supply, I have nothing at the output, I did the classic tests and replaced some components such as the mosfet that was shorted, a couple of resistors burnt following the short of the mosfet, the integrated td3843 which in turn it gave me a short between the ground and the output (the IC is under the big capacitor) I replaced the capacitors on the secondary for safety, the big capacitor looks good by testing it, when I power the big capacitor I have 320 volts , I got stuck because I can't understand what the problem is, a hand would be appreciated thanks
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Old 01-30-2021, 11:21 AM   #2
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Default Re: Dual output power supply 5 and 12 volts (no output)

In your pictures there is a missing VCC capacitor in the primary. Did you replace it? There should be a resistor connected between the source of the mosfet and hot ground, then there should also be a resistor betwwen the source to the CS pin of the ic, check if it is open. You could also have a damaged optocoupler

Last edited by R_J; 01-30-2021 at 11:35 AM..
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Old 01-30-2021, 12:08 PM   #3
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Default Re: Dual output power supply 5 and 12 volts (no output)

is this crap for an external harddrive case?
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Old 01-30-2021, 12:51 PM   #4
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Default Re: Doppio alimentatore in uscita da 5 e 12 volt (nessuna uscita)

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is this crap for an external harddrive case?
yes you got it right
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Old 01-30-2021, 12:55 PM   #5
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Default Re: Doppio alimentatore in uscita da 5 e 12 volt (nessuna uscita)

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Originally Posted by R_J View Post
In your pictures there is a missing VCC capacitor in the primary. Did you replace it? There should be a resistor connected between the source of the mosfet and hot ground, then there should also be a resistor betwwen the source to the CS pin of the ic, check if it is open. You could also have a damaged optocoupler
yes the capacitor, it was removed to test it (but it's good), now I'm sending you a photo without the big capacitor so you can explain better what I need to check
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Old 01-30-2021, 01:07 PM   #6
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Default Re: Doppio alimentatore in uscita da 5 e 12 volt (nessuna uscita)

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Originally Posted by falcontech View Post
yes the capacitor, it was removed to test it (but it's good), now I'm sending you a photo without the big capacitor so you can explain better what I need to check
I would not trust the tester, just replace the cap. The resistor between the CS pin and the source of the fet should be under the large capacitor.
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Old 01-30-2021, 01:18 PM   #7
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Default Re: Doppio alimentatore in uscita da 5 e 12 volt (nessuna uscita)

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Originally Posted by R_J View Post
I would not trust the tester, just replace the cap. The resistor between the CS pin and the source of the fet should be under the large capacitor.
if you talk about the bigger resistance that has already been replaced
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Old 01-30-2021, 01:19 PM   #8
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Default Re: Dual output power supply 5 and 12 volts (no output)

is there a way to test the optocoupler or do I just replace it?
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Old 01-30-2021, 01:28 PM   #9
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Default Re: Doppio alimentatore in uscita da 5 e 12 volt (nessuna uscita)

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if you talk about the bigger resistance that has already been replaced
What do you mean by "bigger resistance" Did you check the resistor that connects the fet SOURCE pin to the IC's "CS" pin.
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Old 01-30-2021, 01:31 PM   #10
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Default Re: Dual output power supply 5 and 12 volts (no output)

I mean the bigger one that you see now in the picture after removing the big capacitor, is that the one I should test?
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Old 01-30-2021, 01:32 PM   #11
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Default Re: Dual output power supply 5 and 12 volts (no output)

however the resistors have all been replaced, except the smallest ones that look good to the eye
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Old 01-30-2021, 01:37 PM   #12
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Default Re: Dual output power supply 5 and 12 volts (no output)

in the photo all the resistors that I have replaced
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Old 01-30-2021, 04:40 PM   #13
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Default Re: Dual output power supply 5 and 12 volts (no output)

Download the ic's datasheet, maybe that will help. When the mosfet shorts, and the source resistor (mofet source pin to hot ground) goes open, there is over 300vdc applied to the current sense (CS pin) of the ic, this usually opens the resistor that connects the mosfet source to the ic's SC pin If this resistor is open the NEW ic can not monitor the mosfet current and may not start up.
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Last edited by R_J; 01-30-2021 at 05:15 PM..
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Old 01-31-2021, 05:46 AM   #14
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Default Re: Doppio alimentatore in uscita da 5 e 12 volt (nessuna uscita)

Quote:
Originally Posted by R_J View Post
Download the ic's datasheet, maybe that will help. When the mosfet shorts, and the source resistor (mofet source pin to hot ground) goes open, there is over 300vdc applied to the current sense (CS pin) of the ic, this usually opens the resistor that connects the mosfet source to the ic's SC pin If this resistor is open the NEW ic can not monitor the mosfet current and may not start up.
in this photo I have tried to trace which resistance you are referring to starting from the CS pin of the IC, I would not be wrong but there is no resistance that goes directly to the source pin of the mosfet, the one I see in the path that I have drawn for you, at the time of the inspection it was burned out and was replaced (220K 0hm), the other that was replaced part from the large capacitor pin and reaches the source pin (0.49 ohm)
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Old 01-31-2021, 05:52 AM   #15
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Default Re: Dual output power supply 5 and 12 volts (no output)

the original mosfet was a stk0460 and I replaced it with an FTP04N60C
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Old 01-31-2021, 11:27 AM   #16
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Default Re: Doppio alimentatore in uscita da 5 e 12 volt (nessuna uscita)

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Originally Posted by falcontech View Post
in this photo I have tried to trace which resistance you are referring to starting from the CS pin of the IC, I would not be wrong but there is no resistance that goes directly to the source pin of the mosfet, the one I see in the path that I have drawn for you, at the time of the inspection it was burned out and was replaced (220K 0hm), the other that was replaced part from the large capacitor pin and reaches the source pin (0.49 ohm)
Your tracing is wrong. You need to look at the board again. Have you looked at the datasheet for the ic ????
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Old 01-31-2021, 11:51 AM   #17
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Default Re: Dual output power supply 5 and 12 volts (no output)

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Originally Posted by falcontech View Post
the original mosfet was a stk0460 and I replaced it with an FTP04N60C
I was just going by the number you told me.
The the mosfet should work ok.

Last edited by R_J; 01-31-2021 at 12:57 PM..
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Old 01-31-2021, 12:02 PM   #18
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Default Re: Dual output power supply 5 and 12 volts (no output)

absolutely it is the insulated type, otherwise I would have put the plastic insulation on the screw mine exactly is 04n60c3
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Old 01-31-2021, 12:22 PM   #19
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Default Re: Dual output power supply 5 and 12 volts (no output)

Fuse would have blew if the insulation wasn't there anyway?

Looks like there's a pull up to shut down if the resistor blew (incidentally it seems like I see two resistors in parallel there I think, this should be the 0.49Ω resistor? Which should be a fusable resistor? And it looks like a 100Ω resistor there in parallel with something else that I can't see underneath?)

Hmm...
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Old 01-31-2021, 12:42 PM   #20
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Default Re: Dual output power supply 5 and 12 volts (no output)

The way the current sense circuit works is they monitor the voltage drop across the SOURCE resistor(0.49Ω) of the switching mosfet. this small voltage is fed to the CS pin3 of the ic via a RC filter, resistor (1kΩ)? with a small capacitor.

The LARGER resistor is NOT connected to the smaller resistor!!!
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Last edited by R_J; 01-31-2021 at 01:11 PM..
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