Bad PSU Thermistors blowing, no multimeter

[AlwaysCheap]

Hi guys, I am experiencing the same weird problems too with my thermistors what gives?

Here is how it all started: I got the brand new PSU(Approx 650W "App650PS") about 3 weeks ago without guarantee because it's a replacement for another malfunctioning psu(Cooler Master 600W) that lasted for about 2 years.
A few nights back I was gaming, enjoying my new PSU and "BAM" the PC turns off without any warning. I try to turn it on and it won't start but the Mobo light is lit "green". Decided to pull the plug and let everything cool for an hour or so.
After the 1hr wait I plugged the extension cord back in the wall socket and tried to turn on my PC. At this point all I heard was a pop in the PSU and the lights in the whole house went. It tripped the breaker for the whole house.

After opening the PSU it up(no warranty so why not ) I noticed the fuse(5x20mm 6.3A) inside had blown,went quickly to the store and got 3 replacements just in-case. Put in the replacement and "BAM" it blows the second fuse together tripping the breaker for the whole house

Googled around and found some usual culprits that might be causing the issue. Since i'm kinda cash strapped and don't want to wait for a cheap multimeter from AliBaba(which could take weeks) I decided to replace some parts for the PSU which I thought may be defective. (All this without using a multimeter).

I first started by replacing the crappy Chinese bridge rectifier(KBP 307 3A) https://cdn.badcaps-static.com/pdfs/...ce07994762.pdf
to a better Diotec brand with higher ratings(KBU8K 8A)

Soldered it on and "BAM" still the same problem, blown fuse + tripped house breaker circuit.

Next I decided to change the filter cap from a cheap (HQX 334 250V - 275V X2)275VAC0.33UF 275V334K X2 to (.47uF 275V X2)

Soldered it on and "BAM" still the same problem, blown fuse but this time it blew out an NTC thermistor too (MF72-5D11 5 Ohms 4A)(Blew it to pieces) with a loud bang + tripped house breaker circuit.


Went to the store again(became a regular idiot by now) and got a replacement with higher ratings (4.7 Ohms 5.1A)

Soldered it on and "BAM" still the same problem, blown fuse but this time it blew out the other NTC thermistor next to the one I replaced, it is also a (MF72-5D11 5 Ohms 4A)(Blew it to pieces too) with a loud bang + tripped house breaker circuit.

What could be the cause of this ? I would appreciate any help you guys could provide to guide me in the right direction, I still can't test any component because I don't have a multimeter or anything similar.

So far I have not been able to find the psu's schematic anywhere, I have a few diodes(higher ratings) and resistors that I can replace, is it worth a try to replace the one's on the pcb?

[budm]

Re: PSU just "exploded" what's the cause?

"What could be the cause of this ? I would appreciate any help you guys could provide to guide me in the right direction, I still can't test any component because I don't have a multimeter or anything similar. " Not until you have test equipment to look for shorted circuit. I hope the circuit does not have further damage when new parts are installed and blew up again.

[CapLeaker]

Re: PSU just "exploded" what's the cause?

kinda pointless without a DMM.

[stj]

Re: PSU just "exploded" what's the cause?

a meter will be cheaper at this rate than constantly blowing stuff up!

[Per Hansson]

Re: Bad PSU Thermistors blowing, no multimeter

I created a new thread for you, no need to resurrect the other one

I would suggest that you make a bulb tester so you don't have to blow your PSU into a thousand pieces each time you try it:
https://www.badcaps.net/forum/showpo...4&postcount=70

More threads I found while looking for the one linked above
https://www.badcaps.net/forum/showth...054#post335054
https://www.badcaps.net/forum/showth...424#post184424
https://www.badcaps.net/forum/showth...149#post261149
https://www.badcaps.net/forum/showthread.php?t=32748
https://www.badcaps.net/forum/showth...045#post279045
https://www.badcaps.net/forum/showthread.php?t=30986

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

I created a new thread for you, no need to resurrect the other one

I would suggest that you make a bulb tester so you don't have to blow your PSU into a thousand pieces each time you try it:
https://www.badcaps.net/forum/showpo...4&postcount=70

More threads I found while looking for the one linked above
https://www.badcaps.net/forum/showth...054#post335054
https://www.badcaps.net/forum/showth...424#post184424
https://www.badcaps.net/forum/showth...149#post261149
https://www.badcaps.net/forum/showthread.php?t=32748
https://www.badcaps.net/forum/showth...045#post279045
https://www.badcaps.net/forum/showthread.php?t=30986

Thanks for creating a new thread and for all the helpful info to point me in the right direction. I'll try it and see how it goes before my Multimeter gets here, I agree it's way better than blowing new stuff up

[Joe Black]

Re: Bad PSU Thermistors blowing, no multimeter

Someone near you HAS to have a multimeter you can borrow.
Maybe one of switching transistors is shorted, but still cant verify without DMM.
Also, plus 1 for the bulb…

[eccerr0r]

Re: Bad PSU Thermistors blowing, no multimeter

Yeah, very likely the input primary switching transistors shorted out but there may be something else that caused the transistor to short... Multimeter is mandatory. Even an analog one will help. Or even getting a PP3 battery, LED, and resistor may be helpful to see if something's shorted (though you will have to test both ways because this will forward bias semiconductors...)

The fuse in these switching psus are there for protecting your house, not the PSU. When it blows, the PSU is guaranteed to need repair... and usually it's a semiconductor.

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

Yeah, very likely the input primary switching transistors shorted out but there may be something else that caused the transistor to short... Multimeter is mandatory. Even an analog one will help. Or even getting a PP3 battery, LED, and resistor may be helpful to see if something's shorted (though you will have to test both ways because this will forward bias semiconductors...)

The fuse in these switching psus are there for protecting your house, not the PSU. When it blows, the PSU is guaranteed to need repair... and usually it's a semiconductor.

Great !! They will be the first I will check once I get my hands on a DMM. You are referring to these transistors right? If one is faulty I can change both to a higher rating ?

Currently the stock ones are and from the looks of it they look pretty weak.

What is the criteria for choosing a good transistor, do we consider the volts or watts? e.g Drain-Source Breakdown Voltage V(BR)DSS 500V or the Total Power Dissipation in Watts?

Too bad I don't know how much juice the transformer can produce.

Could this be a good replacement?

[eccerr0r]

Re: Bad PSU Thermistors blowing, no multimeter

What you can do now is make the dim-bulb tester, use a 40W bulb, and start pulling parts out of the power supply and then test if the PSU input is still shorted. If the bulb still glows brightly, then pull out another part after turning it off, and retest. The last part you pulled before it stops glowing brightly is a suspect part. Be careful of the MOSFETs, static can destroy them, but likely they are already fried.

Of course, there are parts that will obviously turn out the light like pulling the fuse, the inline noise filters(s), the bridge rectifier, so don't pull those ... You really need the multimeter to really see what's bad.

I'd try not to swap the parts too much from stock, you may get strange operation if you choose mosfets that have higher gate cap than what it was designed for...

[TELVM]

Re: Bad PSU Thermistors blowing, no multimeter

Our American friends are giving great advice suggesting the dim-bulb tester, which is a simple and useful tool.

How to build a dim-bulb tester

But on their side of the pond they enjoy sockets with unambiguous polarity.

On Hungarian Schuko sockets with ambiguous polarity, and without a DMM to clear the ambiguity up, there is a 50% chance of reversed polarity and placing the bulb on the wrong (neutral) wire.

[goodpsusearch]

Re: Bad PSU Thermistors blowing, no multimeter

So? The result will be the same. The bulb will limit the current and keep the psu from blowing.

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

Our American friends are giving great advice suggesting the dim-bulb tester, which is a simple and useful tool.

How to build a dim-bulb tester

But on their side of the pond they enjoy sockets with unambiguous polarity.

On Hungarian Schuko sockets with ambiguous polarity, and without a DMM to clear the ambiguity up, there is a 50% chance of reversed polarity and placing the bulb on the wrong (neutral) wire.

Thanks for the heads up, I was going to use just two wires without ground/earth since the wall socket only has two holes. As for which is the neutral and which is the live is a little confusing right now.

I am in the process of putting one together now with some old parts lying around(it's not finished yet still have to connect the bulb to the circuit).

Bad idea?

[AlwaysCheap]

Contraption Complete !!

Thanks guys for all the feedback about the dim-bulb tester, finally got it up and running. I tried connecting a DC 5V clock to it and the clock worked but the bulb didn't light up. When I tried unscrewing the bulb the connection was lost once the bulb was loose, if screwed back in, the clock works again.

Does it light up always or only when on load? I thought it should be dim, right?

I'm using a 40W bulb at the moment for testing, any input is much appreciated.

[budm]

Re: Bad PSU Thermistors blowing, no multimeter

Plug in a working PC power supply by itself without forcing it on so that it will be just the standby power supply section is running then observe the brightness of the lamp.

[stj]

Re: Bad PSU Thermistors blowing, no multimeter

connect the earth.
use the screw on the wall socket if you have to.

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

Thanks STJ, there is a screw for the earth terminal in the socket used for the Dim-Bulb tester. It's on the bottom of the socket, I could connect a wire to it, but where can I connect it from there?

Forgive my weak physics application of theory

[stj]

Re: Bad PSU Thermistors blowing, no multimeter

the screws holding the 2pin socket on your wall are probably earthed through the back-box.

show a foto of the 2pin socket.

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

I forgot to mention that I will connect the DB-Tester to an extension cord and not direct to the wall. The extension cord is earthed and fused. any ideas?

Edit* : Is there any risk if I connect it to extension cord the way it is?

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

What you can do now is make the dim-bulb tester, use a 40W bulb, and start pulling parts out of the power supply and then test if the PSU input is still shorted. If the bulb still glows brightly, then pull out another part after turning it off, and retest. The last part you pulled before it stops glowing brightly is a suspect part. Be careful of the MOSFETs, static can destroy them, but likely they are already fried.

Thanks for the tips and helping troubleshoot my PSU , I managed to test my PSU without it behaving like a "Flashbang".

Here are my results so far:

Removed blown NTC = "Bright Bulb/ still shorted"
Removed both MOFETS and of the big 200V Capacitors = "No bulb light at all"

Is it because of the removed cap? or did removing the MOFETS also play a part in this.

Hope my Multimeter gets here soon