Bad PSU Thermistors blowing, no multimeter

[budm]

Re: Bad PSU Thermistors blowing, no multimeter

Check the resistance between the Source pin and the Drain pin pf the power MOSFET to see if it shows really low resistance (<10 Ohms)

[Joe Black]

Re: Bad PSU Thermistors blowing, no multimeter

Cap. Put it back and try again.
When you removed it, you totally cut off current flow.

Budm, he doesnt have a DMM (yet)

[eccerr0r]

Re: Bad PSU Thermistors blowing, no multimeter

Took out too much stuff in one step. But likely it's the MOSFETs that are shorted now and sticking the 200V capacitors back in, you'll get a blink of the bulb that will then go dark.

I would have thought that the NTC thermistor should also make the bulb go dark because it's in series with the transistors... that doesn't quite make sense unless you removed a capacitor and not NTC. Trace out the circuit carefully...

[budm]

Re: Bad PSU Thermistors blowing, no multimeter

Cap. Put it back and try again.
When you removed it, you totally cut off current flow.

Budm, he doesnt have a DMM (yet)

I know, I am just telling him what he should do once he has the meter to do some testing.

[eccerr0r]

Re: Bad PSU Thermistors blowing, no multimeter

Oh another warning, if you take the mosfets out, the capacitors may be holding onto very high voltage (unless if there are leak resistors in the design). Test before touching!!!

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

Just removed the two MOFETs on the heatsink and one Big Cap(Yeah I read somewhere CAPs can kill, always extra careful around these babies).

There are supposed to be two NTCs but I removed one becuase it was blown and tested with just one while the other parts(MOFETs & Cap) were still intact in their original places.

Result = Bright Bulb

I think I will wait it out till my multimeter gets here then test the MOFET for resistance as budm suggested. I'm really not comfortable with these crappy MOFETs in my PSU and if I'm already tinkering deep in it then I'd like to go the extra mile and make it an efficient one.

After googling around for this particular model of PSU(app650ps) I found that it fails after 2Weeks of operation on most users PCs.
Eager to learn how this stuff works sorry for the noobish questions that have been asked a thousand times already:

If I were to replace them what do I search for in the meantime? The stock MOFETS TO - 220 are rated 500V but the ones I come across in the shop are like 75V. For example this one
. It has a higher output in Watts and Drain to source. Gate Threshold Voltage = 2V to 4V

VS

the stock . Gate Threshold Voltage = 3V to 5V


What should I look for when searching for a replacement, any suggestions for the specs of the replacement MOFETs?

I know these cheap psu's don't put out as much as advertised(crappy parts), is there any overhaul in the components I can do to get as close or even exceed what it was really designed for? If I recapped the whole thing with good Japanese or German Caps + New MOFETS, is it really worth it? Is changing the ICs required too?

[stj]

Re: Bad PSU Thermistors blowing, no multimeter

the psu is hitting the mosfets with about 360v.
if you use a 75v one it will explode so fast you will still have your finger on the switch!

[eccerr0r]

Re: Bad PSU Thermistors blowing, no multimeter

Are you sure you're dealing with NTC thermistors and not MOVs? Don't get them mixed up...
NTC thermistors are in series, MOVs are in parallel with the supply.

[stj]

Re: Bad PSU Thermistors blowing, no multimeter

thermisters have a cement coating - nice rough finish.
usually green or black.

mov's have a resin coating - nice and shiny.
they are usually yellow or red.

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

Are you sure you're dealing with NTC thermistors and not MOVs? Don't get them mixed up...
NTC thermistors are in series, MOVs are in parallel with the supply.

Hope i'm not mixing them up, one of the round topped black thingies blew(visible in 4th pic), after that I replaced it and the next one blew when trying to test it before using Bulb-Tester. That is an NTC right?
This is the stock component that blew http://www.alldatasheet.com/datasheet-pdf/pdf/619191/ETC2/MF72-5D11.html
Replaced one of them with this before the other "stock" one blew too.
http://pdf1.alldatasheet.com/datasheet-pdf/view/182818/EPCOS/B57237S0479M000.html

They are next to the Transformer plug in the pic(facing each other). When they blow they go out with a bang, like a firecracker Edit* coming to remember........ I replaced one yellow filter cap from a .33uF to a .447uF. Could that have caused them to blow one by one? But the PSU was already blowing fuses and shorting the house mains before i replaced it, after replacing the filter cap that is when the NTC's started popping. Hmmmm....

I also removed both MOSFETS TO-220s from the heatsink + one big Capacitor that was in the way.

[budm]

Re: Bad PSU Thermistors blowing, no multimeter

"I replaced one yellow filter cap from a .33uF to a .447uF. Could that have caused them to blow one by one" That yellow rectangular cap CX1 is the ac line noise X/Y safety cap, that cap can be left out of the circuit and the circuit will still function, the main filter caps are those two large cylinder caps sitting next to the main bridge rectifier. Are those the two caps you said you have removed?
So what are the designator used on those two disc device that blew up, TH1?. MOV1?

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

That yellow rectangular cap is the ac line noise X/Y safety cap, that cap can be left out of the circuit and the circuit will still function, the main filter caps are those two large cylinder caps sitting next to the main bridge rectifier. Are those the two caps you said you have removed?

I removed one large cylinder cap(Picture 1 bottom) and the transistors on the heatsink(Picture 3). Also removed one burst NTC(Picture 4) and left the good one inside(there were two)
I had previously replaced the yellow rectangular cap from a .33uF to a 447uF thinking that was the issue.

[budm]

Re: Bad PSU Thermistors blowing, no multimeter

There should be designators printed on the board to tell you what kind of the component are, I cannot see the printing on #4 image. Do both of the NTC have the same designator, I.E TH1, TH2?

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

Yes they do : TH1 and TH2, the second image is the filter cap I replaced(maybe the old one is working too, dunno yet)

[budm]

Re: Bad PSU Thermistors blowing, no multimeter

OK, both of them for sure are NTC, I do not know why there are two one them, they usually use just one that has one to reduce the current surge due the main filter caps being charged when power is first applied, so if the POWER MOSFETs are shorted that will cause the NTC to blow up.

http://www.ametherm.com/inrush-current/
http://www.ametherm.com/inrush-curre...ters-pcim.html

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

sounds like those MOSFETS are toast, I will check them just to make sure. I was looking around and can't find anything in that exact specification. I might have to replace them with something else.

[eccerr0r]

Re: Bad PSU Thermistors blowing, no multimeter

So it's verified that the mosfets are toast (all parts reinstalled except for mosfets and your dim bulb does not stay on?)

Anyway if you can't find exact, just get one that has about the same voltage (500V should be safe), about the same current, same threshold voltage, and especially same gate capacitance (don't go too far off). Rdson is important as well, mostly for efficiency, but it's less important IMHO when making a sub, just don't go too crazy. Another issue I just spotted, the GPT13N50 has integral gate protection diodes and the gate voltage to source max is +/- 30V compared to the IRF1405 which appears to be +/- 20V to source... This also depends on the circuit itself, perhaps it already has the protection diodes on the board and the ones in the MOSFET are superfluous...

[AlwaysCheap]

Re: Bad PSU Thermistors blowing, no multimeter

Thanks for the info, actually haven't put the big cap back in yet, I'll try it and see if the bulb comes on(hopefully it's just the MOSFETS).

There are two single directional diodes on the board right before the transistors, could they be protection diodes? They are black with a silver strip on one end.

[stj]

Re: Bad PSU Thermistors blowing, no multimeter

the green cap behind the transformer in that foto looks like it's just starting to dome!

[eccerr0r]

Re: Bad PSU Thermistors blowing, no multimeter

Generally you might be able to just get another 13N50 clone to see if it works.

Rohm R5016FNX, ZDX130N50; NTE2939 might work. But I didn't really look at the spec sheets that carefully...