# Trying to identify a blown thermistor

#### SailCS33

Joined Mar 18, 2021
37
Hi everyone,

I bought a cheapy 15v/40A power supply from Ali, which immediately started smoking and within few minutes loud pop and the show was over. Upon checking inside I see there is a blown thermistor which I cannot identify capacity nor type.

It is located on the "neutral" AC side.

Wondering if anyone can help with this so I can buy a replacement.

Thanks!

Val

#### ericgibbs

Joined Jan 29, 2010
13,633
hi Sail,
A link to the Ali power supply type description or a datasheet would help.
E

#### AlbertHall

Joined Jun 4, 2014
11,504
Could we at least have a picture of the faulty area or should we just guess?

#### AlbertHall

Joined Jun 4, 2014
11,504
Did it blow the fuse?
Good picture.
Next a picture showing a larger area of the board including the blown component and the mains in connections, please.

#### SailCS33

Joined Mar 18, 2021
37
Did it blow the fuse?
Good picture.
Next a picture showing a larger area of the board including the blown component and the mains in connections, please.

No the fuse is not blown. No other components appear to be damaged at least on surface.

#### SailCS33

Joined Mar 18, 2021
37
One other area of concern. 4.4Mohm (???) resistor has paint flaking off.

#### AlbertHall

Joined Jun 4, 2014
11,504
The blown component is I think and NTC for limiting surge current. The replacemnt should be one the same diameter but I don't know what value you need.

#### prairiemystic

Joined Jun 5, 2018
189
These aren't worth repairing. They are cheap because, well they are super cheap and no way they can meet even half of rated power.
There was likely nothing wrong with the NTC, it just acted as the fuse when the mosfets shorted. The cracked resistor is likely a side effect, which means Q1 has failed shorted. R20, Q2 likely dead as well.

Check out DiodeGoneWild similar experience and diagnostics, repair. Pretty good to watch.

#### SailCS33

Joined Mar 18, 2021
37
By the way, you are spot on. I just checked after your posting. Blown mosfet.

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#### Ian0

Joined Aug 7, 2020
3,245
The blown component is I think and NTC for limiting surge current. The replacemnt should be one the same diameter but I don't know what value you need.
Look up NTC inrush suppression thermistors in a catalogue, find those with the same diameter. Choose one for the appropriate current. Often the maximum size of capacitor is listed.
They blow up quite often, especially in 110V power-tool transformers.

#### SailCS33

Joined Mar 18, 2021
37
Question for everyone:
The blown up component has D13009K markings. Quick google search says it is " HIGH VOLTAGE FAST-SWITCHING NPN POWER TRANSISTOR ". What I dont understand is it says 12A 120W rating...The PSU is labeled (and paid for) as 40A. Is that ok for this transistor to be 12A rated?

Thanks again

Val

Last edited:

#### Ian0

Joined Aug 7, 2020
3,245
That's 40A output current at 15A = 600W.
The transistor is on the mains side. If your mains is 230V, then it gets rectified to 325V. If it's in a half-bridge circuit, then it only takes 4A, so 12A should be plenty, but I wouldn't fancy its chances being only 400V rated - I bet the SOA curve is a bit narrow.

#### SailCS33

Joined Mar 18, 2021
37

#### Ian0

Joined Aug 7, 2020
3,245
I on 110-120v
So half the voltage and twice the current. 400V Vceo limit is no longer a problem, 8A peak current is within the limits for a 12A device, though I suspect Hfe would be falling quite rapidly.