Trying to identify a blown thermistor

Thread Starter

SailCS33

Joined Mar 18, 2021
32
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
 

AlbertHall

Joined Jun 4, 2014
11,222
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.
 
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.
Fake (wax filled) capacitors, used parts, $26. Note the transformer's insulation later failed, not to electrocute but still.

12V 50A 600W power supply - full load test

12V 50A 600W power supply - failure analysis & transformer autopsy

Also scope analysis etc. he did. Good learning.
 

Ian0

Joined Aug 7, 2020
1,911
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.
 

Thread Starter

SailCS33

Joined Mar 18, 2021
32
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
1,911
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.
 
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