The post investigates a cheap Chinese made 12V, 1 amp mosfet based smps circuit which can be modified into 24V 1 amp, or 12V 2 amp smps circuits also. The mosfet used is STB9NK60Z which is a highly advanced, rugged 600V 7A device specially manufactured for high, unpredictable, voltage environment circuit applications.

Reverse Engineering a 12V 1 Amp SMP Adapter Physically









Coil Details:


The proposed 12V, 1 amp mosfet based smps circuit utilizes a single E-core transformer, the winding details may be understood from the following info:



Wire thickness for the above secondary winding = 0.6mm, no. of turns = 12



Wire thickness for the above primary winding = 0.25mm, no. of turns = 12



Wire thickness for the above primary winding = 0.25mm, no. of turns = 170

Feedback from one of the dedicated readers of this blog Mr. Debabrata Mandal:

I bought one today which is exactly the same, well, almost... cost me 100₹
Tested it @shop for 10 sec & it ran fine, brought home, plugged in & was just bout the test with multi & there was a popping sound though the indicator led was still glowing
Opened & found that the electrolytic @ 12v side blew, a 16v cap blew, so i tested & found output as some 22-23v

Well i couldn't check properly cause as soon as am turning it on, in 1 sec the mosfet is becoming burning hot

Also i dunno bout the transistors s9014 & s8550, wat type or how their pins r arranged
There's just 3 components diff...

100e/.5w instead of yr 150e/.5w (resistor)220e instead of yr 270e (resistor)s9014 instead of yr s8050 (transistor)

Without taking anything out of the board, i tested the diodes/zener... they r fine it seems
So without taking out the transistor/mosfet if u could tell me how to check them w/o having to turn the power on... cause tat’ll melt the mosfet for sure

Or watever idea u can share to debug... also, how do i tweak the circuit to 14v (13.6~14.4) & 1.1A (>1.05)

Possible Solution:


The mosfet should not become hot as long as the output is not loaded or short circuited.
If it's getting hot without any load at the output could mean a faulty primary section.

Confirm the status by doing the following steps:

Cut the PCB tracks of the secondary winding terminals such that it becomes completely isolated from the circuit board, confirm the continuity with a multimeter.

Next connect a 25 watt bulb in series with the input AC to the smps and switch ON power. If the 25 watt bulbs glows or if the mosfet shows significant heating would confirm a faulty primary stage.

The next step would be to remove the transistors one by one and replace them with new ones and apply the input voltage to check the mosfet condition.

If the heating persists then finally you can go for a mosfet replacement with a new one.After all these procedures are completed and the problem is fixed, we can go ahead to check why the secondary is generating a 24V output.

This could be due to a wrong winding data or may be once the primary stage is resolved as above, the output would also settle down with the correct output.

More Inputs from Mr. Debrata

Well i already took out the 2 transistors + mosfet + transformer... & checked...all fine... checked the bloody resistors & caps... all fine put them back & started checking the pcb itself...

i found a short as in attached pic... after soldering they didnt cut short tat leg & it protruded & touched the copper film... so i cut it short & checked... now 2.3v but still mosfet BURNING...

amazing... finally nothing to do, i replaced the blown cap with 1000u/16v keltron & whala... damn thing got fixd

No more heating, stable 13v, exactly what i wanted for my project, am attaching project photos... maybe u can put it up as a project actually done by a hobbyist visitor

I was thinking to buy a led bulb/tube for aome time but suddenly last evening i just decided to make it, re-live the old days, 12pcs 1w led with heatsink @ ₹7/piece + 12v 1A smps + ₹10 steel plate...

Analyzing the Issue


Wow that's indeed very interesting., so the problem was in the filter capacitor, once it got fixed, the opto-coupler could receive the feedback input from it and in turn helped to regulate the mosfet conduction.....
Anyway all's well that ends well.
And yes your LED lamp looks darn good, congrats on that.
Thanks for the feedback.

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