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12V, 24V, 1 Amp Mosfet SMPS Circuit

12V, 24V, 1 Amp Mosfet SMPS Circuit

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 it in & was just about the test it with multimeter & there was a popping sound, though the indicator led was still glowing

Opened it and found that the electrolytic @ 12v side had blown, another 16v cap was burnt, in this condition I tested the output & found output at around 22-23v

Right now, I cannot check it properly because as soon as I turn it on, within a second the mosfet starts getting burning hot.

Also I can't figure out the transistors s9014 & s8550, what type or how their pins are configured?

There are just 3 components different than the above design

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

Without taking anything out of the board, I tested the diodes/zener, they looked fine to me.
So without taking out the transistor/mosfet  could you tell me how to check them w/o having to turn the power ON... because that could melt the mosfet for sure.

Or whatever idea you can share to debug this... 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

OK I already took out the 2 transistors + mosfet + transformer and checked all looked fine, checked the resistors and capacitors, all seem to be fine and then I put them back & started checking the pcb itself...

I found a short, after soldering they didn't cut short that leg & it protruded & touched the copper film, so i cut it short & checked, now the output showed2.3v but still mosfet kept getting Hot.

Amazing... finally nothing to do, i replaced the blown cap with 1000u/16v keltron and what? The problem got fixed.

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.

Thanks for the feedback.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

51 thoughts on “12V, 24V, 1 Amp Mosfet SMPS Circuit”

    • Olupot, I don’t think a 12 amp modification would be possible from this circuit, unless thorough calculations are done for achieving the inductor dimension and the mosfet rating correctly


    • Thanks for information..
      From market i have a similar circuit..
      But has few changes
      1. In place of 150k 1/2w they usr 100k 1/4w
      2. 2A272 in place 222/1k
      3. And 1.5e 1/2w in place of 22e 1/2w

      • Thanks Sumeet, slight changes might be possible for different versions, depending on the manufactures research and improvements…

    • Dear sir
      Thanks for circuit is working great..
      As i increases the secondary wire thikness or added parallel two wires.. the 'Amp' decreases.
      Can suggest solution

    • Dear Sumeet,

      That's very strange, winding with parallel secondary wires should increase current….make sure the wires are thin and seamlessly merge within each other and fit tightly inside the bobbin…if the parallel wires are not cleanly wound and do not blend smoothly then that might create opposite results.

    • Sir, will the circuit performance will remain unaffected if we use 103/lkv instead of 102/1kv.
      Please suggest

    • Sorry I am not sure about that because I have not yet studied the circuit deeply…however it may not be recommended to change anything in the original design

  2. Dear,
    Is it that the mosfet in the shown smps is taking the switching pulses from series connected transistors to drive the trafo?
    i want to know working difference between transistor and mosfet,
    according to me, i feel that mosfets cannot switch on/off on their own, they need a low current switching pulse at their gate and amplify it then send o/p to drain and to trafo……is this right??…….just want to clear this thought.

    According to me, when a transistor is used for switching, it does not necessarily need any switching pulses from any oscillator circuit, it directly starts switching as it gets input power taking simple example of a joule thief circuit.
    In the joule thief circuit, the transistor is switching the trafo on its own…..but what if there were a mosfet in place of transistor? would switching action take place or not?
    Dear, please correct me if i'm being misunderstood.

    • Dear Sherwin,

      Your assumption are incorrect.

      The mosfet and the transistors along with one of the primary winding of the transformer become an oscillator circuit. Basically it's the mosfet and the trafo primary which become the main oscillator components, the transistors are just for assisting the mosfets in the process.

      Transistors also required external osculating frequency just like mosfets, transistor have no power or property of oscillating alone without an external configuration.

      in a joule thief the transistor oscillates due to the feedback from the coil winding, the above smps may be also running on the same principles.

      mosfet require 10v to trigger optimally so if the joule thief is powered with above 6V then probably a mosfet would also work

  3. Dear,
    I have the following queries;
    1. What is the idea behind insertion of paper or keeping an air gap between the two core pieces in ferrite trafos?
    actually, i've opened various smps power supplies and tried to study their config…..i noticed some trafos had a paper stuck between the core pieces, some had air gap and some had
    nothing between the core pieces, stuck firmly.

    Hence, please help me…… in what way should i make the config and which config is proper, since i'm rewinding the trafo for the emergency tube light circuit.

    The trafo is quiet big……salvaged from an 12v 5a smps and managed to remove the core pieces, here it was an EI core type. this trafo had a thick paper between the core pieces.

    2.Secondly, does it matter whether i wind primary first or secondary second or vice-versa, and what about winding direction of both coils?

    3.Again, which winding configuration is more efficient at the primary of trafo, a two wire or a three wire?

    Please help.

    • Dear Sherwin
      the paper ensures a gap between the ferrite core joints and restricts them from getting stuck magnetically while the inductor is in operation.

      The gap is crucial, if it's not maintained will cause the magnetic effect to go beyond the correct optimization point and lead to "shorting" of the effect.

      This will result in the inductor functioning at much lower saturation levels and below the optimal points.

      No it doesn't matter which winding is wound first, primary can be wound over secondary or secondary over primary according to me.

      two wire configuration is more efficient

  4. Dear Sir
    Please let me if I want to purchase the above circuit diagram complete with assemble please quote me the rate my qty will be 100-500 per month on regular base. We are manufacturing medical equipements and we are using same circuit purchasing from local market in adaptor cabinets please quote us the rates so we can purchase from you.

    Ketan Khakhar
    email: krupaelectrodevice@gmail.com
    mobile: 09409648515 / 8153084584.

  5. With ref to… https://homemade-circuits.com/2013/10/12v-24v-1-amp-mosfet-smps-circuit.html

    I bought one today which is exactly the same, well, almost… cost me 100₹

    Tested it @shop for 10sec & 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 couldnt check properly cause as soon as am turning it on, in 1sec the mosfet is becoming burning hot

    So…. help…. @my hobby days there was no smps so i donno how to fix them & its 100₹ non-refundable so i dont wanna throw it away

    Also i donno bout the transistors s9014 & s8550… wat type or how their pins r arranged

    Theres just 3components 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)

  6. Hi sir, I see you again on this SMPS project is a nice work but I don't understand the circuit diagram, so I need the schematic diagram of this project. Continue on your good doing God help you thanks.

  7. Hi Swagatam sir, wishing you & ur family a very Happy Durga Puja…may Maa Durga fulfill all ur dreams…my best wishes r always with u…keep doing the good work sir…

  8. Hey mr. swagatam love your work and designs. But could not find your contact details and email address on your site to contact to you about a project.

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