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12 volt 5 Amp SMPS Repaired

By on October 19, 2018
power supply drive led chain











smps repair

The complaint of this small switch mode power supply (SMPS) with the output of 12 volt and 5 amp was no dc output. I powered it up and test the output and indeed it was no output as seen from the photo below:

no output voltage in smps

Just like any other electronic equipment with the complaint of no power, the first component to check is the main fuse. In this power supply design, it did not use the normal slow blow fuse, in fact it used a low ohm resistor as a fuse.

I checked on the fuse resistor and found that it was open circuit. From many repair histories, when the main fuse have problem, usually there will be a or some components that have shorted drawing too much current cause the fuse to blow.

fuse open in smps

After found the fuse to be open circuit, I checked on all the primary side components hope to find bad component.

check components in smps

My guess was right and I found a shorted power FET in the primary side and other components seems to be working fine including components  at the secondary side.

shorted fet in smps

I replaced the power FET and soldered a 40 watt light bulb in series with the main fuse line to check and make sure no other shorted components in the power supply once the power supply is powered On.

40 watt series light bulb trick

The result was the power supply worked again and I put in the new resistor fuse and put back the power supply casing.

I turned it On once again and saw the power LED lighted up at the secondary side-see the photo below:

working smps 12 volt 5 amp

Next, I used this power supply to power up the LED light chain as seen from the photo below:

smps power supply repaired

Conclusion- This repair case history is pretty straight forward. What you need to know is the safety and be able to test electronic components. I can recommend Jestine Yong’s ebook in how you can be very good in testing electronic components and start to repair work. If you want to be a power supply repair expert where you can repair power supply fault fast, you can get his SMPS repair ebook here. Ok see you again in the next article.


This article was prepared for you by Suranga Bandara who owns an Electronics repair shop in Anuradapura, Sri Lanka.

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  1. Albert van Bemmelen

    October 19, 2018 at 10:37 pm

    Another pro repair! What type was the defect power fet?

  2. Joseph Watananga

    October 19, 2018 at 11:28 pm

    Excellent work!!!! I love it hey.Keep up the good work.I am learning.

  3. Robert Calk Jr.

    October 20, 2018 at 12:04 am

    Good job, Suranga.

  4. Saqib

    October 20, 2018 at 8:52 am

    Can we use this power supply to power car audio headunit? As these power supply are also available in DC 12V 10A.

  5. beh

    October 20, 2018 at 11:43 am

    Hi Suranga
    Indeed your repair reports are useful for me.

    • Finn

      October 20, 2018 at 2:57 pm

      Fine article.

  6. Justice

    October 20, 2018 at 4:09 pm

    Great and interesting article. Power field-effect transistors are like that... Thanks Suranga keep sharing with us your expertise--Justice from Durban in South -africa.

  7. Humberto

    October 20, 2018 at 9:39 pm

    Good repair Suranga, as usual.

  8. Akure Olayinka Shadrach

    October 22, 2018 at 4:57 am

    Hello techies here am a beginner(Student) in electronics repair please how can i know a sorted transistor? Educate me please.


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