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12V 10A switching power supply (with schematic and explanation)

12V 10A switching power supply (with schematic and explanation)

The schematic in my DB of reverse engineered schematics:

Today I made a teardown of an industrial switching power supply module S-120W-12 in a metal housing. The input is 100-240V AC mains and the output is 12V DC 10A 120W. It came from Ebay and it cost me only $8. In the listing it's called "switching power supply LED driver adapter", but it may have a lot of uses other than just powering LED strips. The SMPS is based on the old good UC3842 control chip, TL431 voltage reference and an 817 optocoupler. It has an open loop protection added, made of discrete transistors. It also has a softstart, a proper fuse, NTC thermistor, safety capacitors, snubber networks, EMI filters, good isolation distance and a relatively safe-looking transformer. The only problem of this switching power supply is the way the power transistor and diode are mounted on the heatsink! Just the sharp corner of it actually touches the aluminium plate! This video got quite long,...


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  1. Date: July 7, 2018 at 05:24
    Author: Ahmed Ali

    good explanation so far for me on youtube for SMPS working.
    hope to have a good article of your on these.

  2. Date: July 8, 2018 at 12:42
    Author: TTV

    hi, any idea how to control output current for a CC type option

  3. Date: July 8, 2018 at 23:42
    Author: roy Yung

    I totally disagree with your inpit power calculation as Ohms law states Wattage (or power) is Volts * Amps. This however, according to Ohms law, does not apply to A/C circuits. The formula you are using calculates VA or VoltAmps which is a vague approximation of Wattage.
    In A/C circuits, the frequency of the A/C signal and components used in the circuit (aka front end of power supply) determines cirrect wattage. You are using a formula for D/C circuits or an A/C circuit with purely linear load (ie toaster, or incandescent lightbulb)
    Even Electric meters on the side of the house do not measure wattage, they measure Amperes. The meter is calibrated with a PF (or power factor variable) which in the USA IS .95. Inductive and capacitive loads can vary this reading depending on frequency applied.
    Do not calculate "efficiency" by this method as it is wrong

  4. Date: July 12, 2018 at 12:03

    Actually the benefit of smps is that its idle current is very low and doesnt heat when idle. But in transformers for example like 5 amp its idle current will be very high as compared to smps and will heat up!

  5. Date: July 12, 2018 at 17:14
    Author: Adrian Bertossi

    The double diode that appears in your schematic diagram with a question mark is : MBRF20100CTG

  6. Date: July 21, 2018 at 09:58

    Thank you and well done! very good job. just i am wondering how you measure the power in `watt` with yellow multimeter setup at V~ and you said when is loaded power input drawing is 71.1 W ?
    Could you please elaborate on the issues more and to measure Quiescent Power Consumption.
    I know you are busy, so I really appreciate your time and effort. …

  7. Date: July 25, 2018 at 07:37
    Author: Big Foot Dave

    Excellent video on flyback switching power supply. I have 42 years of experience in the electrical and electronic field from power plant, consumer electronics, computer repair, aerospace CNC/NC maintenance, 17 years repairing and maintaining light rail commuter trains, attended 5 years of college of electrical engineering and numerous vocational and military schools. After saying that I want you to know you have provided answers to detail questions concerning switch mode power supplies which I have not been able to find. I have been working with switch mode power supplies for the last 36 years and have never seen this subject explained in detail and confidence the way you have done and now it’s all perfectly clear to me now from an design, engineering and repair point of view. The snubber network, continuous mode and discontinuous mode, the zener protection and the secondary regulation via 431 transistor using the voltage divider network is all perfectly clear to me now. Thank you very much for this video and keep putting out the videos. Knowledge and understanding of the buck, boost, flyback switchmode power supplies employing the principle of (Pwm) pulse width modulation is essential for today’s Technicians and engineers. Good job and I will be following you.

  8. Date: July 26, 2018 at 21:16
    Author: Godfrey Carino

    Connot even listen to this mans voice his english accent is very funny?????????

  9. Date: July 29, 2018 at 15:50
    Author: defaultuser000

    I didn't know you had a YouTube channel. I've built a few SMPSs with the information I learned from your website.

  10. Date: July 31, 2018 at 17:22
    Author: Ajay Verma

    How can we correct our 12 v 5 amp smps supply it comes only 9 volt and op is fliker, no preset for setting

  11. Date: August 10, 2018 at 01:12

    I have this same power supply.
    It is ok and giving exect output 14v. But when i put some load on it it goes offwat happend witg it?

  12. Date: August 14, 2018 at 08:47
    Author: mark yu 0405

    I also have this model of psu however after a long amount of use the output became 4vdc instead of 12vdc, could it be the transformer?

  13. Date: August 26, 2018 at 10:52
    Author: Byron Herrera

    Muchas gracias por tan explendida forma de enseñarnos tanto en prueba como teóricamente el funcionamiento de la fuente!!! bravo!!!!

  14. Date: August 26, 2018 at 22:37
    Author: Dominik Gula

    Najlepšie video zo všetkých… Ve?mi pekne vysvetlenie princípu a všetkých detailov… Viac takýchto videí prosím 🙂 Inak videá sú všeobecne ve?mi dobré a zaujímavé, dobrá práca 🙂

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