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0-300V Adjustable MOSFET Transformerless Power Supply Circuit

0-300V Adjustable MOSFET Transformerless Power Supply Circuit

The post explains a simple mosfet controlled transformerless zero to 300 volts variable voltage and current power supply circuit which can be used for many different applications.
To protect against my higher voltage research projects from shooting up in smoke permanently, I developed an easy circuit which is able to render a variable voltage supply of 0 to 330 Volt.

But please be cautioned, the circuit is not isolated from mains potential, and therefore can inflict a lethal shock.

The supply is short-circuit proof: the current is restricted to approximately 100mA.

Circuit Operation

The design does not require a transformer, rather a 100 watt bulb is introduced at the input in order to provide ultimate safety in case of a short circuit or a component failure.

The mains voltage from after passing through the lamp is rectified with bridge D1 (1Amp / 500V) and C1.

T1 is configured as a source follower: the source of T1 complies with the voltage of the wiper of R3. D2 is insured to safeguard the gate of T1.

T2 and shunt resistor R2 establish the current limiter. Whenever the output current results in being excessive, T2 quickly discharges the gate of T1.

This stops the current from increasing any further. The value of R3 was basically identified experimentally; however it actually depends  on the Hfe of T2 which means you may need to adjust the value of R2 appropriately.

Keep in mind T1 requires a large heatsink: in nastiest situation T1 would probably disperse 330V x 100mA = 33Watt!

You may try mosfets such as a BUZ 326 (400V/10.5Amp) or you may likewise use an IRF740 (400V/10Amp).

The output impedance of the power supply varies according to the beta of T1, therefore the bigger the MOSFET, the lesser the output impedance!

Circuit Diagram

Video Proof:

This power supply can be used to obtain a regulated power output, variable right from zero to 300 volts maximum. All the devices should be mounted on heatsinks.

Using a Combination of BJT and Mosfets

Circuit Operation

The next transformerless 0-300V variable power supply circuit diagram can be understood with the following points:As can be seen in the figure, a high voltage transistor BF458 is used as the main load handling device.
Its base bias is controlled by another high voltage transistor BF337 whose emitter is clamped to a stable 24 volts.An FET is used for selecting the base current of the transistor BF337 via a pot of 1M.

This setting adjusts the base current for the BF337 which in turn restricts the main transistor BF458s voltage and current flow to the output.

The input to the circuit may be derived directly from the mains AC after proper rectification and filtration using a bridge network and a 10u/400V capacitor.

The entire circuit is extremely dangerous to touch, due care should be maintained while making and testing this circuit.



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!

85 thoughts on “0-300V Adjustable MOSFET Transformerless Power Supply Circuit”

  1. Good day,
    I really need to lower the mains voltage (230Vac) to 100V but i don’t want to use this power supply as it is now because it’s current is limited to 100mA and i need about 40A. If i put more transistors in parallel and of higher current ratings and remove R2 and T2 i’am sure the current will go higher, right sir?

      • Good day sir,
        I tried this schematic and as i said i used two paralleled FDL100N50F mosfets and a 5W 12V zenner. This is the problem: when i connect a 2000W load such as a resistor to it, the voltage immediately rises to 187V and won’t let me adjust it from the potentiometer. What can i do to be able to adjust the voltage?

        Thank you,

        • Good day Razvan, as you can see in the video, it works smoothly for a resistive load, so it should work for your application also, unless something is incorrect in your design.

          I would recommend testing it with a 220V bulb using a single MOSFET, to check if there’s anything wrong in the configuration

  2. Hi Sir,
    SMPS circuit is design with high frequency transformer and KA5M0***R fet for spindle motor drives but the problem is there so much power dissipation across FET so what step should be taken to resolve that problem.
    best regards

    • H Sam, a power device will start getting too hot if the load exceeds its optimal ratings. Make sure to use an FET whose current and voltage ratings are twice that of the load or the motor rating

  3. Dear

  4. Hello Eng. Swagatam,
    Find below the manner I calculated the wattage:
    Number of Batteries in a pack: 16 pieces of Battery
    Ampere of each Battery: 1300mAh
    Voltage of each Battery: 1.2 volts
    Voltage of battery pack (total voltage used by the cordless saw): 19.2v=16 pieces of Battery x 1.2 volts (Voltage of each Battery)

    Amperage of the tool (total):20.800 = Ampere of each Battery: 1300mAh x 16 (Number of Batteries in a pack)

    Wattage of the tool (total): 399.36 = total voltage: 19.2v x 20.800amps (Amperage of the tool (total))

    My Summary:
    Voltage: 19.2
    Amperage: 20.800amps
    Wattage: 399.36watts

    Eng. as a matter of fact I am amateur in electronics. I started it as a hobby so please I stand to be corrected on the above presentation.

    In case it is correct, kindly modify the suggested circuit to output at least 400watts/20v power. If there are any mistakes please tutor me on it.

    Now concerning the winding of the transformer, is it possible to recommend already made one for me to purchase. Getting the items required for the transformer to DIY is posing a challenge. Many thanks. Danny

    • Thanks Danny,

      When supply sources are joined in series, their voltages will add up, but current specs won’t.

      So in your case the total voltage will be 19.2V but the current will be only 1300mAh.

      You cannot draw more than 1.3 amps (max) from this series battery set up for your saw machine.

      And therefore a 1 amp current would be more than enough to charge this battery series, without issues.

  5. Hi Eng. Swagatam,
    Thanks for the numerous circuits and electronic designs you have freely placed at the public domain for our viewing and experimenting with.
    I need a 19v to 19.2v transformerless DC power supply to operate a cordless circular saw whose battery packs are very dead. I hope you can help with a circuit design. Many thanks

      • Glad to receive a fast response from you.
        Unfortunately the cordless saw does not carry any of the requested information on neither the device itself nor the accessories like the battery pack nor the charger set. I saw nothing too in the manual.
        Here are links to the manuals: https://www.alltradetools.com/pdfs/new/archive/Kawasaki-19.2V-3-to-5-Hr-Battery_840638_eng.pdf

        However I want to open one of the two battery packs to see the voltage and amps of each battery as well the number of batteries in each pack. I welcome any advice from you sir. Many thanks. Danny

        • Surprisingly the linked articles also do not have any reference to the current requirement of the unit. However I assume it to be no less than 10 amps or even higher. Your battery specs might help us to get the required information.

          • Yes, I from what I saw in the battery pack embossed on each battery may be of help.

            Kindly peruse these information:
            Number of Batteries in a pack: 16 pieces of Battery
            Ampere of each Battery: 1300mAh
            Voltage of each Battery: 1.2 volts
            Voltage of battery pack (total voltage used by the cordless saw): 19.2v

            Eng. I hope these current information may churn out something good.
            Many thanks for your time and patience. Danny

            • Yes Eng. but my issue is not a charger problem. The power tool came with 2 battery packs but are all dead. They are very expensive to replace so I have decided to convert it from a cordless to a corded tool. That is I want to use an AC – DC power supply instead of a battery pack. So what I need is a circuit that can provide the 19.2v (20v) and the accompanying respective wattage which I guess is about 400w. Many thanks

            • Sorry Danny, I got lost, I mistakenly imagined that the batteries needed to be charged. I do not have a 20amp SMPS circuit , and even if I had I wouldn’t have recommended it due to its complexity. The easiest option is to get a 14V 20 amp iron core transformer, and rectify its output using 50 amp bridge rectifier and a 2200uF/50V filter capacitor. This would perfectly take care of your 400 watt saw and drive it smoothly without any issues, although the size of this trafo could be too large.

  6. Thanks a lot for this circuit? Mr swagatam, i can apply this in my led strip wit 72 leds, each 3 led will light in 12 volts, in total series of led will consume of 208 volts. i ask one more question is this safe for my back light led tv as a subtitute in broaken inverter? thanks a lot for your reply, GOD bless!

    • Thank you Armand, I am sorry since this design is not mine I cannot suggest anything with confidence. Although others have built it successfully I still wouldn’t want to risk anything by suggesting on assumptions.

  7. Can we modify it to have adjustable current too. Where do i need to put a current limit potentiometer. Using direct potentiometer isn’t suitable as i require 3 to 5 amps power.

    • you can modify the R2 section with a rotary selector switch and attach the desired number of resistors with it, and then you can select one of those for achieving the required current control limit

  8. Hello sir ,
    I need to make a circuit which switchover automatically between 11.1v 8800mah li ion battery and smps 12v 2amp circuit.At the load side I have used stepper motor ,TFT graphic lcd and wifi unit which is powered by smps supply. Is it possible to charge a battery with this smps supply and make switching to battery supply in the absense of smps supply.Charger circuit is also required.I am new to electronics please help me to find out charger circuit+auto switch circuit.

    • Hello Sham, you can try the second circuit from the following article


      please adjust the opamp preset through proper understanding and thinking, otherwise you will keep struggling with it.

      The idea is simple.

      pin#3 of the opamp is the battery voltage sensor.

      when the voltage on this pin#3 goes higher than the pin#2, pin#6 switches to the supply level and vice versa, since pin#2 is fixed at 4.7V implies that as soon as pin#3 voltage becomes higher than 4.7V, pin#6 instantly becomes high, switching OFF the PNP transistor

  9. sir can i ask a favor ? could you make for me a circuit intended for testing a led strip for led tv back lights? i made this circuit above i used a 1:1 ratio transformer it works well even in a single beads with out damaging it but my fear is the electric shock.

  10. hi sir swagatam is this circuit could be use in testing led strip?
    or how to modify this circuit for the purpose of testing led back lights strip without shock

    • Hi Elmar, shock cannot be avoided in this design, it’s better to use an SMPS based power supply instead of this one if shock is a concern

  11. Hi Sir ..Your circuit ideas are awesome.

    I am designing a variable dual power supply circuit with high current output .. I have a 300 W transformer. I was initially thinking of making the circuit with LM317 and LM337.. but it is very difficult to get LM337 in our place. So I have designed the circuit with 7805 and 7905 Ics. By putting them in dual mode with the ground as common I got +5V 0 -5V dual power.. By connecting 2 pots between the Battery ground and the Ground pins of 7805 ( pin#2 ) and 7905 ( pin#1 ) I could tweek the output voltage from 5V to the full 15V range which was exactly my requirement.. The circuit is successfull in delivering the voltage range.. But the real problem happens next I am not able to take enough current from the output. I cant run high current loads with it. To compensate that I have used 2 to 4 no.s of TIP3055 Ics after 7805 regulator and there was improvement in the current. But not completely.. Now I need the help of you to design the current amplifier section at the output of the negative voltage regulator Ic ( 7905 ). I have several 2N3055 NPN transistors and MJE2955 PNP Transistors with me.. please help in this problem.. I have sent you an email comprising the designed circuit

    • Hi RT,

      Your 2N3055 connections are correct and they should be able to deliver high current, but make sure to add a low value (may be a 22 ohm2 watt) resistor with the base of each transistor, otherwise they may get damaged if the load is bigger.

  12. Please the 12v is it as a result of 12v zener diode please how come the 12v and why not use 330v which is the required output voltage

  13. Please sir can u elaborate more on how to calculate the value of R2 because if i multiple the value of R2 being 3E3 by 100mA is not giving me the 330v. And in one of the comments i read u said for 300mA R2 will be 2ohms u get by dividing 0.6/0.3 i want to know the 0.6 please can u give me steps on how 2ohms is gotten
    Thanks and best Regards

  14. Hi
    I'm looking to make a 430V 150ma supply for a tube amplifier. If I use a 350V transformer to supply the input voltage, increase the voltage on C1 to 600V and use a higher rated MOSFET, should the above layout work? What would you recommend as a suitable MOSFET? Any issues that you can see in an audio application?

  15. Hi dear Swagatam
    i need a transformless power supply for my leds which change 230 v ac to 0-35 v or more
    but please simple and without transformer

    • Hi Ronald, hum will depend on the filter capacitor value, if it is properly optimized then the hum can be controlled to the desired limits.

  16. Please sir, what modifications can i make in the circuit above in order to get a regulated 12v, 300mA at the output? And i don't need the light bulb at the input.

    • Jideofor, I cannot suggest much regarding this circuit because it is not designed by me and it can be dangerous if anything goes wrong, especially because it is not isolated from the mains in any manner…

      it's better to go for a capacitive power supply

  17. if you have excluded the current limiting stage then initially try a resistive load with relatively high resistance such as a 40 watt bulb at the load side or a 25 watt soldering iron etc.

    In this situation the input 100 watt bulb can be eliminated.

  18. Hello, and thanks for the diagram. Have tested it and the setup continues to burn the irf740 as soon as I come over about 250DC primary. Has cooling, and tested with bulb at about ½ wattage. Have tried with less zener, but even 10v burns it. What could be the reason?

    • Hello, the circuit won't activate until a load is connected at the output, if it's burning without a load then something could be seriously wrong with connections or the device, make sure that the fet "source" is towards the load.

      and remove C1 initially and check the response without it

      also you can try connecting the zener across the gate and bridge (-) of the supply.

  19. I would need something to provide -70, +70 and ground

    with this circuit i was wondering if I use the ground from the supply along with each output if that would work.

  20. Can i get a circuit like this that can produce dual polarity voltages for various amplifiers from probably 0-100v at about 3 or more amps, the supply in my country is 110v

  21. Hi sir I want to make a variable power supply for 1Amp load.
    My purpose is to test relays and adjusting 555based, Opamp based circuit….
    Please sir help me…

    • hi Swagatam 🙂
      I want to ask.
      1. if the input should be given 100watt bulb
      2. if the circuit is safe to touch the output.
      3. and for how many watts the resistor wear.

    • Hi Muhamad,

      1) yes 100 watt is required for safeguarding the mosfet
      2) No the circuit is extremely dangerous to touch
      3)R2 is 2 watt rest all are 1/4 watt

  22. Hi!

    Thanks for sharing. I'm actually designing a similar circuit but with SCRs to improve efficiency, did you think about doing one like that?

    Kind regards,

    • Hi, yes I have a SCR design using a zero crossing detector stage, but I am not very sure if it would be entirely safe.
      The above design looks more convincing to me.

  23. Thank you Mr. Majumder for publishing this ckt. However I require a perfect ckt. for charging my 2X1.5 V chargeable batteries i.e. a 3V charger directy from 220V AC main supply, those are generally supplied with chinese make re-chargeable torches (easily available in the local market). My two sets has been damaged & I am unable to use them.

    • You can use your cell phone charger for charging the 3V batteries by adding 4nos diodes in series with the positive line. The diodes can be 1N4007 type.

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