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Variable Voltage, Current Power Supply Circuit Using Transistor 2N3055

Variable Voltage, Current Power Supply Circuit Using Transistor 2N3055

In this post we learn how to make a simple variable power supply circuit using transistor 2N3055 and some other passive components. It includes variable voltage and variable current feature, fully adjustable.

Main Specifications

1) Adjustable from 0-30V, 0-60V, and 0-100V, and 500mA to 10 Amp as per user preference
2) Short Circuit Protected when mounted on proper heatsink
3) Ripple free, with less than 1Vpp
4) Output is Stabilized and filtered DC
5) Short circuit LED indicator
6) Overload Protected


A power supply circuit that does not include the features of a variable voltage and current control can by no means be considered truly versatile.

A variable workbench power supply circuit explained in this article is not only specified with a continuously variable voltage control but is also equipped with the feature of overload or continuously variable current control.

Circuit Diagram

variable power supply circuit using transistor 2N3055

How it Works

A keen look at this 2N3055 based variable voltage current power supply circuit using transistor 2N3055  reveals that it’s actually only an ordinary stabilized power supply circuit, however it yet still provides you with the proposed features very efficiently.The voltage variations are made by using the preset P2, through a feedback configuration employing the components D1, R7, T2 and P2.

The inclusion of D1 makes it sure that the voltage can be lowered right down to 0.6 volts, which happens to be the forward voltage drop of the diode.

If any other specific minimum value is required then the diode can be replaced by zener diode having the required specified value.

Therefore in this variable power supply circuit using 2N3055 transistor, the transformer being a 0 – 40 V, the output becomes variable right from 0.6 to 40 volts maximum, that’s very handy indeed.

For implementing the current control feature, T3 along with P1, R5 and R4 are involved.

The value of R4 is specifically becomes responsible for defining the maximum allowable output current.

P1 is set to choose the maximum range within the value that’s marked or identified by the resistor R4.

PCB Design

Parts List for the 2N3055 variable power supply circuit

  • R1 = 1K, 5 watt wire wound
  • R2 = 120 Ohms,
  • R3 = 330 Ohms,
  • R4 = to be calculated using Ohms law.
  • R5 = 1K5,
  • R6 = 5K6,
  • R7 = 56 Ohms,
  • R8 = 2K2, P1,P2 = 2k5 presets
  • T1 = 2N3055,
  • T2, T3 = BC547B,
  • D1 = 1N4007,
  • D2, D3, D4, D5 = 1N5402,
  • C1, C2 = 1000uF/50V,
  • Tr1 = 0 – 40 Volts, 3 Amp

2N3055 Pinout Details

If you have any doubts regarding this variable voltage and current power supply circuit using transistor 2N3055 circuit please do not hesitate to ask then through the comments below.

Original Transistor Power Supply Diagram:

The above design was inspired from the following circuit which was designed and presented in the elektor electronics magazine by the elektor engineers:

Simplified Variable Power Supply Design using 2N3055 and 2N2222 Transistors

The above designs were assessed and simplified with more effective results by Mr. Nuno. The revised and simplified design can be viewed in the following diagram:

The design features an over-current shut down with LED indication.

Video clip of the tested prototype:

For PCB Design and other Related Data, you can Download the following ZIP File:

PCB Design for the above Circuit

Another Similar Power Supply Design as Referred by Mr. William C. Colvin is presented below for the viewer assessment:

2N3055 power supply 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!

184 thoughts on “Variable Voltage, Current Power Supply Circuit Using Transistor 2N3055”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Good evening sir,
    I want a power supply that can give me from 0V to 50V and 20A. What do i have to change/modify in your circuit so i can achieve that?

    Thank you,

    • Hi man its an awsome circuit but i used different transistor in place of 2n3055 beacose i don’t have that one and all the other components are the same but doesn’t work.Just stays at 19 volts and my input is 20 and when i turn the the pots doesnt change the voltage.Is it of the different transistor or other thing please reply.

      • No i think about the big 2n3055 transistor i have change it with a different one and the circuit doesn,t work how is supose to work.Thanks for fast reply.

      • Thanks Dzoro, can you please give the number of the transistor which you used instead of the 2N3055, I’ll check it out!!

          • It’s an NPN transistor so it should work although not as efficiently as 2N3055, because your transistor has very low gain compared to 2N3055. If the output is not varying there could be some fault in your circuit connections.

            • Yes i know and i found the problem it was the r2 resistor it was conected directly to the positive rail and i corected it and now it works flawlesly and the minimum voltage i can get is 1.3 volts.And one more question if i use mine transistor as darlington pair is the gain go up and also my efficienty.
              Sorry for my english

            • Glad to know it’s working now. Your transistor is high voltage transistor so not ideally suited for this application, still if in Darligton mode the current will go up many times. You can try it.

            • I have other transistors do you mind checking them and tell me which one is the most suitable for this psu.
              Here are the transistors:
              Please help

            • You can remove R5 and replace it with direct link and check what happens

              The voltage developed across R4 due to overload will be divided by P1, and when this potential becomes sufficient enough to trigger T3 only then the current control will kick in. So set up the T3 area to implement this optimally

            • When there’s an overcurrent the voltage at the base of T3 should be 0.7V, then it will conduct and control over load. This 0.7 will come from R4 when overload happens.

  3. good day sir.
    i need a 0 to 60 volt, 0.5 to 100 amp, variable volt and variable current power supply circuit diagram. must have fairly low ripple and if i set the the current level to say 20 amps at 5 volt, the power supply must be able to keep up supplying the 20 amps at 5 volt for +/- 1 hour. i will have to be able to use it at much higher amps/ lower volts and vice averse. volts and amps output must stay at set levels as close as possible with only minimal adjustment for amps as the resistance of anodizing bath changes. Yes, i want to use this power supply for dc anodizing aluminium and as a very small or big bench top power supply. i wind my own transformers, so i can make myself a very powerful transformer for this application. at the moment i use a 0 to 15 volt 30 amp regulated power supply that was given to me, i had to replace the rectifier. but i can not control the amps and when things go wrong, they go wrong with a big bang form available amps.
    please help sir.

      • Good day.
        Thank you sir for this circuit diagram, i really appreciate your help.
        I will compare the components that needs to be changed to what my local electronics supplier have in stock and buy what i need for my application and if i run into trouble, i will ask for your help and advice. The circuit looks fairly easy to build so i think i should not have any problems.
        I am busy building your water level indicator with pump control. i will give you feed back once i am done with the water level project. i must say that i am modifying this circuit a bit to suite my own needs so let us see how good or bad i am with electronics.

        Thank you.

  4. Hi!

    I like to build the power supply, but the PCB layout in the article has text in it, is it downloadable in KiCad format?

    • Hi, the PCB is only for reference, downloading it may not help…you can show the design to a PCB manufacturer who can replicate the design through a software

  5. Hello sir Swagatam,
    I have been able to build the power supply successfully using the following:
    – 60V transfo
    – Darlington pair of T1
    – 0.4ohm for R4
    – 5K pots for P1, P2
    – 10K, 5W for R1.
    Other components are the same with what is in the schematic.
    Initially when I turned on the power supply after completion, it worked very well. I got 1.2V to be the lowest voltage and got 44V as the highest voltage (this is due to the mains supply voltage which wasn’t up to 220V). I varied it back and forth and it worked well. Then later, I turned it on again and saw smoke coming from either the 1N4007, one or two of the BC547 or R4 (those components are close to each other on the board). I couldn’t say for sure which of them smoked. When I turned it on again, none of those components smoked but R2 burnt out. I changed the 1N4007 and changed R2 to 1W, but it burnt out again. I changed R2 for the third time and it burnt out again.
    Now I am confused and don’t know which component(s) could be causing the problem.
    Please I need you to help suggest which of the component(s) I need to change or how to troubleshoot the problem.
    Anticipating your usual prompt response. Thank you sir.

    • Hello Godson, as you can see R3 is in series with R2, so R2 should not burn. It may perhaps burn if T1, T2 are burnt, just make sure T1/T2 are still OK in your circuit. Alternatively you can try increasing the value of R3 to 1K and check the response

      • Hello sir Swagatam,
        Thanks a lot for the reply. I thoroughly checked the circuit and discovered that one of the 2N3055 that I used as Darlington pair was faulty, so I removed it, and used only one because I couldn’t immediately purchase another one. Surprisingly, it worked well. I also changed T2 and T3 to 2N2222, and R3 to 1k. Everything worked perfectly without any of the components heating up or burning out. I got above 50V as the maximum voltage at the output. With the 5k pot that I was using, the voltage change was too rapid and I couldn’t get some precise values easily, so I decided to change the pot to a 10k multi-turn pot. The voltage change was now much slower and I could get precise values easily. Surprisingly, I was able to get up to 72V at the output.

        I will like to eliminate the current-varying pot and its associated components because I don’t really need them for my application, and also, it appears that that part isn’t working well (from comments above). I just want to get the maximum current that is being set by R4 and the transfo. So my question is, which components do I need to eliminate?
        Anticipating your usual prompt response.

  6. Hello sir Swagatam,
    Thanks so much for this schematic. You recently referred me to it but I’ve not been able to build it because I hadn’t got all the components. I recently got the components, but some are different from the ones in the schematic. I need you to please help me to verify if the components that I bought will work fine.
    1. I need up to 60V at the output, so I got a 60V, 3A transfo
    2. For R1, I got 1K, 5W, and 10K, 5W resistors
    3. For R4, I got 0.39ohm, 3W
    4. For D2-D5, I got 1N5401
    5. For C1 and C2, I got 1000uF/63V
    6. For P1 and P2, I got 5K pot. I couldn’t find 2.5K pot.
    All other components are the same with those in the schematic.
    Kindly tell me if those components will work fine and give the desired result. Thanks in anticipation.

    • Hello Godson, all the specified components will work satisfactorily, preferably try to use Darlington pair for T1, and you may increase 1K upto 10K if possible

      • Hello sir Swagatam,
        Thanks a lot for the reply. I am working in it now.
        I would like to include digital voltmeter and ammeter and also an LED indicator to show when power is on. Connecting the voltmeter is straightforward.
        How do I connect the ammeter?
        How do I connect the LED knowing that the output voltage is variable?
        I am using a Darlington pair for T1 as you suggested. Will they be heatsinked?
        In the main specifications above, you wrote that the circuit has a short circuit LED indicator. It is not shown on the schematic.
        Please reply. Thank you sir.

        • Godson, the ammeter can be simply connected in series with the load positive wire.

          In can connect the LED parallel to the voltmeter, but with a rather high value resistor such as a 15K or 22K.

          T1 will always need a large heatsink, even if it is a Darlington.

          the diagram suggested by Mr. Nuno has the short circuit indication feature.

  7. DearSir,
    Assembled the circuit Transformer used is 80V AC @ 500 ma. MJE13009 BJT used for T1 and 13003 BJT used as its darlington drive.T2 & T3 used are also 13003 for the high voltage input.R1 tried from 1K to 3.3K/5W. R2,R3,R4 as specified but 2watts used.I have the following observations.Pl. guide in details.
    1> R1 gets very hot & inturn T2 also gets hot. 10W of R1 also gets very hot after 3~ min.
    2>There is no response for current control. I have checked the circuit N-times as per your circuit.
    (2ohm/3w for 300ma output)
    3>When there was overloading (24v,700ma dc motor) R2-120ohms burned alongwith T2.[Not able to confirm whether it
    was due to overload or the growing hotness of as mentioned in point no. 1].
    4> 2K pots give output upto 65~68v Dc,10K pots are giving the desired 100Vdc output.Why so?
    I would like you to help specify the values of R1,T2 for this 100v circuit.Why current control is not responding & lastly how the overload feature functions. And Sir one last querry which shortcircuit protection circuit can be used for such a high voltage.
    P.S. My circuit for 0~40VDC @3amps is working fine ,Hence I am baffled with 100v,500ma circuit.

    • Dear Prashant, If you have used a Darlington pair then the R1 can be increased to a higher value, may be upto 4k7 5 watts or even 10K 5 watts, once this is done T2 will be safeguarded, and will not become hot.

      Current control idea very straightforward. When overload is detected or over current is detected, the voltage drop across R4 reaches to a point which becomes enough to trigger T3 which in turn shorts T1’s base emitter and stops it from conducting any further until the situation is corrected.

  8. Dear Swagatam,
    The circuit works very fine upto 50v .But now a requirement has arised for to increase the voltage as 0~100vdc & current from 0~500ma max. 1K/5w became very hot and T2 burst. 2N2222A also gets very hot.I have used 2N3773 from my stock[160V transistor as 2N3055 is only rated at 60VDC]. My Transformer is wound at 80V/40VA at 230V AC. Pl. clarify.
    1>Is it ok to use 2n37733 as T1 for a variable output of 100 vdc ?
    2>What is the value of R1 to be used in case of 100~110 vdc at input ?
    3>What should I use in case of T2 for the above output ?
    4>What will be the effect of 5K pots as 2K2 are not available for the above needs ?
    Kindly help Asap as the project is in incomplete state.
    Thank You !
    Prashant Dalvi

    • Dear Prashant, you can use any 150V transistor for the purpose. preferably use Darlington, 2N3773 will also work.

      try the last diagrams, because they have used Darlington topology for T1, which makes sure that R1 can be a low wattage and high values resistor

      R8 must be included otherwise T2 will burn

    • I have updated the original diagram from elektor electronics magazine at the end of the post, which you can refer to for your analysis…

      However your design also looks good to me, except the current control stage, which according to me can be made even more reliable by a little modifications.

      I’ll try to do it soon and post it under the above article, then you can try this new version of your design.

  9. Hi:
    I’m sorry about my poor english. I made a test circuit on a breadboard exactly with the same values of schematic. The Vcc IN is 40 V / 2.3 A and i used 0R27 5W on R4. It worked fine, but I have some observations:
    1) R1 (1K 5W) goes hot, very hot and T2 gets a litle hot to. Is it possible to use a darlington configuration for T1, increasing R1 and reducing power dissipation?
    2) VCC Out is 8-38 , burt i think that D1 should make a drop to 0V6. Is it correct?
    3) It seemns that P1 has no control over current. I concted a 12V 1A DC motor and manipulating P1 there is no diference at output.

    I apreciate an opinion.
    Best regards,
    Nuno Figueiredo

    • Hi Nuno,

      you are correct with your analysis.
      R1 does get hot, therefore it must be a high wattage. Not sure about T2, since I did not check this in my prototypes.
      D1 does make a drop of 0.6V and in my prototype I could get the output voltage to go as low as 1V

      yes Darligton would allow R1 to be of higher value and avoid its heating up.

      However the current control feature for me too did not seem to show any response, I am wondering how it is supposed to work? As far as I can imagine, the drop across R4 would trigger T3 which would force T2 to conduct more so that T1 base conduction is inhibited…but this action doesn’t seem to be working effcetively.

      I have updated the original diagram in the article, tested by the elektor electronic engineers, for your reference and for analysis by all other readers….

      • …sorry, for the current control I think when T3 is triggered it will short circuit T1 base with its emitter and this would stop T1 from conducting, this makes more sense…

      • Thank you for the feedback
        I’ll try a darlington with 2 3055 (only because I have both), recalculating R1; |I’ll also change t2 to 2n2222.
        The current limiter i intend to study and understand before make changes because I’m not a pro, just an hobbist that loves electronics.
        Best regards.

        • OK, that’s great, but using 2N2222 also for the Darlington will look much better because two 2N3055 can occupy a huge amount of space and also the response will be not as good as using a 2N2222.

          I wish you all the best…

          • Hi, mr. Swag:

            I followed your advice, because also had more than one 2n2222. Reserching on the web, I made a modification on the circuit and got an efetctive short circuit protection device but, with few components, I couldn’t make a control of current limiter. I’d like to send you a schematic but there is no email here. As you have my email, in case you wish, send me an email and I’ll replay with schematic.
            Basicaly, I made a darlington in T1, increased R1 to 3K3 and it get cold. I also extracted R3, R5, P1 and R6 (base of T3). T3 became also a 2n2222 and it’s colector was linked with base of barlington; it’base was linked with emiter of t1; it’s emiter to +V Out (anode of D1).
            I also created an led device to advise if it is in short circuit: a resistor (calculated by ohms law – [vin-vled]/iLed. In my case 10K), a let and a npn general purpose (bc 547 it’s good). The resistor was linked in colector of t1 and the outher side to led’s anode. Led’s cathode to npn’s colector witch it’ś base is to emiter of Ti and it’s emiter to Vcc out (D1 anode).
            I’m not sure if I was clearly. I tryed this circuit with 40 V cc and got 0.8 until 38V with limit of 2.3 amps. In case of SC, led becames bright and voltage drop to ~0 Vcc.
            (BTW: i discovered in my first test that couldn’t reduce vcc min to 0,6 because de cable I used to conect d1 to pot was damaged.)
            I hope this information

            • That’s superb Nino, please send it to my email, I’ll be most happy to check it:



              By the way my email details are provided at the end section of the right sidebar or in mobile right at the bottom of the page, inside “contacts”

  10. Hello Sir Swagatam
    Thanks for your tireless generosity educating and sharing your knowledge and insights.
    Could you please explain how in the above circuit the value of R4 is calculated using Ohms law; i.e. how is the figure of 0.6 arrived at, and by using this calculation what will the short circuit current be at any given voltage setting?
    E.g. 24V, R4: 0.15 Ohms for max 4 Amps!
    Many thanks.

    • Thank you Husmukh,

      the value 0.6 is the optimal conduction base drive voltage for T3.
      for the shown calculation the short circuit current would be 4 amps.
      However this will depend how P1 is set…if P1 adjustment is set at the opposite end, then the 4 amp will not be effective, and the circuit would be operating at the full input current level

      • Many thanks for your prompt reply.
        So would the inclusion of say a 1k Ohm resistor between P1 and the output of R4 prevent the accidental short circuit overload of of T1?
        Or, if the load V, I, is not that critical, then could the value of R4 be set higher to say 4 Ohms 5W or higher for a ‘child-save’ operation?
        I’m building this for the kids to play with and would like it to be absolutely fool proof so they can’t blow it up, but use it to learn about electronics.

        Kind regards!

  11. Hi Swagatam, I need a 100A power supply, i am using a microwave oven transformer with a welding cable as a secondary and some 100A rectifier diodes, years ago i attached it directly to 3x 40W laser diode bar arrays in series, while it was 5 volts (voltage drop across each array is ~2V) the current was too high and it fried all three (i had no way of measuring current, multimeter only went to 10A.) my question is can i use this NPN BJT:

    in place of T1, T2 and T3 instead of a power mosfet across T1? the highest current power mosfet i can find on ebay is 75A which is less than what my new 100A laser diode bar array requires. Or can i use two power mosfets in parallel across T1?

    I am also waiting for 4x 200W 1k wirewound potentiometers to arrive and some more transistors so i can put the pots in series with a toggle switch to switch from low range to high range to get an equivalent of the 2.5k pot you specified. (the wirewound pots only go up to 1k so i'm putting them in series pairs with a 500ohm resistor for the range i don't need)

    i figure R4 will be .6 milliohm according to your formula, so i bought two 1 milliohm wirewound resistors to put in parallel for .5milliohm = 120A max. i have bought 100W wirewound resistors for all the values in your diagram.

    I have a Nd:YAG laser crystal i'm converting from arc-lamp pumped to diode-pumped for power conversion efficiency reasons, it's application is a direct metal laser sintering machine, i have an f-theta lens, mirrors and collimating lenses and i programmed a .DXF file parser in java processing that sends coordinates and laser on/off commands to an arduino DUE controlling a MCP4822 DAC with op-amps, which sends bipolar analog signals to my galvanometers. I have not made the build chamber or powder dispenser yet but after this power supply i will start buying the stainless steel i need.

    eagerly awaiting your response, i've bookmarked this page and will continue checking for the next month or so while i wait for my packages to arrive. your expertise means the world to me.

    • Hi, I am not sure whether T2 and T3 can be replaced with a mosfet or not, but T1 perhaps can be replaced with the mosfet, you can try this with a low current version without changing T2/T3 values, if everything works OK then you can add more mosfets in parallel to increase the current output.

      By the way you can also experiment with the following easy concept first, here you can easily replace the BJTs with mosfets:


    • thanks heaps for your quick reply, and i was only meaning to use the mosfets in combination with T1, i was a bit hazy on whether use the high current BJT i mentioned in the first link for T1, T2, and T3 or if the BC547B you specified will still work for T2 and T3.

      sadly the variable voltage alone is no good for my purposes, as i need to vary the current as i only have 1 working 60W laser module left which i will use at slower scan speeds. I will be upgrading to the 100W ones later by replacing the emitters i have soldered to the anodes with new ones, on alibaba they're $30 – $50 each but there's a minimum order quantity of ten. Then i will need to change the voltage from 2 to 6V, and the current from 60A to 100A. There is a 50% conversion efficiency from watts to light watts and a 30% conversion efficiency from 808nm to 1064nm through the Nd:YAG crystal, so i'll need 600W. also going to have three groups of three 100W emitters that are pulsed alternately, going to use three more of those BUT30V BJTs, a 555 timer, a 2 bit shift register, some EXNOR gates and some AND gates for that.

    • You are welcome Ananke,

      OK in that case you can try the design which is explained in the above article, I think you should go with BJTs for T1 instead of mosfets, this is to help reduce complications.

      You can replace T1 with 15nos of TIP142 in parallel (on heatsink)

      This will call for R1 to be beefed up appropriately in terms of wattage, although the value may not require much change due to the darlington nature of the TIP142.

      and yes when the circuit is dealing 100A, then T2/T3 might also need to be upgraded for better safety. You can use 2N2222 for T2/T3.

      Please let me know if you have any more doubts regarding the application, I'll be happy to help!

    • Excellent I'll try that, I'll let you know how I go in a month or so when these things get delivered. I've bought a 1k 100W wirewound resistor for R1, do you reckon it will suffice?

    • When finished I'm going to try using magnetite instead of metal powder in the powder dispenser and a reductive gas such as carbon monoxide instead of inert gas like argon, to selectively reduce and melt the magnetite ore into something like pig iron, with wire bristles on the leading side of the powder dispenser which comes before the powder recoating blade to brush off any slag. I have a few refrigerator compressors I got cheap at the local metal recyclers for drawing a vacuum on the build chamber before filling it with carbon monoxide, and for compressing it into a second tank for impure CO gas with CO2, which can be reduced back to CO using hydrogen gas and electrolysis with a zinc cathode, probably using the same power supply as this one.

    • sure, thanks! 1K 100 watt should be more than enough for R1.

      You project looks very interesting, I only hope the design fulfills your project requirement,

      do keep us updated.

      Wish you all the best

  12. Yeah i´m pretty sure that the 2N3055 not genuine, so maybe they are all super dodgy. I know that the 2N3055 also comes in a TIP package, maybe i could try one of those. Oh and by the way my R4 is 0.3ohms 50W! (yes 50W, its huge). so if i have done the math right it should be limited to 2 amps? but for some reason T1 shorts out internally causing the output of the rectifier to go straight to R2 and T2 and then it burns. (this is my current theory anyways)

    • yes that might be possible, the TIP version are usually more reliable.

      for the max output current you can simply use Ohm's law to find out how much the resistor can handle….

      yeah..if T1 is dodgy then there's no way you can save your circuit or any of the other associated components.

    • Ok i have done some more troubleshooting now. When i increase the resistance of R4 to something a lot bigger like 24 ohms then i am able to adjust the current. but any low value resistor i try the current limiting just isnt working. and if i short the outputs now it alwauys just lets 2 amps through with no option to limit it.

      is there any other resistor i can change for this to work with the normal R4? if i change R3 or R5 will that have any effect?

    • You can try eliminating R5 entirely and see if that helps, although the circuit should have worked with its existing form, still you can try the above modification and check the results.
      Alternatively you can use a Darlington for T3.

  13. I just soldered together this circuit, and when i first tried it out my R2 burned instantly. I found out that it was a dodgy 2N3055 that caused that to happen. So i switched out the 2N3055 and now the voltage adjusting is working. But the current limiting is not. I tried to just put a high watt 24ohm resistor across the output and then turn the current pot but nothing happens. and if i short the output then it destroys the 2N3055. Can you make a educated guess as to what may be causing this? i dont even know where to start troubleshooting this. Thanks

    • As explained in the previous comment you can connect a high watt bulb and then adjust the current pot and check the illumination level on the bulb, it must vary accordingly.

      The 2N3055 is never supposed to burn even if the output is shorted because the current limiter resistor is in series with the emitter of the transistor, which ensures that the current never exceeds the transistors max limit, ofcourse this limiting resistor must be calculated such that it limits anything above 8 amps, and the transistor must be mounted on a large heatsink.

      When I constructed this circuit it started working immediately with no issues,and since then I used it for many many years with absolutely no maintenance and all technical negligence, yet it showed no signs of getting damaged, ultimately I had to dispose it off in a good condition, since I wanted a new one with more features.

      in your case pleae make sure all the BJTs are good and the connections are perfect, if possible make it on a veroboard and never on breadboard, because breadborad will always produce messy and confusing wiring making troubleshooting difficult.

    • Di, how can R2 burn when R1 is present in series with the positive supply…check your circuit you might have done something incorrect.

    • yes sir i change my 2n3055 then r2 safe but Current can't adjustabe i use 3 Amp and current output 2,5 A what i must do sir? thank for respon

    • Is the voltage adjustment working?? first check this,if not then your circuit some fault….if yes then you can proceed with the current checking

      check current by connecting a 12V car bulb at the output and a n ammeter in series with the car bulb and then adjust the current potentiometer to see the response in the meter and the bulb brightness

  14. Dera swagatam thanks for all ur kind. But im still have problem adding volt amp meter digital on this schematics. Owh i like the calculator on this mobile version but i cant find the search bar on mobile version. Ow one more thing can you help me how to make smps power suply with overload short circuit protection variable volt 0-100 and 0-100amp using the simplest component cos its so hard and expensive to buy regular transformator. Ty very much

    • Dear GK, you can connect an ammeter in series with the positive output of the power supply, meaning in between the positive line which connects with the load positive.


      voltmeter can be connected directly across the (+)/(-) output terminals or simply across the lad terminals.

      100V, 100A SMPS will be extremely difficult circuit…actually I already have this design in my blog titled 100 amp welding inverter circuit

  15. Good day again, Sir. Just found out that the transformer that I have has a rating of 24V 3A, would that still be ok? My output voltage requirement is just 0-20 or 0-24v 2A. What modifications do you recommend for the lower ac input. As always, thank you very much for your time.

  16. Hi mr.Swagatam
    I made this circuit using 12v 8amp transformer.And the C1 overheat and smoked.The output is about 1.2v.Nothing happen when i turn the pots.I use same part with your part list.Can you help me mr?
    Thanks a lot

    • Hi Romeo, not sure what might have gone wrong? however a capacitor will blow if the supply input is higher than its rated breakdown voltage rating…

      make sure the 2N3055 is a genuine one….

  17. Good day sir!!
    I will test this on 12amp trafo,
    But i think the C1 Capacitance is too small, should i increase the capacitance value??

  18. Good day Sir,
    I will be using a 30V 3A transformer, my desired output is just 24V 2A, what modifications should I do to the circuit? Thank you so much.

  19. Hi Sir,I want to make this project,but i have a few question:

    I didnt find the 50v 1000uf caps, can i just paralelling 2 500uf or 5 200uf?

    I didnt found BC547B can i just use 2N2222 or any other suggestion for the transistor sir??

    I will test this circuit on 10 amp output transformer, should i increase the C1 value?
    Thank you very much sir

    • Hi Raihan,

      yes you can use capacitors in parallel for achieving the shown values

      use a couple more transistors in parallel for achieving 10 amp.

      make sure each of the transistor has its own separate base resistor.

      mount them over a common heatsink

  20. Nice blog sir!!
    I have transformer with output 50v 5amp .Can above circuit handle it?
    Do i need any modification?
    Thank you sir!!

    • Hi, you can connect more numbers of 2N30055 in parallel with the existing one and reduce the values of R1,R2,R4 appropriately, for increasing the net output current.

      use a common heatink for them without any mica isolator

      No difference except that the above is a single supply and the other one is a dual…

  21. To keep thing simple, and the parts count low, I think it's better to replace 2N3055 by some darlington transistor: like TIP142. It is V(br)ceo 100V, Ice 10A, initial Hfe= 1000. And what's really good in our cace, the Hfe rises with collector cureent rise. Such, Hfe@3A=3200, Hfe@6A=4000, than Hfe going down, but still OK – 3000 at 10A.

  22. Here you suggesting that PSU for 50V 10A
    I prefer to read component's datasheets and appnotes instead of comments. This is far better source of correct information.
    The numbers above – just paper and pencil math. Not more. But flaws pretty visible.
    I'm working with LTspice and ISIS Proteus about 4-5 years. I used OrCad from 2003.
    Yes, there are cases, when simulator can be wrong: SMPS, self oscillating generators, UHF. But I never ever saw linear DC circuit, that filed on simulator, but shined on prototype board.
    PS: Even if that circuit was flawless on paper and simulator, it requires custom made transformer, otherwise, there will be a trouble with heat dissipation.
    For general purposes, 0-30..50V, 3-10A output, Buck pre regulator + Linear post regulator is the way to go. Here SMPS will reduce input voltage to 1.5-2V above Vout required. Than good linear regulator will add transient response, and reuse SMPS residue noise and ripple products, with total heat dissipation around 15-30W.

    • Yes, the title of the linked circuit is slightly misleading.

      Actually I tried to provide an easy dual power concept which could be modified by an user for getting higher current, perhaps by changing the transistor specs or by using a Darlington version.

      Alternatively the 2N3055 transistor could be reinforced with an outboard PNP current booster for enhancing current, so it is possible by some thinking.

      The same can be done with the circuit explained in the above article.

      As I said earlier simulators are only for those engineers who know exactly how to operate it, for new hobbyists it could give confusing or incorrect results.

      Efficiency is not an issue here, a quick and easy design with maximum features is the main idea behind the proposed circuits

      I do not prefer simulators because I trust my brain more for simulating my designs.

      By the way the above power supply design is not my design, it was done by the elektor electronics engineers.

  23. you say that circuit is correct and working, and simulators are wrong…
    let's take a look at datasheet of 2n3055. we are interesting about DC current gain.
    At at3A gain is about 40, at 10A the gain falls to 10-12.
    So Ib for 3A Ic should to be 3/40=0.075A at least, and 10A/10=1A Ib to put 10a to load.
    The power transistor biased via R1 R2 with Rtotal 1120 Ohm. Lets assume Vin=40vac*1.41=56Vdc. That network capable to transfer 56V/1120Ohm=0.05A to base of T1.
    0.05A*40..50=2…2.5A output capability. And all above not taking in account current limiting circuitry.
    now let's imagine you loaded that PSU with 3V 3A at output. Assuming Vin = 40VDC, than
    40-3=37V drop accross T1. Pd at T1=37*3A=111Watt – you are almost out of 2n3055 SOA

    • I have tested and used the circuit for 5 years without problems, instead of complaining why don't you just build it and test it yourself if you are capable of doing it.

      I never said this will would supply 10 amps, in fact I said it won't.

      read the comments properly and then shout!

      and yes simulators will give stupid results unless you know how to use it, simulators are meant for experts who have a sound knowledge of practical as well as theoretical electronics.

  24. And sir, how to calculate the R4??Does the P1 and P2 need calculated too??if yes how to calculate it?Thanks sir!

  25. Sir, i have 50v 10amp tranformer.i want this project max output will be 50v 10 amp too.Will this project need mods for my transformer?if yes, please tell me what shoukd i change.Im a beginner.Thanks sir!

  26. Sir, I have a 18-0-18 volts 3 Amp,Transformer, I want make this project, can I do this, and please tell me what value of Transistors,Diod,Resistors, and Preset Resistors is required. Please Sir calculate the value and kindly tell me…..because I am begginer in this course…this cerkit is very much required…..Thank you Sir

    • Hi Arun, yes you can use 18-0-18 transformer and no changes would be required for your design…you can make it exactly as recommended in the diagram….

  27. hello sir swagatam,i want to regulate an output of 25 volts,24 amp transformer which is available in mostly in our local shops here.is it ok to use this bridge rectifier that i bought? it's a KBPC3510W that can handle 35 amp and 1 kvolt which is way higher than 25 volts.and lastly for calculating R4, 0.6/24=0.025 ohms and 0.6*24=14.4 watts which i couldn't find any where, any suggestion for this? and by the way i'm gonna use TIP35C in replacement for 2N3055. i look forward to hearing from you soon thank you

    • Hello Jerico,

      you can use 4nos of 0.1 ohm/5 watt resistor in parallel for achieving the 0.025 figure.

      TIP35 will do in place of 2N3055, just make sure to mount on a big heatsink

  28. hi Swagatam, i build it, but i can't get it to regulate voltage well, it only goes down 15% of the voltage i suply. i built it several times but i can't get it to work, any tip of what i may be doing wrong??

    • Hi marsonob,

      It would be difficult for me to judge the fault just by assuming…

      you can check the diode D1 polarity, may that's causing the problem…not entirely sure though

      if you have built it many times and still you are not able to succeed then that's extremely strange, because I could make it work at the first go..and it worked forever for me until I myself dismantled it.

      • hi Swagatam, thanks for replying!! i couldn't make it work on the breadboard so i solder it to a pcb and the voltage regulation got fixed (probably i connected something the wrong way) but i still can´t regulate current! the only thing i change is R4(2x 0.1 Ohm resistors in parallel) and i can't get more than 0.8A on the output. everything is connected the right way, what could be wrong?

        • I am sorry marsonob, I cannot troubleshoot your circuit it's difficult to judge what could be the fault or which part could be malfunctioning.

          you can do one thing replace the 2N3055 transistor with a smaller NPN such as a 2N2222 or any similar and verify the pot function (without a load at the output)

          if it works then you can proceed by replacing back the 2N3055 and check the same

  29. Hi Swagatam, while I have been researching variable PS I have come across digital units that use a DAC. My question is how would you connect the output of a DAC to control the voltage & current for a PS? Preferably using MOSFET due to high current >25amps. Thanks again

    • Hi Ben, I guess you are referring to the SMPS units, right?

      The DC from and SMPS is no different from a rectified DC from a trafo, in fact the SMPS output will be perfectly stable in contrast to a trafo.

      so it can be used in the same way as one would do for a trafo based DC

  30. Ben, Ry is already present in the above transistor circuit in the form of R4…so you can completely ignore Ry in the LM317 circuit and concentrate only on Rx and the mosfet configuration.

    • OK so I'm clear. The drain is to be connected on the right side of R4 essentially on the +ve output rail. The gate connects the the emitter to the emitter of T1, and finally the source would connect to the collector of the transistor, the input rail? Then do I move R4 to the collector side of T1?

    • I think if the drain is connected on the right side of R4 then R4 would have no effect on current regulation, so I assume the connections to be in the following manner:

      drain connected with emitter of T1

      source connected with the DC supply input (bridge/C1 positive).

      gate with collector of T1.

      and Rx between gate and source.

      Rx could be a 1 ohm. 10 watt resistor to begin with.

      and R4 will need to be appropriately found through some trial and error.

  31. Hi Swagatam, thanks for your post. I'd like to use this circuit to control a set 48v 50amp switchmode power supply. Would there be any other modifications apart from R4 and changing or adding more transistor in parallel for T1? Would a MOSFET be a better option as I would be drawing big currents around 30amps at around 24V. Thanks

    • Thanks for that Swagatam, my obvious question would now be how would I implement both circuits together as I'd like to variably control both current and voltage. Thanks again

    • Ben, from the mentioned link follow the second last circuit and apply the Rx, Ry and the mosfet across T1 in the above transistor power supply circuit.

      Just place Rx, Ry, and the mosfet across the T1 (consider T1 as the LM317 IC)

      the voltage and the current can be varied in the existing way, through the indicated pots.

    • Swagatam, the Ry resistor is still being used in the last schematic, but instead it's now near the transistor. Do I leave it out or follow that schematic exactly as drawn. Thanks

  32. Hi Swagatam,
    As I suspected, the five 2N3055 I bought on Amazon.com (of all places!) are likely counterfeits! It must be a new batch, since nobody has reported on these, yet. I am so PO that I made a YouTube video about it: https://youtu.be/5GbBN0fuhtY . These fakes must have been the source of all the problems with this circuit. I've ordered new ones from Jameco – a reputable supplier – and will try again when they arrive. Thank you and Merry Christmas!

    • Thanks very much MD for updating the info, I am sure this will help the folks around, and stop them from becoming the victims.

      Merry Christmas to you:)

  33. Hi Swagatam,
    In your drawing, have you accidentally swapped R3 and R5? In other words, is R3 = 330 Ohms in line with R2, while R5 = 1.5 kOhms in line with P1? I do confirm another reader's observation that R2 (the 1/4W 120 Ohms resistor) burns almost instantly. Connections are correct.

    • Hi MD, I am sorry everything's correct in the above drawing and R2 can never burn absolutely. R3 and R5 values are correct in the diagram

      I have used the above circuit for 5 long years and never faced this problems.

      Just now I checked it with original diagram which was published in elektor electronic mag, and found everything was correctly configured in the above diagram.

      At the most you can try increasing the 120 wattage to 1 watt, however a 1/4 watt burning indicates a serious fault in the connections which might not help even with a 1 watt resistor either

    • I did. I put nice, fat 10 watt resistor for R2 and it works fine. Can't say the same about the power supply, though. I think I now the answer to this mystery. The reason why yours works so well is because you must be using high resistance loads. Relatively high, that is. If, for example, my load is 600 Ohms then the circuit works beautifully. However, with 10 Ohms load things look differently. I can reach maximum of about 11 V and slightly below 1 A (BTW, I use R4 = 2 Ohms). In just few seconds the T1 = 2N3055 burns out. I checked all CBE currents and voltages and these are well within the transistor's tolerances. Without a better explanation, I suspect thermal runaway – although I have very good heatsink…

    • I am afraid that's not correct, the above design is supposed to be a robust and reliable design and is rated to accept heavy loads up to 5 amps and above, depending on the value of R4 and P1.

      The only things which are supposed become hot are the transistor and R1.

      I have used the circuit in the most rough environments and still nothing went wrong with it….actually the 120 ohms does not need to be a 10 watt resistor, a 1/4 watt is just enough.

      I think you should verify your 2N3005 connections or its quality..there could be something wrong around this device in your circuit.

    • Ouch, you may be right: I bet this must be shitty 2N3005 that I bought on-line. Recently, market has seen some cheap copies of the real thing. It should not be burned by 1 A current! I have to order these from somewhere else and verify that's the case. If yes, then I'll post the name of seller publicly. I bet many others suffer the same problem.
      Where did you buy yours?

    • Actually I never buy from online shops, rather I trust my local retailer more, who will mostly have two variants for the same part, one will be cheaper and the other costlier one, and I make sure I buy the costlier one because the cheaper one could be the fake one.

      For a 2N3055 transistor, the ST make (STmicroelectronics) will be normally the good one, and this can be further confirmed by checking the print quality and the leads of the component…the print quality will be hard etched and distinct while the lead will be much thicker, made of copper and very stiff.

      An example image can be seen below.


      In the online worked too there are quite a few reliable stores such as digikey, mouser, onsemi, etc these sources will most probably never cheat you.

  34. Hello sir , can you tell me what did you use T1 transistor for ? İ dont want just to copy your design but to understand its concepts fully, before going through with it. Can you eleborate your circuit's operation please ?

  35. Hi swagatam, i get a maximum 33v dc and 1.6v minimum in simulation using mutisim? Why is that? I used correct values of the components given above. And also how do I check the current? Where should I connect my meter in the circuit? And are thosecapacitors electrolytic? Thanks and have a good day!

    • Hi William,

      It's because your software is showing the wrong results…may be you can try putting higher values for the potentiometer.

      It should be from 0.6V to 40V approximately…you can check current by directly connecting the meter prods across the output terminals, and by keeping the meter in the 10Amp or 20Amp DC range

      capacitors are all electrolytic type.

    • Thanks for your reply sir! What simulation software did you use for this circuit? I am using multisim, I have doubke checked my connections and values of components given in your schematic but I still wrong results? What other methods should I use to check it? Thanks and have a good day!

    • Thanks Williams, I never use softwares, I depend on my mind simulation.

      the 33V could be due to the heating up of the transistors which might cause some drop in the voltage.

  36. Hi sir,
    I am Pradeep Sajjan, I want to build a power supply of variable voltage (0 to 60V) and variable current(200mA). can u pls send me the circuite design for it. I will be extremely greatfull

    • all ersistors work like potential dividers for feeding the transistrs with the required correct potentials.

      5K will also work but 2k5 pot will produce a neat zero to end calibration across the scale (dial) of the pot, and 5k will finish the max range somewhere in the middle of the dial, not at the end…

  37. thanks Hadayet,

    I think the above circuit can be used for your purpose, just make sure R1 is increased to 10K so that it does not get too hot.

  38. hi, Swagatam Majumdar,
    I am Hadayet, nice blog. need 60 to 70v source to 52v 12A Output volt and current regulated step down converter circuit diagram. If u post the diagram, then i m so proud of u. thx

  39. hello sir. thank you for your quick reply.
    I got a transformer rated 35v 6A. I want to have maximum 4A in my power supply. so i calculated R4 which is 0.15ohm 3watt. is that ok?
    is it any problem to have 6A transformer but using max 4A out of it for the circuit?

  40. hello.
    what types of transformer i can use and what ratings?
    is it possible to use 10k preset with a fine tune preset? then what change?
    hope for a quick reply.

    • hello, the transformer rating will depend on what max current and voltage you may need at the output, if its above 60V/10amps then probably the transistors will also need to be upgraded accordingly

      a 1K fine tune preset can be included in series with P1/P2

  41. This circuit looks to be exactly what i'm after (variable voltage and current) for making an electro-etcher.

    I'd like to be able to find the minimum volts necessary for current flow and then increase the amps without increasing the volts.

    I've been trying to draft this in Qucs (Quite Universal Circuit Simulator). It doesn't have potentiometers so I've just put resistors either side of where the pot would be (their resistance adds up to 2.5k).

    I'm not getting the results expected. Any chance you could help out with parts that will allow up to 24V and 4A?

    • R4 could be calculated by using the formula 0.6/amp, so for 4amp max it could be around 0.6/4 = 0,15 ohms @ 0.6 x 4 = 2.4 watts, ideally this could be a 0.1 ohms/3 watt resistor

      Frankly, I don't believe in simulators, these are like blind persons with sticks, not reliable in most of the cases… the above design has been tested by me, and it worked quite well as per the proposed specs.

    • Thanks for the fast reply. I like to see things simulated before diving in, but I guess I should just break out the soldering iron.

  42. Hi Swagatam Majumdar,

    I´ve searched for a basic voltage and current control PS, as a replacement for my "gone wild" lab PS. As i do want to build it up myself and do it all discrete, yours looks like just what i need…I already got two toroidals each is 2x18V/5Amps. So i´ll do a breadborad and see.
    Thanx for this post,

  43. Sadly transformer rated at 0-40v 3 amps isnt available anywhere in our downtown in Bacolod…the only transformer with rating of 3 amps is in 0-36v. Can I use this? What modifications will I make? please help me..Thanks! 🙂

  44. what I meant in #4 is that you can change the wave of the power supply, like if you want 12v square wave for a siren or a sine wave for lights , etc… and how about the fuse? or short circuit protection?

    • yes wave making circuits are called function generators.

      fuse could be used and must be rated at sightly higher than the specified load current.

  45. Good afternoon sir!
    1. What is the wattage of resistors?
    2. If 3 amps is my max current o/p, do I still need R4? and if yes is my calculation correct

    – 0.6/max current output (which is 3 amps) = 0.2 ohms (and how many watts)?

    3. Can I add fuse and what rating and where will I put it?

    4. and if possible, can you add another feature
    – variable wave (ex. sine, square, triangular, etc.), and
    – short circuit protection

    It's okay for me if the cost rise.
    thanks. 🙂

    • eshkariel,
      all resistors are 1/4 watt except R4.
      R4 is for protecting the load, it may be included if required.
      yes 0.2 is correct, wattage will be 0.6 x 3 = 1.8 watts or 2 watts

      you mean function generator"? you can get plenty of such circuits online, just google "function generator"

    • I am building one like this.
      In your part lists R1 is 1K 5W wirewound, but in this comment section you said that all resistors are 1/4W, which one R1 should be? It is hard to get 5W cement resistor here, is it okay if I wound 10 of 10K 1/2W in parallel to get 1K 5W?
      Then if I want to connect digital voltmeter-ammeter, which has 3 large wire (yellow-red-black) and 2 thin wire (red-black) which is supposed to be independent power when the voltage is lower than the voltmeter power rating, how should they be wired? Thank you

    • the resistors which not specifically mentioned are all 1/4 watt.

      you can use 1K 5 watt carbon or metal, wirewound is not essential. higher wattage resistors than 5 watt will also do.

      yes 10nos of 10k in parallel will do.

      I am not sure about the meter connections…it'll need to be confirmed by you through physical testing

    • Okay. Hope I don't smoked my meter. I will search more information on the net and post back here when I've done. Thanks in advance.

    • It works. I've got 2.18V to 14.15V. Only that my digital voltmeter won't light when the voltage is below 4V. The transformer I used has multiple output, I also try connecting its thin wires to the lower voltage output to get an independent power source, but still can't light up the voltmeter below 4V. I will try another configuration later. All of all, thanks for this circuit.

    • voltmeter should be connected right across the output (+)(-) terminals while the ammeter should be connected in series with the positive (+) output line.

  46. R2 can never burn, because it has protections in the form R1 and R3, please check the connections, something might be wrong.

    I have built and used this circuit for over 5 years.

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