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# 0-300V Adjustable MOSFET Transformerless Power Supply Circuit

Last Updated on November 23, 2022 by Swagatam 216 Comments

This simple MOSFET controlled transformerless power supply circuit can be used for delivering a continuously variable 0 to 300V DC output and a current control from 100 mA to 1 Amp.

To protect against my high voltage research projects from going 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.

WARNING: ALL THE ABOVE CIRCUITS CARRY LETHAL MAINS VOLTAGE AND THEREFORE ARE EXTREMELY DANGEROUS. IT CAN KILL ANYBODY, IF TOUCHED ANYWHERE ON THE CIRCUIT IN POWERED CONDITION. OBSERVE APPROPRIATE PRECAUTIONS TO AVOID ANY MISHAP.

## 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

UPDATE:

The above design could be much simplified as indicated in the following diagram. The bridge rectifier has been eliminated which drastically reduces the stress level on the MOSFET. However, the ripple generated due to a half wave rectification may be significantly higher. The output 10uF filter capacitor helps to reduce this to some extent. The value of this capacitor could be increased to higher levels for improving the DC quality.

The input series lamp can be added, although this may not be required due to the presence of the current control stage in the design. However, for better safety a fuse may be added in series with the input line.

The output load specification must not exceed 100 ma

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.

### You'll also like:

- 1. Simple Voltage Multiplier Circuits Explored
- 2. 1.5V Power Supply Circuit for Wall Clock
- 3. High Current Transformerless Power Supply Circuit
- 4. How an Autotransformer Works – How to Make
- 5. Voltage Regulator Circuits using Transistor and Zener Diode
- 6. Make this 3.3V, 5V, 9V SMPS Circuit

#### About Swagatam

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!

### Have Questions? Please Comment below to Solve your Queries! Comments must be Related to the above Topic!!

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Alastair Clark
6 days ago

Hello again swagatam. I finally have a working circuit using a 6r125p mosfet, 500k potentiometer and sa5.0a unidirectional tvs diode across gate/source. I figured out I was mistaking fast switching diodes with zener diodes leading to most of my issues. In a source follower configuration and using a potentiometer to control the gate. It only gets around 3-4v. Which doesn’t fully open the fet. Is there a way to increase the gate to source voltage? Or would it be better to switch to a logic level mosfet? With the sa5.0a I was expecting 9.2v since that’s the clamping voltage of… Read more »

Author
Swagatam
6 days ago
Reply to Alastair Clark

Hello Alastair,
Sorry, I can’t figure out the issue you are facing with your circuit. In the video you can see that I have already tested the circuit and it worked fine for me.
I am not sure why a TVS diode would be required across the gate/source of the mosfet. According to me it should be a 12V or a 15V zener diode. The job of the zener diode is to make sure the gate source voltage does not exceed the zener value.

Alastair Clark
1 month ago

Okay well that makes much more sense why the failures seemed so random. I knew there was a maximum gate voltage but I just figured the response was quick enough to not have to worry about it. So basically the gate resistor is to save the potentiometer from overpowering itself in this case? And I could base the size of my gate resistor on the size of my potentiometer? Is there any benefit to decreasing the size of the gate resistor? Also when changing the configuration to parallel do I need multiple Zener diodes? And are there any other things… Read more »

Author
Swagatam
1 month ago
Reply to Alastair Clark

The gate resistor is for protecting the zener diode. The potentiometer value is for protecting the potentiometer itself from high voltage and for preventing unnecessary heat dissipation. These resistors are irrelevant to the mosfet if the supply is a constant DC. If the gate signal is in the form of frequency then gate resistor required to be as small as possible. In parallel mode the whole setup works like a single large mosfet, so a single common resistor and a single zener diode should work OK. No other precautions are required for the mosfet except that its specifications which must… Read more »

Alastair Clark
1 month ago
Reply to Swagatam

Hello again and thank you for all of your help. I have a couple Toshiba 2sc5200 transistors available. Would this work as a replacement for bc547?

If so will the current limiting resistor still be calculated the same way?
If you were to add multiple transistors in parallel would this increase the power handling capability of the circuit even further? While still allowing you to control the available amperage?

And if you changed/added a pot to the base of the transistor, would this allow for an adjustable current as well?

Alastair Clark
1 month ago
Reply to Alastair Clark

I just realized the transistor only discharges the gate and doesn’t actually provide any power. Would a 2n222 be a sufficient replacement?

Would the current limiting resistor still be calculated the same way?

And would adding a pot after R2 or replacing R2 with a big enough pot allow for an adjustable current regulation?

Author
Swagatam
1 month ago
Reply to Alastair Clark

The mentioned transistor will not work in place of BC547, since their specifications is completely different.
You can try 2N2222 instead.
Current limiting will still work even if the BC547 is replaced with an equivalent.
Increasing the number of mosfets will increase the current handling capacity of the circuit proportionately. The BC547 would still control the current normally.
Adding a pot at the base of the BC547 may be possible, but the control will not be accurate. It can done as explained in the following post:
https://www.homemade-circuits.com/simple-current-sensor-circuit-modules/

Alastair Clark
1 month ago
Reply to Swagatam

Thanks again for all of your help. I’m still having issues getting my head wrapped around the use of mosfets. Specifically the source follower diagram is using an npn in a high side configuration vs the suggested npn configuration using low side. Everything I’ve read said that an npn should be used in a low side configuration connecting the ground to the load. Because you need a higher voltage at the gate than your supply. And in a low side configuration you don’t. Another question regarding the zener diode. And the configuration of the mosfet. The transformer im using also… Read more »

Author
Swagatam
1 month ago
Reply to Alastair Clark

The high side, low side and bootstrapping issue arises when the drain/source of two mosfets are connected in series. But in our example there is only one mosfet, which is configured as a source follower. Source follower configuration makes sure that the source voltage follows the gate voltage, so the gate voltage can be varied effectively to vary the source voltage (load). That is the main benefit of using a source follower in a variable power supply like the above. You can use a 10V at gate and 100V at drain with common ground but then how will vary the… Read more »

Alastair Clark
1 month ago
Reply to Swagatam

Okay starting to make sense of things now. So thats why pnp transistors have their source in different position. That definitely helped solve quite a bit of my confusion. I asked about the separate voltage because I remember seeing others use this method with an arduino or a separate power supply for high voltage applications. I’ve also seen extremely basic examples using an irf540, 100k pot, and 10k gate resistor. And that’s it. This was also for a 0-100v regulator. Ideally I’d like the simplify the circuit until I get a better understanding of it. I’m slowly getting there… Is… Read more »

Author
Swagatam
1 month ago
Reply to Alastair Clark

Actually I was wrong, the 10V on gate and 100V on drain cannot be use and will not work. Because with 10V on gate the source will generate less than 10V regardless of the 100V on drain. So the load will get a voltage less than 10 V and thus cannot be operated. This is how a source follower is supposed to work. It follows the gate voltage and generates a source voltage which is always lower than the gate voltage. Zener diode cannot be replaced with a resistor, The zener diode makes sure that the gate/source voltage never exceeds… Read more »

Alastair Clark
1 month ago
Reply to Swagatam

I think I’ve figured out most of my issues. Here’s the post I referenced. And I had to go searching through the data sheet to figure it out. I’m using 2 13n50c mosfets. The body diode allows positive biased voltage to flow from source to drain. And I was not able to turn off or adjust the mosfet, similar to how you just explained. So instead of positive voltage from gate to source. I ended up with an extremely voltage from gate to source. And the mosfet latched to power on mode. After I turned the circuit off I was… Read more »

Alastair Clark
1 month ago

Hey, I’m working on a 100v adjustable power supply based on the 0-300v circuit and I’m having some issues. I’m powering the circuit with full bridge rectified ac from a 72v transformer. And have 2 – 330uf smoothing caps before the fet. Originally I was using too small of a potentiometer. Currently I’m using a 500k pot with 2 – 0.5w 1M resistors in parallel for the gate resistor. This confused me quite a bit since most of the data sheets and circuits I found had a really small gate resistor? I’ve blown up enough pots now to know that’s… Read more »

Author
Swagatam
1 month ago
Reply to Alastair Clark

Hi, thanks for the question, I will try to solve your problem. A small resistor at the mosfet gate is relevant only when the gate signal is a pulsating frequency. If the gate voltage is a constant DC then the value of the gate resistor does not matter. Moreover the potentiometer value has to be high in order to safeguard the pot from high voltage. Another thing is that MOSFET gates cannot tolerate voltage above 20 or 30V across their gate/source terminals. Therefore a zener diode is a must across their gate/source pins of the FET if the supply is… Read more »

Tinu
3 months ago

Hello Swag, well done Sir, please can I convert 60V from 5battries of 60ah each to 12v to power 12v appliances. Thanks

Author
Swagatam
3 months ago
Reply to Tinu

Hi Tinu,
Unfortunately the circuit which I have can handle a maximum of 55 V only…60 V is just a bit higher for this device:

https://www.homemade-circuits.com/0-to-50-v-adjustable-switching-power-supply-circuit-using-ic-lm2576/

Moses
6 months ago

Good day sir! To get 200mA output current out of the circuit instead of 100mA I intend using 2N2222A in place of BC547. My question is do I need to adjust the value of 3.3ohms resistor.

Author
Swagatam
6 months ago
Reply to Moses

Hello Moses, the current is not relevant to BC547 rating, so you can use BC547 for 200 mA also.
For 200 mA current the 3.3 ohm can be replaced with a
R = 0.6 / 0.2 = 3 ohm.

sudarsono
7 months ago

I am interested in your post and I want to try it for experimentation as a smd led light driver, in your opinion which diagram is better than the two diagrams above, the first or the second, then in the first diagram there is a 100 watt lamp, can the lamp be replaced with a resistor as a safety (how much resistor value should I use) so that the circuit is safe, in the first diagram what do you think if the output is also installed a 10 uf 400 volt capacitor as in the second diagram. thank you for… Read more »

Author
Swagatam
7 months ago
Reply to sudarsono

The lamp cannot be replaced with a resistor, otherwise the resistor will become too hot and burn. However, the bulb could be replaced with a high voltage PPC capacitor. A 1uF/400V will produce 50 mA at the output and so on. Yes in the first diagram also a 10uF/400V or higher value capacitor could be used at the output. In that case the capacitor after the bridge could be removed.

sudarsono
6 months ago
Reply to Swagatam

I have changed the first diagram and I installed a 105 j 400 volt capacitor and I parallel it with a 470 Kohm resistor, then I removed the 150 uf 400 volt capacitor and at the output I installed 10 uf 400 volt, I will practice it on the pcb hopefully the experiment is successful, thank you friends. Unfortunately I can’t send you a diagram change, buddy

Author
Swagatam
6 months ago
Reply to sudarsono

It’s fine you can go ahead.
However be extremely careful with the circuit. It is not isolated from mains AC and therefore can inflict a lethal electrical shock if touched in an uncovered and switched ON condition.

mark smith
8 months ago

Hi,
I would like to use a transformer both works 220v or 110v to power my heater 150w also I would be needing 12VDC to 3.3v via a regulator to power esp modules, Kindly could you let me know which transformer diagram is recommended to use. Thank you

Author
Swagatam
8 months ago
Reply to mark smith

Hi, what is the voltage and current specifications of the heater?
For the 12V to 3.3V conversion you can use an LM317 adjustable voltage regulator circuit.

mark smith
8 months ago
Reply to Swagatam

Hello Mr. Swagatam
Actually, I am making an IoT prototype in which power would be required.
For esp chip, at the same time for heater blanket. there would be 2 rods of 75watt each for the blanket Volts 110v 1.25amp also I would be needing 12v for modules.

Author
Swagatam
8 months ago
Reply to mark smith

Hello Mark,

If your input is 220V and the heater is 110V then you will require a 220V to 110V 3 amp transformer for powering the heaters. For getting the 12V you can simply use a 0-12V/220V/1 amp transformer.

Алексей
7 months ago
Reply to Swagatam

Нагреватели последовательно. Будет как 1 .12 v любой маломощный трансформатор. Конструкция меньше будет.

محمد فؤاد الخليلي
8 months ago

Hello Swagatam I am a hobbyist of electronics and I was taught by qualified experts I am now working on an available project that lights the lights at night and turns them off at daybreak, and they are several designs, including using transistors and others with integrators. Of course, these circuits need to supply power from 9 volts to 12 volts, but the important thing is that these circuits control the relays, and these relays need a relatively large current that exceeds 1 Ampere What is your advice to use suitable feeding circuits to carry the relay I salute you… Read more »

محمد فؤاد الخليلي
8 months ago
Reply to محمد فؤاد الخليلي

Knowing that I am very interested in feeding from the general current directly to reduce the size of the device and reduce the material cost as well. Several feeding circuits have been tested from the main current 220 volts AC, but the current is so small that the relay cannot work and move the internal contacts to connect the lighting
Greetings again
May God bless you in all your endeavors and projects

Author
Swagatam
8 months ago
Reply to محمد فؤاد الخليلي

Thank you محمد فؤاد الخليلي,

I appreciate your interest in electronics. It is strange that your relays require 1 amp. Even a large relay rated at 30 amp will not require more than 100mA current for its coil activation.

Anyway, for a 1 amp current I will never recommend a transformerless power supply, which is explained on the above article.

For any load above 200 mA I will recommend an SMPS circuit, therefore for your application also I would recommend you to use an SMPS circuit.

محمد فؤاد الخليلي
8 months ago
Reply to Swagatam

thank you so much
Please give me the simplest design
For SMPS so that I can easily implement it with a small final size and relatively little material cost
Thank u

Author
Swagatam
8 months ago
Reply to محمد فؤاد الخليلي

I would rather suggest you to buy a readymade SMPS instead of making one, because making a SMPS can be quite complex and will require advanced knowledge in the field of electronics. However if you are interested to build it you can try one of the circuits from the following article:

https://www.homemade-circuits.com/how-to-make-simple-12-v-1-amp-switch/

Max Weber
8 months ago

Hello,
my name is Max.
I am currently building a simple tube tester (EL34/KT88) and need a constant 250VDC anode voltage that does not collapse at 200mA.
Where can I find a corresponding circuit diagram based on IRF840?
Can you help me??
LG
Max

Author
Swagatam
8 months ago
Reply to Max Weber

Hi, instead of using a mosfet power supply I would recommend using two small transformers connected back to back o get the intended low current 220V output, as shown below:

Habtamu Tamiru
1 year ago

ok sir! thank you!

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