250 to 5000 watts PWM DC/AC 220V Power Inverter

Picture of 250 to 5000 watts PWM DC/AC 220V Power Inverter
3524 pwm transistor ex.jpg
3524 pwm V2-MOSFET.jpg
3524 pwm mosfet ex.jpg
3524 PCB.bmp
This is a heavy duty design of a Pulse Width Modulator DC/AC inverter using the chip SG3524 .
I've been using it as a backup to power up all my house when outages occur since aprox. 6 years non stop.

If you like the work and intend to build the circuit don't forget to click on the "follow" button so I know how many people benefit from the design, Thanks.


1> The schematic circuit design is for a 250 watt output, while the pics are of my 1500 watts inverter that i built, to increase the power of the circuit you have to add more of the Q7 and Q8 transistors in parallel, each pair you add will increase your power by 250 watts, ex: to get 750 watts of power from the inverter you need to add in parallel 2 of Q7 and 2 of Q8 to the original design.

2> If you increase the power transistors you have to enlarge the T2 transformer to match the new needs, the circuit's transformer is rated 25 amps to handle 250 watts of 220v, for every 1 additional amp you need on the 220v side you have to increase 10 amps on the 12v side, of course there are limits to the thickness of the winding so if you need more than 750 watts i recommend that you use a 24VDC supply instead of 12 volts:

DC voltage and Transformer "T2" winding recommendation:
Power     Supply     Winding
750w       12VDC     P:24V "12-0-12" / S:220V
1500w     24VDC     P:48V "24-0-24" / S:220V
2250w     36VDC     P:72V "36-0-36" / S:220V
3000w     48VDC     P:96V "48-0-48" / S:220V
3750w     60VDC     P:120V "60-0-60" / S:220V
4500w     72VDC     P:144V "72-0-72" / S:220V
5250w     84VDC     P:168V "84-0-84" / S:220V
*The transformer should be "center tapped" at the primary side.
**You can make the secondary 110v if needed.
***The transformer in the pic is a custom made (48V center tapped / 220v ) 2000 watts, weights like 10 kilos.

3> R1 is to set the PWM duty cycle to 220v. Connect voltmeter to the output of your inverter and vary VR1 till the voltage reads 220V.

4> R2 is to set the frequency to 50 or 60 Hz (R2 range is between 40Hz to 75Hz), so guys that do not have a frequency meter are advised to blindly put this variable resistor mid-way which should drop you in the range of 50~60 Hz.
If you want you can substitue the variable resistor with a fixed resistor using the following formula: F = 1.3 / (RxC)
in our case to get a 50Hz output we remove both the 100K and the variable 100K both from pin 6 and we put instead a 260K fixed resistor and we leave the 0.1uF (the 104 cap) as it is, this change should give out a fixed 50Hz as per the formula :
1.3 / (260,000 ohm x 0.0000001 farad) = 50Hz
But in reality it will not exactly give 50Hz because the 260K resistor has a specific error value margin so does the capacitor, that's why i recommend a variable resistor so that accurate calibration can be achieved.

5> Use either tantalum or polyester film "as in pic" for the 104 caps, ceramic disc caps change value once hot and this in turn changes the frequency of the inverter so they are not recommended.

6> Pin 10 of the SG3524 can be used to auto shut down the inverter, once a positive voltage is given instead of negative to pin10, the SG3524 will stop oscillating. This is useful for persons wanting to add some cosmetic makeup to their inverters like overload cutoff, low battery cutoff or overheating cutoff.

7> Wiring connections on the power stage side should be thick enough to handle the huge amps drain from the batteries. I marked them with dark black on the schema also I included a pic so you see how thick those wires must be.

8> The design does not include a battery charger since each person will be building a custom version of the inverter with specific power needs. If you are ordering a custom made transformer you can ask them to take out for you an additional output wire on the primary side to give 14v (between point 0 and this new wire) and use it to charge a 12v battery, of course this needs a seperate circuit to control charging auto cut-off. But anyway this is not advisable because it will shorten the life of the transformer itself since using it as a charger will toast the enamel coating layer of the copper wires over time. Anyway .. YES can be done to reduce cost.

9> A cooling fan will be needed to reduce heat off the heat sinks and transformer, i recommend getting a 220v fan and connecting it to the output T2 transformer, when you power up the circuit the fan will start this will always give you a simple way to know that 220v is present and everything is OK.. You can use a computer's old power supply fan if you like.
Note that the fan must suck air out from the inverter case and NOT blow inside, so install it the correct way or it will be useless.
Also note how I fixed both the heat sinks and where the fan is, in a way that the fan sucks hot air from like a channel between the 2 heatsinks. 

10> 2 circuit breakers are recommended instead of fuses, one on the DC side and one on the AC side, depending on your design
Ex: for a 24vDC ( 1500 watts design ) put a 60Amp breaker on the DC side and a 6Amp on the AC side.
For every 1amp of 220vAC you will be draining like 8 to 10 Amps from the 12v battery, make your calculations !

11> The 2 Heat sinks should be big enough to cool the transistors, they are separate and should NOT touch each other. "see the pics"

12> Important: If you're building a big design that uses more than 24VDC as power source, make sure not to supply the driver circuit with more than 24v maximum. (EX: If you have 4 batteries 4x12 = 48v , connect the v+ supply of the driver circuit to the second battery's (+) terminal with a thin 1 mm wire which is more than enough. this supplies the driver circuit with +24v while supplies the power transformer with +48v) "see the batteries pic example"

13> "Optional" : Deep Cycle batteries are your best choice, consider them for best results .. read more

14> Be cautious when building this circuit it involves high voltage which is lethal, any part you touch when the circuit is ON could give you a nasty painful jolt, specially the heat-sinks, never touch them when the circuit is on to see if the transistors are hot !! I ate it several times :)

15> The optional "Low voltage warning" is already embedded in the PCB layout, you can disregard it and not install it's components if you do not need it. It does not affect the functionality of the main circuit.

16> The Motorola 2N6277 is a durable heavy duty power transistor, it is used in many US tanks for it's reliability but unfortunately it is a very hard to find part, instead you can substitute each 2N6277 with 2 x 2N3773 or any equivalent.

17> I've included an optional "Battery level indicator" circuit diagram that has 4 LEDs, you can see it installed on the front panel of my inverter pic, it is functioning great and shows precisely how much juice the batteries still have. I have included a small relay that is powered by the last LED to auto shutoff the inverter once last LED is off.

18> Also included an optional "Overload circuit", it is very easy to build and can be calibrated to the desired overload current threshold cutoff point through the potentiometer VR1.
R1 is rated 5watts for inverters upto 1000 watts. For bigger versions of the inverter like 1000 to 3000 watts inverters, replace R1 (1 ohm, 5watts) with (1 ohm, 17watts) which should handle loads upto 10 VA.
Make sure you install a proper relay to handle big current drains.

19> Please guys take your time to read and understand my notes, browse and read the posts and questions asked by others because there are many useful information listed in replies. The main reason for me not answering your question is because it has already been asked before and answered upon.

20> It would be nice and inspiring for others if you take some photos and show us how you built your version, any additions to the circuit are mostly welcomed to be listed here, we can all benefit from them.

21> Please click on the "I've made it" button/icon if you did build the circuit so I know how many people benefit from this design.
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JohnA135 days ago

Hello Nick,

Are you saying that this your circuit drawing above can handle 12 Volts, 24 Volts etc or do you another circuit drawing that can handle 24 Volts and above. pls reply.

MohamedA1910 days ago

Hello Nick,

One more question, How this low battery circuit will work on more than one 12v battery.

MohamedA1910 days ago

Hello Nick,

I need to detect the AC main for turn ON/OFF inverter, could you please suggest that how can I achieve this?

dejanb17 days ago

Hello Nick! This is a great project!

Before i start to build this inverter, for my solar powered home i need information how much current inverter will draw from the battery, with no load at 220v in 1500w 24VDC version?

Best Regards!

josfemi made it!19 days ago

i never had issues with feedback as many people complain and it has greatly work for me, with little modification, nick pls i do i get the programing for the sine wave pls

herryau24 days ago
Hello Nick,
I have made the inverter based on your design with the mosfet version. But i found that the output voltage drop around 10 volt after i connect with 5 watt lamp. I use 2k2 variable resistor as well to set 220vac.and i use 4 irfz44 mosfet and standard transformer 5 amp. Could advise what should i do to avoid the voltage drop?
nickk (author)  herryau24 days ago
please upload some pics to see your setup.
herryau nickk21 days ago
Hello Nick, please find the picture about my project.
Vickylbee1 month ago

Hello Nickk, please help! On no-load my inverter works well with 201Vac output which is ok for my use. But once i load it the voltage drops drastically to about 120Vac. I have bought a brand new 75ah battery, wound a custom made 12-0-12/240/12/14Vac transformer that could accommodate even my battery charger when completed, and connected everything as you shown and described. I suspect my feedback section is not working because i adjusted R1 from min to max and measured 1.42 and 2.04Vdc respectively. I used 7812 and i noticed Vdc at pin 11 and 14 remained around 4.5V and didn't change when i varied R1 from min to max.

What could be the problem this time, please?

nickk (author)  Vickylbee1 month ago
how many amps is your custom made trans? can u upload a photo to see its size .
Vickylbee nickk1 month ago

See pix...

vicIMAG2093.jpgvicIMAG2094.jpgvicIMAG2095.jpgvicIMAG2096.jpgvicIMAG2103.jpgVickylbee InverterIMAG2097.jpg
nickk (author)  Vickylbee1 month ago

1- Is your 12vdc battery fully charged? connect voltmeter and see if it drops below 12vdc while inverter is ON and loaded.

2- Disconnect the 12vac "feedback" that comes from your custom made transformer and connect an external 220/12v small transformer "0.5 amp is enough" to the feedback section.

3- Did you install a 2.2K variable "R2" ? because this part is hard to find, or did u install something else?

4- What is the load that your putting on the inverter?

5- I noticed that your wiring that goes to the power section "trans and transistors" is not thick enough, this also causes a drop in voltage, it happened to me when i was in the testing stages. See on the schematic diagram where you're supposed to have thick wirings, they are marked in Bold. also see my pics and notes on the thick wiring and have an idea how thick the wires should be.

Please check the above steps and send feedback.

Vickylbee nickk23 days ago

Hello Nickk,

Firstly my response to your above questions are:

1- Yes, its a new, fully charged 75Ah, 12V battery and it does not drop below 11.8V when inverter is ON and loaded

2- I installed a small transformer at feedback section and the outcome was the same

3- Yes, its 2.2k variable. All components were of the values in your schematic

4- I tested with 200W incandescent bulb and a 100W soldering iron

5- Thanks- your observation helped. I increased the wire size as you can see in picture

Infact, i reconstructed the circuit but noticed the same problem- my feedback section at maximum duty circle drops output voltage to 147V from 210V when i load the inverter with 300W bulb and soldering iron load. I noticed when i varied R2 with no load my output voltage varied from 210v to 145v. Varying R2 doesn't give higher than 210v. I think the problem may be from the feedback section but i can not find it.

What could be the problem- please help!

The waveforms from top to bottom are that of the output voltage, IC pin 11 output, and the main transformer input

Vickylbee nickk1 month ago
I have added photos. The transformer is 1kva, battery 75ah, power trans 2n3773. Also see waveform from 3 channels of my oscilope- blue (top) T2 output, blue (middle) T2 12v feedback, and grey (bottom) T2 input.

So what could be the problem, please.

Thank you, Sir, for this useful circuit But it could not run a fluorescent bulb or desktop computer, From what I see in the pictures output is square wave not modfid sin wave ?? Does not it
herryau24 days ago
Hello Nick,
I have made the inverter based on your design with the mosfet version. But i found that the output voltage drop around 10 volt after i connect with 5 watt lamp. I use 2k2 variable resistor as well to set 220vac.and i use 4 irfz44 mosfet and standard transformer 5 amp. Could advise what should i do to avoid the voltage drop?
(removed by author or community request)

I THINK YOUR PROBLEM IS OVER-BIASING. I cooked 56 MOTO transistors. I have just ordered 80 more. I also barbequed parts of my two middle fingers!! NEVER GAVE UP!

Please use separate two test transistors at a time to save transistors in case something goes wrong.

1. Ensure ALL connections are as shown in the schematic diagram. Double check. Ensure the collectors from the TIP122 drivers are connected to the respective sides of the transformer. Do not crisscross them.

2. Set the duty cycle variable resistor R1 to half way in the middle

3. Connect 50K variable resistors (betters if you can find the variable resistor that have two mounted on the same tuning nob so that you can vary them at the same time) to the TIP122 emitters so that you can limit the biasing current into 2n6277.

4. Connect voltmeter to transformer output and a clump meter to read current on the buttery circuit

5. Now slowly adjust the variable resistors you have connected in 3. until the transistors just switch on. Always keep an eye on the meters. Check that the TIP122 are not heating up. If one heats up at low current, then it is not driving its respective 2N6277 transistor bank.

6. When you have about 180V on your AC output, your feedback transformer my produce sufficient voltage to drive the feedback circuit. At that stage, try to set the duty cycle to 220V by using R1. If it doesn't respond, increase the biasing current a little bit more. Very slowly.

7.Try little loads and check current on the clump meter.

8. If this doesn't help then try to test with a different transformer. I had a transformer that cooked my transistors because it was pushing 10A on the buttery side even when no load connected to the output.

THIS WAS WHAT I DID. It helped me learn how this beautiful circuit works. Never give up.!!

nickk (author)  chivindwe1 month ago

Nice .. !

Although I do not recommend step-3, this is not a solution. The circuit should work 100% with no problems if done as you said by step-1

It is possible that i've always damage my transistors 'coz its cheap? I bought 2N3055 cheaper 50% than the original.

I double checked all of my wiring connections. No wrong at all.. I already energized my circuit using a power supply and having an output voltage of 235V max.. and connecting some loads like electric fan and laptop charger.. It works! but when I connect it into a Battery,,, my 40 transistors got burnt :(

Huhuhhuhuhu Please need some help.. There's only little voltage variation when I vary R1.

gal-jabi1 month ago

thhhhanks man

instruct8392 months ago

thank you sir Nickk for sharing this good inverter. i am trying to make this one for my house using a solar but the voltage value of electrolytic capacitors are missing and the value of bridge diode also is missing. can you please tell me what is the value of said components.. thank you

nickk (author)  instruct8391 month ago


I got suck with my inverter. Everytime I power up it using a power supply +24V.. it energize my transformer.. But when I power up with a car Battery, my power transistors got burnt.. I have 40 burnt transistors 2N3055 :(

Someone please need help. Is there any modification at the power stage when we use 2N3055 as our power transistor?? please help Nick :(

NelsonC12 months ago

hi nick, i'm planning to used IRFP260N to produce a 1500 watts 24 volt how many mosfet can i put in the circuit. thanks!

where the same intention my friend. i am also trying to make this with mosfet for me trying 750w so i will be connecting soon the 6pcs IRFP250. but i have problem buying some parts because some parts electrolytic capacitors has no voltage value and the bridge also has no value. please tell me my friend if you know.. thank you

Vickylbee2 months ago

Hello Nickk, it works! I found out the problem was that one of the T2 input diodes, D1, had failed close (shunt) and thereby shunting one section on T2 input which resulted in giving half of the expected output voltage. Then i removed D1 and D2 and it works well. At last!

I will now add other protective function, voltage display, buzzer, fan and more cosmetics to it.

Thanks Nickk for sharing your knowledge. I have actually attempted to construct different PWM inverter circuit over the years and this is the one that has worked! Thank you, thank you, and thank you...

i cant find any diode D1 D2 you mention on this circuit

They are at the transformer T2 input side

yes my friend i found it. i am sorry.. i just focusing on mosfet diagram whic is D1 and D2 are not included.. i trying to do this project but i don't know the voltage value of this electrolytic capacitor and the bridge diode. may i know my friend where i can get the value of this parts?.. thank you

nickk (author)  Vickylbee2 months ago

Great.. :)

Vickylbee2 months ago

Hello Nickk,

My inverter output is 209Vac but when i load it the voltage drops to about 90Vac, and the feedback transformer voltage also drops. What could be the reason? Could it be that the feedback section is not working or what?

Also, can you please explain how PWM and the output feedback maintains the inverter output voltage? -does it vary the ON/OFF pulse width to TIP122. If yes, whats the maximum ON/OFF duration and how does this achieve the output voltage regulation. Please assist. Thanks.



Vickylbee2 months ago

Hello Nickk,

My inverter output is 209Vac but when i load it the voltage drops to about 90Vac, and the feedback transformer voltage also drops. What could be the reason? Could it be that the feedback section is not working or what?

Also, can you please explain how PWM and the output feedback maintains the inverter output voltage? (does it vary the ON/OFF pulse width to TIP122) If yes, whats the maximum ON/OFF duration and how does this achieve the output voltage regulation. Please assist. Thanks.



eeteddy2 months ago


eeteddy2 months ago

hello nick,

chivindwe2 months ago

Sir, this circuit keeps blowing off my 2n6277 transistors at start up. The two banks of transistors are isolated and I am using a 36V bank of butteries.Could it be that my transformer is too big? I have also since put the cooling fans on top to extract heat instead of blowing from below. I have been testing with a pair of transistors at a time (to get 250W) on this transformer hopping to get satisfactory results before I switch on all the 48 transistors but to no avail. Each time I try to switch on, the 36V burns my test pair of transistors on this huge transformer and that has not made me confident to load the whole set of 48 transistors. It is particularly the base-emitter junctions that gets cooked. I also have the feedback transformer connected ok. Please help.

nickk (author)  chivindwe2 months ago

No the transformer size does not matter. It seems your pinout connections on the 2N6277 are wrong, look into that. Also check if your transformer is functional by disconnecting it and applying to it 220vac and testing to see if you get 36-0-36 on the other side.

chivindwe nickk2 months ago

Thank you Mr. Nickk. I think that my beloved transformer was the problem. Yes applying to it 220vac was giving me 36-0-36 on the other side. However, applying to it 36-0-36 was immediately sucking in 10A even when the other end was still open. This means my test pairs of transistors were being subjected to 360W of power. I have sing ordered a new 48-0-48 to 220 transformer from somebody who winds transformers. I think things will be fine now. I will report back in two weeks time. Have a blessed day today!!

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