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Single Transformer Inverter/Charger/Changeover Circuit

An inverter circuit design  may be quite incomplete if it’s without an appropriate integrated battery charger circuit. Though you may find many inverters having an integral battery charger, the section will mostly employ a separate transformer for implementing it. The following article describes a unique design which utilizes the inverter transformer for power inverting as well as for charging the battery.

The circuit diagram below shows a design where a single power transformer is used for inverting purpose as well as for charging the battery when mains is present.

The good thing about the circuit is that the transformer doesn’t employ separate winding for this, rather works with the same input winding and reverts DC to the battery with the help of a few DPDT relays.

The circuit can be understood with the following points:

The inverter section can be easiy recognized in the diagram, R1 to R6, including the T1 and T2 forms a general astable multivibrator circuit for producing the required 50 or 60 Hz pulses.

These pulses drive the mosfets alternately which in turn saturate the transformer by switching the battery voltage in it. The secondary of the transformer generates the corresponding magnitude of AC which is finally used for operating the connected appliances.

The above configuration suggests an normal or ordinary inverter operation.

By adding a couple of DPDT relays in the above discussed operation, we can force the circuit to charge the battery in the prsence of an AC mains source.

The coils of the two relays are powered through a capacitive low current compact power supply, involving C6, C5, D1—-D5.

The above circuit is connected to a mains AC source, this source is also connected to RL1 poles.

The second relay RL2 is wired up with input winding of the transformer.

In the absence of mains AC, the position of the relay contacts are in the N/C as shown in the figure.

In this position the mosfets get linked with the transformer input winding, and thea battery with the circuit so that the inverter starts oscillating and the output appliances gets the AC power from the battery.

In the presence of mains AC the relay coils instantly get the required DC power and the contacts activate.

RL1 activates and connects the mains input to the transformer, the appliances also get connected with the mains AC in the process.

Also due to the action of RL2 the mosfets get disconnected from the transformer, while the the lower tap connects with D6. Since the center is already connected to battery positive, the inclusion of D6 provides a half wave rectified voltage to the battery, which is effectively filtered by C3 so that the battery is able to get the required sufficient charging voltage.

The above charging process continues until mains is present, so it should be monitored manually. When mains fails, the action reverts into inverting mode without interrupting the appliance operations and by using a single transformer for both the operations.

C4 makes sure that RL1 always activates a shade later than RL2 for safety reasons.


Parts List

R1, R2 = 27K,
R3, R4, R5, R6 = 470 Ohms,
C1,C2 = 0.47uF/100V metallized
T1, T2 = BC547,
T3, T4 = any 30V, 10amp mosfet, N-channel.
C3 = 47000uF/25V
C4 = 220uF/25v
C5 = 47uF/100v
C6 = 105/400V
R7 = 1M
D1—D5 = 1N4007
D6 = 1N5402
RL1, RL2 = DPDT, 400 OHMS, 12V, 7 AMPS/220V
Transformer = 12-0-12V, current as per requirement.

For only inverter design please refer to this ARTICLE

166 comments on Single Transformer Inverter/Charger/Changeover Circuit

  1. Hi swagat,
    This is a good idea. But We need 14.3v DC to charge a 12v lead acid battery.If we planning to make an automatic trickle charger, then we need 18v DC. You are indicating that the inverter transformer is 9-0-9 8A. So how can we charge a 12v battery with a 9v transformer.?

    1. Why there is no surge protection that is done because of the capacitor no polarized like 2.2uf 400V between the mains terminals LIVE and NEUTRAL

  2. sir, Can i use the car battery for this circuit and what are modification for this circuit. how many watts output obtain in this circuit by using car battery and i want to run ceiling fan .. pls help for this..

    Thank you ..

    1. Yes you may use a car battery with this circuit.

      You will have to use a 10amp, 12-0-12V transformer, then you would get an output of about 120 watts. you can run a ceiling fan with it.

  3. Helo mr swagatam…please can you help me design a circuit to charge 25 li-on cell battery(3.7v- 800mA each) at the same power source is from 12v- 50AH battery. Also let me know how many amps of the 12v battery would be drawn with this setup per hour…thanks in advance

  4. Dear Swagatam sir,

    Very good morning to you! how are you ? I am Maung Pru, from Bangladesh.

    Thank your very much for your thousands of valuable circuits.

    I would like to have some info on this circuit, would you please kindly advice me that ..

    Q1. using an external DC charger would it be possible to charge the battery from inverter AC out ? while same time powering other lights and fan ?

    Q2. if so, what would be the battery life cycle ? the battery plates may damage ?

    Thank you and with best regards

    Maung Pru

    1. Hi Maung,

      many people have asked this question to me, and the answer is always a "NO!"

      It's due to two reasons:

      First of all charging will require much more time than the back up time.

      If you use high current charging adapter, then all the power will go to the battery, and none to the appliances.

      Second, due to inefficiency the output from the inverter will be always 10 to 20% lower than the total input supply power.

  5. Dear sir,

    Thank you very much for your response with valuable comments.

    for your kind info, we live in hilly area Bangladesh, some time the power go off for 3-4 days. we need the power, which will be constant and non stop supply. but don't have it.

    sir, can it be able to run for at least 3 days ? please help us.

    3000 Watt Generator Powers Itself, Grinder & Drill Press.

    Thank you and with best regards


  6. Thanks for your time and dedication .I hav a 600w 12-0-12 transformer if i use it on this diagram would it work and how many watts do i get at the output? Q2- if i use 3205 instead of irf540n wil it work?

    1. Yes it should work according to me,…with 600 watts naturally you would get something around 600 watts provided the battery is also rated appropriately.

    2. Hello, Engr. Swagatama,
      You have done a great job to save people, thank you.
      I read mechanical engr but have interest in electrical work.
      I want to buid a 50w inverter from the past analysis u have been given before i can build a bigger one for my family use.
      I have been looking for person that will answer small small questions the way u do but i have not seen. Thanks for your time and effort.
      From Tony.

  7. Hi swagatham
    What is the wattage of the circuit given
    And what are the modifications needed for 600w output

    1. Hello brother, as i look the diagram i see that battery charging system is half wave rectification when relay switches by mains 220v doesn't this damage the battery? Your design is simple than mine i designed using microcontroller. But i am still looking for the best charging method with cutt off for a 200Ah battery. I am using irf3205 mosfets and a 600w transformer. Any suggestion ? I will appreciate! Thanks and your posts are always amazing and informative

    2. Thank you Syed, Half wave charging will not affect or harm the battery in any manner, you can improve the DC by increasing the filter capacitor value to some higher value….for example a 10,000uF could be tried t enhance the DC quality for the battery.

  8. Thanks Mr Swagatam, suppose I want to use 12V-0-12V transformer for a 24V-0-24V transformer, how will I do it?. Secondly, how can someone build an inverter of Wattage rating of his choice through calculations of components to be used and the numbers

    1. Acquiring 24V from 12V is difficult and is not recommended, because the transformer can become hot.

      Wattage rating can be controlled by controlling the battery current through some circuit stage.

  9. DEar sir,
    Can i use a 10 A relay instead of the 7 A one used here

    What precaution should be taken in order to safeguard the entire circuit before applying the ac mains to transformer's primary ?

    And one more doubt sir
    What does the connection (which seems like inductor coil )in the upper circuit means ? I realize that these are the connections across two relay pins and the upper circuit is the power feeding circuit for the relays. But i can't understand the 2 pins that i specified here. Please help me sir. I am not at all familiar with relays. Hoping your reply very soon sir……

    1. Dear Arun,

      Use a 100 watt bulb in series with one of the input mains wires for safety.

      The inductors are the relay coils, a relay cannot be activated unless its coils are energized therefore these coils are supplied through the capacitive power supply shown at the upper section of the diagram.

  10. Sorry for disturbing you again sir. But what to do…. I have to be completely free of doubts before going for the design….. This time i am also coming with another doubt which may feel silly to you, please dn' t neglect it since i am not having deep knowledge in Electronics, but i like it.

    I have a 300 W 12 V transformer having 4 thick windings at primary and 5 at secondary. I know it is a multi-tap one. But don't know how to wire them to suit for the single transformer inverter design.. Please help me……

    How can i link the images with the comments in this blog ?

  11. Sir that was not enough….. I need a better suggestion…….
    My transformer is a locally made one. No markings ( even no separate colors for wires ) are there. The only thing that i could see that some set of the terminal wires are thicker than other set. From basic calculations of transformer design, i think the wires to bear large current are of secondary ( 300 W/ 12V ) and they have to be thick enough than the primary. Am i right ? If not please correct me…..
    I am in a little bit confusion that how to connect the neutral and earth lines of ac mains into the transformer if it has 5 primary terminals as i calculated….. Please help me sir… Can you please suggest the exact connection schematics…..
    Since it is an ac transformer, i willn't be able to measure the tap voltages exactly without a brige type rectifiers. Taking each tap voltage by changing the polarities supplied to the rectifier will be very time consuming..
    I couldn't upload the images of the transformer…. SO PLZ HELP ME

    1. you won't need a bridge rectifier at each tap for checking, you can do it with your meter on AC 220V range.

      you can also check the taps by measuring their resistances…the one which gives the highest resistance can be assumed to be the mains input winding, connect a 100 watt bulb in series which testing with mains for avoiding possible hazards.

  12. Very nice circuit sir.
    Can i use a 300 W transformer in this circuit ? How much high will be the currents in both terminals ? Can i use the formula P= IV for its calculation. If i am doing so i will get maximum current
    In primary : 1.304 A and
    In secondary : 25 A…….. Is that will happen ?

    If so, that diode (IN5402) used in Charging section sufficient to overcome this high amperage ?
    What about the diodes at the primary if i am using the inverter to be run on a 300 W load. Reply me soon plz……. Sorry for the bad english…

    1. Thanks Ramsree,

      You can use P = IV, 25amps is right but that might be too high for the battery, because as per rules the ideal charging current should be 1/10th of batery AH, so we may have to upgrade the circuit with an additional current limiter circuit and also some modification in the relay wiring…..

    2. Hey Dear, Did you add current limitation adjustments to the above circuit ever. Let me know. I think the design is wonderful. So we should work on it. Kindly share the link if you already posted about adding a limiter to this circuit.

  13. What modification you mean sir ? How can i modify this circuit to charge a 60 Ah battery bank effectively ? Can i use your one of previous post on ' 3 stage smart battery charger for 12 V ' with this.circuit ? What modifications to be done sir ?

  14. Dear sir,
    Can you please give a model current limiting circuit to fullfill my need ? Can i have it to be used prior to the rectification section ? Thereby i can avoid using high ampere diodes there. that is why

  15. Another doubt too sir,
    If i am using battery rated below 100 Ah along with the current limiting circuit , will it be possible for the inverter to run maximum load that i am expecting with ?

    1. As mentioned in the previous comment, the series resistor with D6 will not interfere with inverter power outputs, it will work with the specified full capacity

  16. Thank you so much for this post. I will like to give you feed back after I complete the work and I will also prefer to know about circuit diagram.

    Kindly, let me ask this question.
    1. Is it possible to use this circuit with 24volts battery without any damage to the cell of the circuit?

  17. Hai sir,
    Can i ask you something……
    What modification to be done to the present circuit if i am using high current transformer ??
    A series current limiting resistor alone will not do this purpose i think. What extra things to be added to the oscillation section to prevent ir from accidental damage due to high current flow. Also i want to know the necessary changes to be made on relay wiring also. Please try to REPLY MY ALL QUESTIONS SIR

  18. Hai sir it is again prithviraj speaking…..
    A serious doubt is now disturbing me….
    As per your suggestion, the inverter o/p after successful testing can be connected directly to the ac wall outlet. But in this inverter schematics, the relay coils are actuated by the ac mains. So if i am gonna follow my strategy, will the relay actuate on getting supply during inverter operation and continously turn ON and OFF ?????

    1. Hi Prithviraj,

      You are right, the above inverer cannot be inserted in the wall socket, because the relays would start vibrating ON/OFF due to the particular configuration employed in the circuit.

  19. Thank you Sir for replying me fast. Now i want to know whether any modification can be done to the present structure to suit my need or can you suggest any other design for me. I know i am disturbing you a lot. But what to do, i willn't get a better sleep if the problem is kept unsolved….
    I am not at all interested in connecting appliances directly to inverter terminal out, because it will cause discomfort in turning a particular device on at a room

  20. Sir i think it is possible to solve the issue by any of these,,,,,,,,
    What about using a descriminator circuit to distinguish the sinewave from the square wave( which is produced by the inverter ) and actuating the relay only on sine wave, i.e, during mains is present.
    Another way i think, is to slightly make a change to the existing frequency of square wave ( which is 50 Hz ) and thereby identifying the power on condition by counting frequency of the incoming signal to the relay coil fed.

    But i don't know how this will take effect or will be effective and also don't know how to arrange these modifications. ….. So can you please help me

    1. I don't think that would help, in that case when the mains comes back it will first clash with the inverter AC and cause undesirable things to happen, a fire, a blown fuse etc. because once the the inverter AC gets into the wall socket it's bound to mix with the mains AC unless a DPDT is employed for isolating the respective ACs manually

  21. Hellow sir, how did you calculate the value of resistors and capacitors incorporating the AMV stage of two.coupled transistors. I have to place available and suitable components there

  22. hello Swagatam,
    I interested in this circuit diagram but being novice, I'm finding difficult to understand the connect basically the operation of dpdt relay.
    how many relays are required in this circuit diagram and how do I connect it pls

    1. I have already explained it in the article, if you have a specific doubt you can ask it.

      if the above basic model is confirmed then it can be modified for 2kva also.

  23. Sir, tanks for the post
    Pls I don't get how to connect RL2 to the two mosfet since one Relay is specified for the two I can only connection one successfully. Pls help explain

  24. Hi sir, I'm unable to lay me hand on DPDT type relay, can I use 4 SPDT relay instead of DPDT relay, pls quick reply will be appreciated, thanks

    1. Hi Kolley,

      the capacitor values cannot be changed, so you'll have to use as suggested.

      the ones which are polarized need to be electrolytic while the others metallized polyester type.

      please note that this project is not for the newbies and requires expert knowledge in the field…the circuit has the potentials of causing fire hazard and explosion if not done correctly.

  25. Hello sir, I just finished testing the circuit,am having two problem,1.the mosfet get hot immediately I connect to d battery power and the wire that come from transformer spark to the extent of getting burnt when I connect it to + side of d battery.2 the is turns on and off repeatedly when I connect it to AC supply pls what can I do to rectify these problems.thanks. Pls a quick reply will be appreciated

  26. Dear Swagatam ,
    I had a home inverter which i was using for charging my car battery. One day by mistake i left battery charging wires on floor ,after that i see no voltage in battery charging wires. I checked the main Ac fuse of inverter ,it is ok but one warning sticker is there on inverter that avoid shorting of charging wires other wise its DC fuse will blow . Kindly give some information on indian make inverters DC fuse location . Inverter make is Reliable . It has only one PCB on charging side supply is going to charging wires through heat sink having 4 MODFET each side.

    1. Dear Hamza,

      I am sorry, it'll be difficult to provide a proper suggestion regarding this since the configuration may be different for different inverters and I haven't yet opened the brand that you have mentioned.
      However a fuse in any inverter will be always in series with the battery line and also could be with the output AC line, you can investigate these two positions, I am sure you would easily locate a few across these lines.

  27. Hi,
    This question is not related to the post. Pls reply, I recently purchased a 12-0-12 transformer, and as I measured the V between both ends,it shows 0 V ( not 24V) and centre tap to each end it shows 12 V each. So what type winding is used in it?

    1. Hi, that's impossible, if the individual winding are showing 12V (from center to out), the outer taps will show the sum of the two winding.

      something may not be correct with your meter or measuring procedure.

      make sure the meter is selected in the AC range.

      you may also check the continuity of the winding using the diode range in the meter.

  28. I have a China inverter and a12v battery charger plz give me some circuit or understood me that if light is gone then automatically my battery give me back up and if light comes again then start battery charge automatically
    I hope u understand my question

  29. Sir, thanks a lot for your work here..

    There's just one thing I'm a bit confused about in this circuit..

    220VAC enters the bridge Rectifier from the mains input,, 220VDC comes out…. But the relays are rated for 12VDC

    I'd like to know which of the components in the circuit does the job of limiting the 220VDC to the 12VDC required by the relay coils…. Thanks

    1. Banimz, the capacitor output has very low current content therefore the 300V DC will automatically drop and adjust to the relay coil voltage.

      no external limiting agent is required for this

  30. Thanks a lot….

    Speaking of that capacitor…. I can't find a ceramic capacitor of 1uf anywhere near… But I do have an electrolytic capacitor of 2.2uf….

    Will this affect the circuit???

    I'm talking about C6…

  31. Okay, thanks…. So now I've gotten a 105/400V capacitor for C6….. And I've finished constructing the circuit…

    Now I noticed something. If I connect power to the circuit before connecting a relay ,,,, the relay will trigger…

    But if I connect the relay before turning on the power,,, the relay won't respond….. Upon testing, I discovered that just about 5V gets to the relay if it is connected before the power turns on…. Almost as if its acting as overload on the circuit……

    What could be the cause of this and how can I rectify the problem??

    Oh, also, both relays don't trigger at the same time,, whether power is connected beforehand or not….. Only one works at a time,,,, if I connect one and it triggers,, the other won't trigger……

    Eagerly awaiting your response…

    Thanks a lot for everything

    1. The relays that you have are not of good quality, they might be having a very low resistance coil, in that case you should think of using a 12V AC/DC adapter instead of the C6 power supply.

      The connections will be identical, the 12V from the adapter will connect with the relay coils including D5, C4….

      D1—D4 and C2 may be removed.

      The input of the adapter will need to be connected across the points indicated "mains input"

  32. Sir, the circuit is now working well…. Except for one minor issue…. My relay coils are 90 ohms instead of 400…… Thus, the current this circuit is producing is too low to power both of them together….. …..

    How can I increase the current in this circuit?? Should I reduce the 1M resistor??? Or something else? Eagerly awaiting your response.. Thanks a lot

  33. Thanks so much sir…

    I was eventually able to get the required current and voltage to drive my relays by using four of the C6 capacitors in parallel…..

    Its all working now.. Thanks a lot sir

  34. Sir thanks so much for your help so far,
    Please help me, I build an inverter using IC SG3524 the inverter is working fine on some appliances but won't work on some like small or big fan, small hair clipper and a desktop computer, pls what can I do to make it work on all appliances?

    1. Hi Kolley,

      It could be due to insufficient battery power or trafo wattage….measure the output voltage of the inverter when you connect those loads, if you find the voltage dropping would confirm an inadequate battery current or transformer wattage.

  35. Thanks for the quick responds
    The output voltage before those load is close to 240 but after connecting the load it drop to 220, which I think should be sufficient but still it won't power fan and clipper

    1. 220V is more than sufficient, that's weird, can't diagnose without seeing it practically…just check if both the power transistors are conducting uniformly, you can do it by connecting car headlight lamps in series with the transformer taps which are linked with the power devices….

  36. hi Sir Swagatam

    i dont know much more about electronics basics, But i want to make this ups for my DSL Router and Laptop Charger , I understand the circuit, but DPDT Relay is confusing me ,

    I have found that DPDT Relay have 6 points , 2 for coil, 2 for NC and 2 for Com,
    Can you Please mention that how to use relay in this circuit and what is treatment for coil points,

    If you Please explain the working of R1 and R2 while marking on a picture of DPDT so that i may knwo which points of relay i have to use and which of them are to be leave as it is ,

    Please help me ,


  37. hello…Mr.Swagatam

    I want to make 1200w inverter but problem is in charging circuit I can't use relay for it because it take 100a in inverter mode and 10 in charging mode

    I use 4xIRFZ44N mosfet
    can I use MOSFET reverse diode in charging mode ????

    1. hello janak, if you are referring to the above circuit, it cannot be used without relays….it would work only with the relays connected in the indicated manner

  38. sir,
    could you help me for making a simple inverter for lighting a 18w CFL ?
    i have a 12V 9ah battery and 12-0-12 3A transformer.
    Hope you may help me

    1. Aminu, the value needs to be little bigger, 105 or 104 will not be suitable, you can try 10uF/250V

      C6 cannot be less than 105, otherwise the relays will not operate, in fact you must use two 105 in parallel

  39. Hy I m bravo pls I don't see dat 0.47uf 100v cap in my area pls can I use 0.33uf electrolytic or pls give me any close value I can use anD for that relay I whan to use small 12v 500ma trafo to power d coils and the C3 I get 4700uf by 16v will all this be ok pls help me out. Tnks

    1. Hi, 0.47uF, and 27K together are supposed to determine the 50Hz freq, 0.33uF will reduce the frequency….in that case you might have to increase the 27K to 33K or 39K to correct the frequency to the 50Hz level

  40. Tnks for the reply
    another question sir, (1) pls what will be the value of r1 nd r2 if I use 0.22uf electrolytic cap for c1 nd c2 to get the 50Hz, (2)what value of zener diode can I use to protect my mosfet from damage. tnks

    1. it will do.

      mosfets already have built in freewheel diodes there's no need of an external diode.

      anyway 1N4007 will not be enough….you can try 1N5408 instead for your satisfaction.

  41. Hi Swagatam, thnx for the huge efforts, very helpful. I've a question plz, I am troubleshooting an inverter, it's not charging the battery anymore, the output goes to the battery is 7.5v, which should be no less than 12v. Plz what could be the reason for that?
    Thanks in advance.

    1. Thanks Taher, please disconnect everything that's connected with the transformer or the source of the supply, and check whether it reads around 14/15V or not, if yes then the problem could be somewhere in the adjoining electronics or controller which will need to identified appropriately….if the source shows less voltage then it will need to be replaced with a new one.

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