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2 Easy Automatic Inverter/Mains AC Changeover Circuits

2 Easy Automatic Inverter/Mains AC Changeover Circuits

I have been put forth with this question many times in this blog, how do we add a changeover selector switch for automatically toggling of an inverter when AC mains is present and vice versa.

And also the system must enable automatic switching of the battery charger such that when AC mains is present the inverter battery gets charged and when AC mains fails, the battery gets connected with the inverter for supplying AC to the load.

Circuit Objective

The configuration should be such that everything takes place automatically and the appliances are never switched OFF, just reverted from inverter AC to Mains AC and vice versa during mains power failures and restorations.

So here I am with a couple of simple yet very efficient little relay assembly module which will do all the above functions without letting you know about the implementations, everything is done automatically, silently and with great fluency.

1) Inverter Battery Changeover

Looking at the diagram we can see that the unit requires two relays, however one of them is a DPDT relay while the other one is an ordinary SPDT relay.

The shown position of the relays are in the N/C directions, meaning the relays are not powered, which will obviously be in the absence of the mains AC input.

At this position if we look at the DPDT relay, we find it to be connecting the inverter AC output to the appliances through its N/C contacts.

The lower SPDT relay is also in a deactivated position and is shown to be connecting the battery with the inverter so that the inverter remains operative.

Now let's assume that AC mains is restored, this will instantly power the battery charger which now becomes operative and supplies power to the relay coil.

The relays instantly become active and switch from N/C to N/O, which initiates the following actions:

The battery charger gets connected with the battery and the battery starts charging.

The battery gets cut OFF from the inverter and therefore the inverter becomes inactive and stops functioning.

The connected appliances are instantly diverted from the inverter AC to the mains AC within a split second such that the appliances doesn't even blink, giving an impression that nothing had happened and the are kept operative continuously without any interruptions.

A comprehensive version of the above can be witnessed below:

2) 10KVA Solar-Grid Inverter Changeover Circuit with Low Battery Protection

In the second concept below we learn how to build a 10kva solar grid inverter changeover circuit which also includes a low battery protection feature. The idea was requested by Mr. Chandan Parashar.

Circuit Objectives and Requirements

  1. I have a solar panel system with 24 Panels of 24V and 250W connected to generate a output of 192V, 6000W and 24A. It is connected to 10KVA, 180V inverter which delivers the output to drive my appliances during daytime. During night the appliances and inverter run on grid supply.
  2. I request you to kindly design a circuit which will change the inverter input from grid to solar power once panel start generating the power and should again revert the input from solar to grid once darkness falls and solar power generation falls.
  3. Kindly design another circuit which will sense the batter.
  4. I request you to kindly make a circuit which will sense that battery is getting discharged below certain threshold value say 180V (esp during rainy season) and should switch the input from solar to grid even though some amount of solar power is being generated.

Designing the Circuit

The 10kva solar/grid automatic inverter changeover circuit with low battery protection which is requested above can be built using the concept presented in the following figure:

10KVA Solar-Grid Inverter Changeover Circuit with Low Battery Protection

In this design which may be slightly different to the requested one, we can see a battery being charged by a solar panel though an MPPT controller circuit.

The solar MPPT controller charges the battery and also operates a connected inverter through an SPDT relay for facilitating the user with a free electricity supply during day time.

This SPDT relay shown at the extreme right side monitors the over-discharge condition or the low voltage situation of the battery and disconnects the inverter and the load from the battery whenever it reaches the lower threshold.

The low voltage situation could mostly take place during night when there's no solar supply available, and therefore N/C of the SPDT relay is linked with a AC/DC adapter supply source so that in an event of a low battery during night the battery could be charged for the time being through the mains supply.

A DPDT relay can be also witnessed attached with the solar panel, and this relay takes care of the mains supply changeover for the appliances. During day time when the solar supply is present, the DPDT activates and connects the appliances with the inverter supply, while at night it reverts the supply to grid supply in order to save the battery for a mains failure back up situation.

UPS Relay Changeover Circuit

The next concept makes an attempt to create a simple relay changeover circuit with zero crossing detector which may be used in inverter or UPS changeover applications.

This could be used for switching-over the output from AC mains to inverter mains during inappropriate voltage conditions. The idea was requested by Mr. Deepak.

Technical Specifications

I am looking for circuit comprising of the comparator (LM 324) to drive a relay. The objective of this circuit is to:

1. Sense AC supply and switch relay 'ON' when voltage is in between 180-250V.

2. Relay should turned 'ON' after 5 seconds

3. Relay should turned 'ON' after zero voltage detection of supplied AC (Zero voltage detector). This is to minimize arching in the relay contacts.

4. Finally and most importantly, the relay switchover time should be less than 5 ms as a normal off-line UPS does.

5. LED indicator to indicate the state of relay.

The above functionality can be found in UPS circuit which is bit complex to understanding since UPS has many other functional circuit beside this. So am looking for a separate simpler circuit which only works as mentioned above. Kindly help me to build the circuit.

Component available and other details:

AC mains = 220V

Battery = 12 V

Comparator = LM 324 or something similar

Transistor = BC 548 or BC 547

All type of Zener are available

All types of resistor are available

Thanks and Best regards,


The Design

Referring to the simple UPS relay changeover circuit, the functioning of the various stages may be understood as follows:

T1 forms the sole zero detector component and triggers only when the AC mains half cycles are near to crossover points that's either below 0.6V or above -0.6V.

The AC half cycles are basically extracted from the bridge output and applied to the base of T1.

A1 and A2 are arranged as comparators for detecting the lower mains voltage threshold and the higher mains threshold respectively.

Under normal voltage conditions the outputs of A1 and A2 produce a low logic keeping T2 switched Off and T3 switched ON. This allows the relay to remain switched ON powering the connected appliances through mains voltage.

P1 is set such that voltage at the inverting input of A1 becomes just lower that the non-inverting input set by R2/R3, in case the mains voltage falls below the specified 180V.

When this happens, the output of A1 reverts from low to high triggering the relay driver stage and switching off the relay for the intended changeover from mains to inverter mode.

However this becomes possible only when the R2/R3 network receives the required positive potential from T1 which in turn takes place only during the zero crossings of the AC signals.

R4 makes sure that A1 does not stutter at the threshold point when the mains voltage goes below 180V or the set mark.

A2 is identically configured as A1, but it's positioned for detecting the higher cut-of limit of the mains voltage which is 250V.

Again the relay switch over implementation is executed only during the zero crossings of the mains AC with the help of T1.

Here R8 does the momentary latching job for ensuring a smooth transition of the switching.

C2 and C3 provides the required time lag before T2 can conduct fully and switch ON the relay. The values may be appropriately selected for achieving the desired delay lengths.

Circuit Diagram

Parts list for the zero crossing UPS relay changeover circuit

R1 = 1k

R2,R3,R4,R6,R7,R8 = 100K

P1,P2 = 10K PRESET

R5, R9 = 10K

D3,D4---D10 = 1N4007

C1,C2 = 1000uF/25V

T1 = BC557

T2 =BC547


A1/A2 = 1/2 IC LM324




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!

88 thoughts on “2 Easy Automatic Inverter/Mains AC Changeover Circuits”

  1. Hi,thank you so much for sharing this.I am doing a project where the battery is now charged from both mains and solar cells.How can I implement a similar changeover?

  2. Hello sir Swagatam,
    Can I use 1N4007 diode as the “blocking diode” in the schematic for all battery voltages? If no, kindy suggest the diode part number that I can use that will be able to handle all voltages. Thank you sir.

    • Godson, it must rated much higher than the charging current coming out from the charger.If this diode is already present inside the charger then no need of connecting an external one.

  3. Okay sir thanks i will be expecting but for the mean time im building a 20kva inverter using center tap transformer with sg3524 ic and i want to use igbt for it can you please give me a good driver circuit to drive the igbt thanks best regards

    • Faith, presently I do not have a correctly calculated filter circuit with me, you might have to do it by some trial and error…however if I happen to find one will surely update it here..

  4. Hello sir Swagatam,
    I want to design an inverter that utilizes the same transfo for charging the batteries through the MOSFETs. The transfo will have a separate winding that produces (14.5-0-14.5V) at the input when mains is applied there. Could you please suggest a changeover switch that I can use? Also I’ll like to add a battery monitor that will stop the charging as soon as battery is fully charged. Please kindly help me with the two schematics or how to go about designing it. Thank you sir.

    • Godson, designing a relay changeover is not difficult. you can try the following steps:

      1) connect a bridge rectifier/capacitor with the charger winding.
      2) connect the coil of a 30 amp relay with the output of this bridge.
      3) make an automatic charger circuit and attach it with the bridge output.
      4) connect the pole of the relay with the battery positive.
      5) connect the N/O of the relay with relay output from the automatic charger circuit
      6) connect the N/C of the relay with the inverter supply positive.

      • Thank you so much for the reply sir. I do appreciate it. My problem/fear of using a relay changeover has always been the huge amps drain from the battery during inverter operation because i cant get a relay around me that will be able to handle such current. But from your explanation above, it appears that i don’t need to tamper with the center-tap of the transfo that is connected to the battery positive. All the 30A relay will simply do is to connect/disconnect the inverter positive supply (driver section) and the automatic charger to/from the battery positive. Is that right sir?
        Can a 30A relay be able to handle the charging of a 200Ah battery?

        • Godson, that will do, you can use the relay to toggle the IC section which will in turn control the mosfet switching as per the changeover actions.

          However the relay has to handle the charging current which could be also pretty high, the ideal minimum being at 1/10th of the battery rating, for 200 AH battery this would be equal to 20amps

          • Oh, very well then. That means that a 30A relay will handle the charging comfortably. One more question sir, do i need another relay to connect/disconnect the negative terminal of the bridge rectifier or should i just connect it to the negative terminal of the battery?
            Please kindly provide me with the link of an automatic charger designed by you. Thank you very much sir.

      • Hello sir Swagatam,
        I have a challenge. It’s very difficult to get a DPDT relay (for this application) in my location. So that leaves me with no other option than to use three SPDT relays for the circuit.
        1. In order to cut cost, is it advisable to use two relays for the circuit, one for the battery and one for the inverter mains changeover, while connecting the neutral from mains permanently to 0V from inverter output?
        2. Supposing the automatic battery charger is for 36V, 48V (and higher) battery, what should I use to get a 12V supply to the relay coils from the bridge rectifier since even the voltage regulators have their own maximum supply voltage?

        • Hi Godson,

          you can work with 3 SPDTs but not with two, and using common neutral is strictly not recommended, and eliminating one relay will not save anything substantial, so it is better to go with 3 relays for a correct implementation.

          for the relays I would recommend using a separate 0-12V/500mA trafo, if the charger is intended to be used then make sure the supply is taken from the trafo secondary directly, and not from the automatic cut off supply of the charger….in order to drop the output to 12V you can use a BJT emitter follower circuit as shown in the following article


          replace the preset with a 12V zener

          • Wow! Thank you so much for the reply sir! I really appreciate it. I’ve learnt what I didnt know before. I think the best option for me here is to use a separate 0-12V/500mA transfo as recommended by you (with its own bridge rectifier) to power the three relays. I really appreciate you sir!

  5. Congratulations on the new look of your site
    thank u so much for the circuit it work for me and you are in deed my hero in electronic please keep the good work going and may Almighty reward you greatly
    Please i have one request can u please give me a good filter circuit i want to use it for my sg3524 inverter i build here is my email faithjumbolives@gmail.com
    Thanks and best regards

    • Thanks Faith, I am glad you liked my site’s new look…

      I am sorry I do not have a tested filter circuit, but you can easily make one through some trial and error, begin with a single LC first at the output side of the trafo

  6. Please for my charging circuit the AC mains Live is through the relay so anytime the the battery is full the relay will open which means there will be no live to power the changeover relays which will make inverter trigger ON in the presence of AC mains. Please​ can this be solved using multiple trafo with different 12V rectified to the relay coils? If not please help me out..

  7. Sir! I am thinking of using the centre-tap and one part of the 12v as ac supply into the separate charging section for charging the batteries during supply from mains. Can I get a design to match this!? I hope to meet you someday

    • Odion, a couple of links were suggested under your previous comments, please go through those links, they seem to closely match your simulation of a single trafo inverter/charger design

  8. Sir please! I need to work with one transformer to minimize costs of production. Any design to work this out? I have done some personal design but they are not promising. I need a charging section that is not done by the inverter mosfets sir but both must use one tranfo

  9. Helo Mr swagatam! I am a fan to your skills. I want to ask; Does this design implies that the same inverter tranfo is used for charging the battery or do I need a separate transformer for that? And if so, can I get a design on how to do this? Thanks a lot sir

    • Thanks Odion, same trafo cannot is not used here for charging/inverting, the diagram suggests only the changeover section from mains to inverter and vice versa, the charger section utilizes a separate power supply which may be a trafo based or an SMPS based.

  10. Hello. I only have one question: the SPDT relay disconnects the inverter under load. This creates a big shock on the MOSFETs from the inverter (not to say a possible arc in the relay, depending on the load of the inverter). I cut the DC connection to my inverner under load once (I put a smaller fuse than necessary on the battery wire) and half of the MOSFETs blew up.
    Wouldn't be much better if you transfer the load first and later cut the DC power to the inverter ?

    • You must use a flywheel diode with relay in order to prevent this. This is caused by the back emf of the relay coil. And diode protects the circuit driving that relay from its back emf. You can use general purpose diode like 1n4007 or 1n4148. Read about Flywheel diode or freewheel diode.

  11. Hello. I only have one question: the SPDT relay disconnects the inverter under load. This creates a big shock on the MOSFETs from the inverter (not to say a possible arc in the relay, depending on the load of the inverter). I cut the DC connection to my inverner under load once (I put a smaller fuse than necessary on the battery wire) and half of the MOSFETs blew up.
    Wouldn't be much better if you transfer the load first and later cut the DC power to the inverter ?

    • you can use freewheeling diodes across the mosfet drain and source to rectify this issue….or to implement your idea you can simply add a high value capacitor across the SPDT relay coil and keep it isolated from the DPDT relay coil through a diode….but the first option looks more logical to me

  12. Swagatam:
    I would like to replace the SPDT relay with 2 scr's. Would you consider a circuit to replace that relay?
    I believe a relay would need to handle 60 amps to be effective for the inverter side… and a smaller SCR for the Charger side.

    Many thanks for the great work you do.

  13. Pls sir, is it advisable for me to connect the coils of a 12v 10amp relay to 14v 1amp dc source and connect its two pole to load and it's two no to mains with it's two nc to inverter will it burn the relay(it's a dpdt relay)

    • why do you think it would burn the relay??…your wiring is correct and as per the shown diagram above so definitely it will work but only as long as the load does not exceed 5 amp or 6 amp max

      you can reduce the 14V to 12V by adding 3nos of 1N4007 diodes in series with the relay coil

    • Hi Swagatm,
      I have rigged up the circuit. Its working fine as per my requirement. I have used single SPDT of 20 A for this purpose. Tested for 15 days.Running smoothly. But only thing is it takes a second to switch over and in the mean time lights goes OFF and ON.
      Thanks for your suggestions.

    • Hi Sham,

      A relay will never take a "second" to operate, it will operate in milliseconds I think the DC input has a capacitor which might be keeping the relay switched ON even after the mains failure….this capacitor value needs to be reduced….or a transistor driver could be introduced for making the response instant.

  14. Hi Swagatam,
    Thats good. So i dont have to spend on DPDT since i have lot of SPDT ones. I am not using generator here. Its UPS( using as inverter ) so my neutral will always be constant. Inverter supports Max load of 600W so no issues with inverter. But when running on mains i think it can go beyond 1000W. So i will use 20 amps relay instead.
    Will update you once its done. Thanks again.

    • Hi Sham,

      are you your inverter has a constant neutral? confirm it first and then go for the SPDT version, otherwise you might burn the switch or the inverter fuse..

  15. Hi Swagatam,
    Cool came across so many changeover ckts … This one is very interesting. I have one question though.
    Can i use SPDT replacing the DPDT one? by connecting the nuteral lines of alternator and main together and only connecting the phase line to SPDT relay and toggling it. Here again, can i use 7 amps relay for switching AC mains? If not what is the suggested amps rating of the relay coil?

    • Thanks Sham, if the generator also has a neutral identical to the mains then you can make them common and use a SPDT switch, however if the generator neutral is not a permanent neutral and keeps alternating from live to neutral then SPDT will not do.

      7 amp relay contacts will do, if the max wattage of the load is not above 1000 watts

  16. pls i have not been able to connect my 3pole contactor? how do i make the connections cox the contactor has L1, L2, L3 in one side and at the other has T
    1L1 3L2 5L3
    2T1 4T2 6T3

    then it has also

    13 NO 21 NC A1
    14 NO 22 NC A2

    pls how can i make the connections on this because my main power grid is single phase and my generator also is single phase. how can i use it so as to b a changeover?
    please use schematic and circuit diagram and symbol to illustrate and explain please

    • I am sorry I don't have much idea about the contactor's operational details so won't be able to explain the connections

  17. Hello Sir, can you tell me which software did you use to make this circuit diagram? I want to use it for my project.

  18. Sir I have bought DPDT relay from the shop however it has two COM (common) pins what should I do with those pins? The relay has total of 8 pins.

  19. sir, can i use solar panel instead of AC main supply to charge the battery? another thing is that i want to use a 10W light as load for this project so can i use a 6V 4.5AH battery?

  20. Sir, the relay coil of the DPDT is shown clearly connected to the 12V DC charger, where is the relay coil of the SPDT connected?

  21. good day sir, i want make a pure or modified sine wave inverter of exactly 1.2Kva that will have battery charging section with couple of only SPST relays to make it fully automatic as described in this post, please i want you to fully guide me on this. thanks

    • Dear Yasir,

      all relay coils are connected with the AC/DC adapter power supply output, meaning with a DC acquired from mains AC.

  22. Sir me bhanu how can i send a E-mail to you.
    I have a simple mobile charging circuit I have already assembled this circuit but i am not getting the desired output so I need help from u………….plz sir give me a e mail id…Thank you

  23. hello sir,
    i think i may have found a solution to this slight problem i told you about last year.
    The problem was that since the battery charger supplies power to both relay coils and being that the battery charger is an automatic one this means that once the voltage is cut off because the battery has been fully charged it will automatically switch off the relays forcing them to switch back to inverter even with AC mains present.
    The simple solution would be that both relays are powered from a separate divider/bridge rectifier network which the appropriate voltage so that even when the battery charger cuts off voltage to the battery both relays will still be powered on and this means that the battery still remains cut off from the inverter and the appliances are still powered from AC mains.

    • Thanks Michael,
      But it's been already solved above as shown in the second diagram.
      We can see that it's done by powering the relay coils directly from the AC/DC main power supply while the battery is being charged through a separate cut off stage.

  24. I have already comprehensively explained it in the above article, you can use the above explained design for your application, it's exactly as per your needs.

  25. Here the charger should a separate DC unit and the over charge relay must be wired with the battery positive, this will enable the over charge feature to activate separately only for the battery, other solutions can also be figured out, there's plenty of options open.

  26. The relay coils are not connected with the battery, it's connected with the AC/DC charger output.

    so as long as AC mains is present the charger will keep the relays energized.

  27. 1)Inverter AC is the AC generated by the inverter.
    2)appliance refers to the household load
    3)Inverter battery input refers to the supply points of the inverter (+) and (-)
    4)"To battery" means to battery which is being used for operating the inverter.
    5)DPDT and SPDT both relays will have it's own separate coils, connect them together in parallel to the points indicated as "to relay coils"

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