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How to Convert an Inverter to an UPS

How to Convert an Inverter to an UPS

An inverter is an equipment which will convert a battery voltage or any DC (normally a high current) into a higher mains equivalent voltage (120V, or 220V), however unlike an UPS inverters may lack one feature, that is these may not be able to switch from mains battery charging mode to inverter mode and vice versa during grid power failure and restoration situations.

Converting an Inverter to UPS

An inverter can be easily converted to an UPS with a few simple modifications or rather additions with their existing circuitry.

The lacking or missing changeover feature in an inverter can be upgraded by including a few number of relay stages within its circuit, as explained in the following sections:



Referring to the figure below, we see that the above requirement is implemented by using 4 SPDT relays whose coils are wired up in parallel and joined with a mains operated DC source, which could very well be the battery charger DC output.

It means during the presence of mains input the relays would be energized such that their N/O contacts get connected with the individual relay poles and the respective electrical gadgets which could be seen connected with the poles..

The left two relays can be seen with their N/O contacts connected with the mains AC input, while the N/Cs are terminated with the inverter mains output.

The relays at the right side have their N/O contacts rigged with battery charger (+)/(-) inputs, and the N/Cs are integrated to the inverter DC input.

The above data ensures the following actions during mains presence and failure situations:

When mains AC is present, the appliances get connected to the available mains power via the left pair of relay poles, while the battery is able to get the required charging voltage through the right hand relay poles. This also ensures that the inverter is cut-off via the N/C points from the battery and is no longer able to operate.

In a situation when mains AC fails, the relay contacts revert to their N/C contacts, giving rise to the following actions:

The battery instantly gets connected with the inverter DC input via the right hand side relay N/C contacts, such that the inverter becomes operative and its output starts producing the required mains back up voltage.

At the same instant the above inverter mains voltage now gets switched to the appliances via the left hand side relay N/C contacts ensuring that the appliances do not experience an interruption while the positions revert in the course of the above actions.

Selecting the Relays

The relays must selected with low coil resistance type so that they operate under higher switching currents, and therefore are able to "hammer" the contacts much harder and quicker compared to the lower resistance coil relays.

This will ensure the changeover time to be rapid within milliseconds which happens to be the most crucial factor with UPSs and inverters needing to be converted into UPS systems.

 

In the above diagram if an automatic battery charger is used, the supply would be cut off once the battery is fully charged, which would also cut off the supply to the relays forcing the inverter to switch ON even while the mains is present.

To avoid this issue, the relays must be powered through a separate power supply as shown in the following diagram. A capacitive type of power supply circuit could be seen here, which makes the design much compact.

Note: Please connect a 1K resistor across the filter capacitor associated with the bridge rectifier, this is to ensure its quick discharging during a mains failure, and an instant switching of the relevant relays.

 

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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!



72 thoughts on “How to Convert an Inverter to an UPS”


  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Good evening Sir i am finished with this circuit but the 4UF capacior as you directe me to use is getting very hot the more i connect the circuit on 220v ????

    • Engr bashir, make sure the capacitor voltage is 400V. I think there may be some problem with your capacitor, otherwise a high voltage capacitor will never become. Try Replacing it with a good quality PPC or MKT capacitor and check again

  3. Good evening sir you said that we should put 1k resistor across the filter capacotor. Do you mean we should put it between the positive side and the negative side of the capacitor???

  4. Good afternoon Sir, Sir i didn’t get 2uf 400v can i substitute it with 2.2uf 400v?? and 100uf 100v with 100uf 160v???

    • Hi Bashir, no problems it will work, however make sure the total current of the relays are not above 80mA, otherwise you may have to use a 4uF capacitor.

  5. Good mprning Sir as i told u last time that i wanted to do this circuit. but i want know if it does not delay before changeover take place. what i mean is that can this circuit change the phase very fast without any interrupt

    • Hi Bashir, the delay will depend on the relay specifications and quality.

      A High quality relay will give minimum response time, however for this the supply source will also need to be from an SMPS based high current power supply.

  6. Good morning Sir this is my second agenda circuit i want to built it and i will let you know if i am finished and if i hv problem also

  7. ok sir, another confused in the place were u wrote TO MAIN 220V SOCKET, and second place you wrote TO MAIN. my question is that which one will go to the MAIN AC GRID??

    • Mains socket is Mains Grid…both are one and the same…the transformerless power supply input will also connect with this mains 220V

  8. Passing inverter battery power via a relay could be a very bad thing considering the fact that the inverter could be drawings up to 90amps from the battery and if the really is rated 20-30a it could mess things up

    • In electronics Everything can be bad and dangerous if done non-technically, a 30 amp relay would burn and fuse at 90amp any sensible person will understand this.

  9. hi sir. please I need a circuit of automatic fan that will cool my heatsink when it get hot.. the fan will automatically blowing when it cool it will stop blow. ..

  10. Hello Swagatam ,
    I have an 5.5kva microtek Inverter which i use to power my cyber cafe.The issue i am facing is as soon as the electricity trips or even fluctuates , a few of my computers goes off automatically although all the pc's have there own UPS (most of them are new or with new replaced batteries). Kindly help

    • Hello Sameer, PCs are generally very sensitive to power fluctuations and even the slightest of power lapses can rattle them…in your case it could be happening due to may be slight inefficiency with the relay changeover actions within the UPSs, which could be difficult to correct or avoid. You can try replacing the systems with SSR based UPSs or alternatively you can try replacing the relays with SSRs after doing some research in the field….

  11. Sir, the normal regulator operate Air Cond fan only at number one, which is the highest among the change. if an attampt is made to reduce the speed, the fan goes off.

  12. Sir I need your assistence.
    I have a Fan removed from dead Air Conditioner I what use it as a room fan but, it fails to work with cieling fan regulator.
    Is there any circuit to achieve this?
    Thank you Sir.

    • Aminu, both the motors are "capacitor-start" type however Air Cond motors are more powerful and include a relay,
      first keep the dimmer at max speed let the motor start and then you can try reducing the speed once the relay has clicked.

  13. Hi Swagatam,
    Your blog makes interesting reading, and your knowledge seems to be phenomenal. I would like an 'idiots guide' to convert my inverter into an 'online' ups. I am not very technically savvy, but can manage my way around circuits – somewhat. I have an inverter for my house. It has two batteries of 500W each, I think. There is this delay in the inverter coming on once the mains fail. I was thinking of buying a new online one, but came across your blog. Would it be possible for me to make an add-on unit that I can use with the existing inverter? Or do I have to tinker with the existing circuitry – which I do not want to do. Some advice would be very welcome.

    Thanks,

    Yogi Bhattacharya

    • Thank you Yogendra, you will have to modify the existing inverter system by opening it and by checking the specific stages.

      You will have to check the relay changeover section and remove the capacitor which may be causing the delay in its operation….once you do this you will be able to find the inverter switching much quicker and making an instant changeover from mains to battery and vice versa.

  14. Hello sir. I would like to make a 12volt DC battery backup system to run 6 Led light each 12v 12wats and one 12v dc fan during power failure.i have 2 sealed lead acid batery each 12v 7Ah. Please guid me how to make this and i need to maintain battery by charging and without over discharge. Sorry for bad english.

  15. sir, if i use dpdt 8 pin relay to change the mode, then one relay can change load from mains to inverter and another one relay(8 pin) can separate battery from charging mode to inverter battery terminal input, when main is out.so, please tell about correction to me.

  16. Hello… Sir, if I connect only one relay (12 volt,200 ohm) on the above change over circuit.then,what is the changes on circuit.

    • hello swapan, for a single relay the specifications should be 4 pole 4 throw type…that is it must have a 4 sets of N/C an N/O contacts and 4 central poles.

  17. Ok, Sir.
    Thank you.

    Sir, can I get one that can safely charge 50Ah to below. like 40Ah, and or 20Ah.

    thank you sir.

  18. Ok Sir.
    Thank you very much.

    Sir, do you have a circuit on transformaless as above that can charge 200Ah 12v battery?

    thanks a lot Sir.

  19. Sir you are great!
    and Iam greatful for the help.

    Sir, can we charge 200Ah battery with transformarless charger circuit?

    thank you once again.

  20. Sir, I did it, and it works very fine.

    Still I want to know what current of relay can handle 200 AH battery?

    Thanks for the help sir.

  21. Godd Evening Sir.
    Sir, I want use this circuit as changeover and inverter switch.
    I have built the circuit, but I used only three relay as above and the design is as follows:
    *the first two relay are remain as it's on the circuit above.
    *the third relay is used as a swicth between the inverter and the battery.
    The positive ( ) were of the battery is placed in NC and the center were of the transfo is connected to the POLE of the relay.

    Sir, I hope there is no problem with this desing. And I hope the relay can hold the current between the center were of the transfo and the positive terminal of the battery.

    Thank you Sir.

    • Hi Aminu, if you do everything correctly then definitely it will work as explained in the article.

      the relay contact current rating should be much above the current consumption of the transformer, to work handle it safely

  22. Thanks Swagatam for clearing my doubt, another method I was thinking to put a diode before connecting UPS with battery, so there is no current to battery from UPS. If it is possible can I use IN007 (small diodes plenty available from old CFL ) may be 10 diode in parallel

    • Yes you can block one of the inputs through a diode, but 1N4007 will not work, even in parallel, because diodes cannot be connected in parallel.,,,you may have to opt for a special 10amp diode…

  23. Hi Swagatam I have 2 PC working on one inverter, distance between inverter and PC is around 4 meter, I have 2 UPS good condition but without batteries, can I connect these UPS to battery of inverter directly, will there be conflict amid charger of inverter and chargers of UPS, if yes how can stop charging from UPS to inverter battery.

    • Technically there shouldn't be any conflict between the two since their polarity would be at the same level. You can prefer to remove the inverter charger as that may be easier to locate than to locate the charger section of the UPS.

      Or you can simply leave them as is, and use the battery as common with the two units.

  24. Hi, how does this circuit handle the fact that the AC mains and the mains from the inverter might be out of phase? What would appliances such as TV's do in a case where these phases are not alligned on the switch over?

  25. hi swagatam
    your idea is very but i have little confusion that all relays are powered by charger. so there may be time lag for relay to denergise during power failure because charger contains large filtering capacitors.

    • Hi Jignesh, an smps normally carries a low value filter at the output, therefore with 4 relay coils in parallel, may be a 100uF or 220uF capacitor could take hardly any time to get discharged, so I don't think that would create any significant lag for the relays.

      we can use low resistance relays (100 ohms) for further ensuring a quick response from them.

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