Automatic Inverter and Mains AC Changeover

I have been put forth with this question many times in this blog, how do we add a change over selector switch for automatic 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 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.

Converting an Inverter into UPS


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:

 



 

Need Help? Please send your queries through Comments for quick replies!




Comments

Swagatam said…
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"
Swagatam said…
yes that's correct.
Swagatam said…
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.
Swagatam said…
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.
Swagatam said…
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.
mexzony said…
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.
micheal
Swagatam said…
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.
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
Swagatam said…
bhanu, you can send it to hitman2008(at)live.in
Yasir Shah said…
Dear sir! The DPDT relay coil is energized by AC or DC.
Swagatam said…
Dear Yasir,

all relay coils are connected with the AC/DC adapter power supply output, meaning with a DC acquired from mains AC.
Pritam Bhowmik said…
hi bro.any harm to operate a relay with higher voltage than its rating?? i mean can i give a 13.7 v to a 12 v relay??
Swagatam said…
hi bro, a couple of volts higher won't harm the relay .. but would dissipate and waste a few tens of milliamps
MUHAMMED ADIO said…
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
MUHAMMED ADIO said…
pls i am sorry for the mistake in the earlier post i want to use only SPDT relays not SPST be
MUHAMMED ADIO said…
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?
Swagatam said…
Hi Muhammed you can try the following design:

https://homemade-circuits.com/2012/02/how-to-make-mini-homemade.html
Swagatam said…
The DPDT and the SPDT have singles coils...both are shown connected to the charger supply.
Dipto said…
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?
Swagatam said…
Dipto, yes that's possible, you can go ahead as per your mentioned plans.
Dipto said…
Thank you sir for your reply.

I have another question is it possible to make a triple changeover? For example if one battery is empty then another battery would give backup by switching automatically. I am interested to add solar & hydro generator with the main supply for my project.
Swagatam said…
Dipto, it is possible to get a split battery changeover actions, you can refer to the following articles for more info:

https://homemade-circuits.com/2014/05/twin-or-split-battery-charger-circuit.html

https://homemade-circuits.com/2013/08/automatic-dual-battery-charger-with.html
Dipto said…
Thank you sir for your reply. The 'split battery charger circuit' you mentioned is exactly what I required. I'll be using solar and hydro to charge the battery 1 & battery 2 respectively, hopefully it turns out well. I will inform you about the outcome.
Dipto said…
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.
Swagatam said…
please show me its pic I'll help you to identify the pins
Dipto said…
The relay looks like this (the link given below).

https://drive.google.com/file/d/0ByOP1BiJWDUaWk51dDJReUtPUEE/view?usp=sharing
Swagatam said…
OK, the common terminals are the poles of the relay shown as black contacts in the above diagram, the N/C are the upper white circles, while the N/O are the lower white circles in the relay contact diagram
Dipto said…
Hello Sir, can you tell me which software did you use to make this circuit diagram? I want to use it for my project.
Swagatam said…
Hello Dipto, I did not use any software, I designed it myself.
Dipto said…
What I meant was which software did you use to draw them?
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
Swagatam said…
I am sorry I don't have much idea about the contactor's operational details so won't be able to explain the connections
Sham said…
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?
Swagatam said…
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
Sham said…
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.
Swagatam said…
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..
Sham AV said…
Yes Swagatam, output of UPS will have constant nuetral terminal... Otherwise phase and neutral will toggle in home AC mains
Swagatam said…
OK, then it's fine to use a SPDT switch...
Sham said…
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.
Swagatam said…
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.
victory said…
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)
Swagatam said…
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
MusicGirl said…
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.
Swagatam said…
Thanks Musicgirl, the concept looks useful, I'll try to design it soon and will publish it in my site as a new article.
Unknown said…
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 ?
Mihai S said…
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 ?
Swagatam said…
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
odion Sireboy said…
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
Swagatam said…
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.
odion Sireboy said…
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
odion Sireboy said…
And sir! Can I get a design of smps that can handle current up to 60Amps?.....thanks alot
Swagatam said…
odion, I have explained one such concept below

https://homemade-circuits.com/2012/12/single-transformer-inverterchargerchang.html

however inverter mosfet is not used for charging in the above link
Swagatam said…
you can try the following design

https://homemade-circuits.com/2014/07/adjustable-0-100v-50-amp-smps-circuit.html
Odion, you can also use this design. And you can modify it to have more power.
https://homemade-circuits.com/2016/10/transistor-ups-circuit-with-charger.html
odion Sireboy said…
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
Swagatam said…
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
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.
Hiram Kumado said…
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..
Faith said…
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
Swag said…
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
Godson said…
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.
Swag said…
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.
Godson said…
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?
Swag said…
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
Godson said…
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.
Swag said…
Godson, Another relay will not be required, the ground can be connected in common with everything.

you can try one of the circuits explained in the following article

https://homemade-circuits.com/2016/05/opamp-low-high-battery-charger.html
Godson said…
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?
Swag said…
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

https://homemade-circuits.com/2015/05/3v-5v-from-9v-using-diodes.html

replace the preset with a 12V zener
Godson said…
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!
Swag said…
Thank you Godson, wish you all the best!!
Faith said…
Please sir can you give me sample diagram of a good filter i still having problems with it below is my email faithjumbolives@gmail.com
Swag said…
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..
Faithy said…
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
Swag said…
OK, that will do, as far as IGBT is concerned you can connect it directly with the SG outputs through appropriate resistors, driver is not mandatory because IGBTs are extremely efficient devices and can work without any external hassles.
Faithy said…
What I'm using is the block module type of igbt. and again the output of the SG enters a transistor bc557 can the output from the bc557 open the gate of the igbt since it below 9v thanks and best regards
Swag said…
you can connect the IGBT gate directly with the IC outputs through a small value resistor, no need of any BJT driver
Faithy said…
Ok sir thanks for your assistance I'm really grateful
Swag said…
You are welcome!
Godson said…
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.
Swag said…
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.
Jay khojare said…
Is it work for household invertes? How much current it will handle?
Swag said…
It will work for all types of inverters. Power capacity will depend on the relay contact rating

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