Switch mains voltage by detecting low DV voltage threshold

[neazoi]

Hi, I have a cheap inverter that is connected to a 24v battery and produces 220V AC.
I would like a circuit (or a cheap product) that will detect if the battery has a low voltage (I will have to set this threshold) and automatically switch the output to the AC mains instead of the inverter output.
Ie. when the battery is not charged, the inverter is bypassed and the output is connected to the AC mains instead of the AC output of the inverter.

The problem is, that I need the AC output not to be interrupted during the switching process. This can be done with a make-before break relay, but the problem is that I do not know if the inverter allows 220V to be present on it's output. By using this relay, the instant time the MAKE connection is done, 220v mains are connected to the inverter output.

Any schematics or cheap commercial products that can do this?

 

[chuckey]

You will find that using a make before break mains change over will cause you big problems. The only way this can work is if you invertor has the same voltage and phase as the mains, else there will be an explosion! The best way of doing this is to feed your mains out of the invertor and change over at the battery level to a mains fed power supply.
Frank

 

[neazoi]

You will find that using a make before break mains change over will cause you big problems. The only way this can work is if you invertor has the same voltage and phase as the mains, else there will be an explosion! The best way of doing this is to feed your mains out of the invertor and change over at the battery level to a mains fed power supply.
Frank

But at the instant time the break connection is accomplished no AC will run at the output and this might interupt power to the loads. Some of these loads can tollerate a very tiny time power interruption but some others can't.
i am not sure I understand the solution you propose, can you please explain this?

 

[red_alert]

What @chuckey has suggested you it's called "online/double conversion ups". It means that you permanently feed your load from the inverter and only switch the battery with a mains powered power supply.

Anyway, the true online/double conversion ups never disconnect the battery and you could do the same.

I guess you already have some kind of charging circuit (solar/wind) for your batteries. Then, instead of switching 230V, you could switch your battery charging circuit with a mains powered one (transformer + rectifiers) when the batteries have been discharged.

 

[neazoi]

What @chuckey has suggested you it's called "online/double conversion ups". It means that you permanently feed your load from the inverter and only switch the battery with a mains powered power supply.

Anyway, the true online/double conversion ups never disconnect the battery and you could do the same.

I guess you already have some kind of charging circuit (solar/wind) for your batteries. Then, instead of switching 230V, you could switch your battery charging circuit with a mains powered one (transformer + rectifiers) when the batteries have been discharged.

Ah, a UPS, I got it.
The solution for switching chargers you propose, keeps the batteries always charged. I do not want the batteries to be charged by the mains, since the mains can be used as it is in the house. A circuit that switches mains when battery voltage is low is ideal. A UPS as said. However big power UPSes are very expensive, just like big power inverters (which in fact are UPSes indeed).

How about a big (depended on the load) capacitor (not electrolytic!) connected somehow with the relay circuit so that it can provide this instant moment power during switching? (assumming no very big loads)

 

[red_alert]

I wasn't talking about a "big UPS" but the inverter you already have.

Anyway, what kind of load do you have that's not tolerating the relay switching time? Most of the home appliances (tv set, pc, chargers) have SMPS power supplies that already have input capacitors.

I had many offline UPS that only had an ordinary relay for switching mains/inverter and I never had any trouble with switching delay.

 

[neazoi]

Anyway, what kind of load do you have that's not tolerating the relay switching time? Most of the home appliances (tv set, pc, chargers) have SMPS power supplies that already have input capacitors.

I am talking about home appliances. This is a very useful information that solves my problem!

 

[Grazza]

I am talking about home appliances. This is a very useful information that solves my problem!

I appreciate that this post has now been answered, but I would like to share with you the solution Im putting together for a similar situation.

I have a solar setup and I wanted to auto switch between the battery 12 volt - 240 volt inverter and the mains supply. I decided to use the LVD feature of the solar controller, so I simply hook the 12 volt output of the solar controller to the coils of a 30 amp electromechanical relay, connect the output of the inverter to the NO terminals, the mains to the NC terminals and the permanent out 240 volts to the COM terminals. So when the solar controller detects low battery voltage (which can be adjusted), the mains supply kicks in and when the battery voltage reaches normal (again this can be adjusted), the inverter kicks in to supply the 240 volts to my appliance.
I have tested this using a bench power supply to drive the 12 volt relay coil and it seems to work with my TV and cable box with no drop out.

I then started to think of safety, e.g. earthing, I wish I hadn't, it seems to be confusing topic in relation to power inverters?

NB: I'm not a qualified electrician, just a hobbyist with a electronics background, so please do not take my solution as reliable or safe.