Hi

I am a retired engineer, my question is as follows:

How do I auto switch over from mains supply, regulated down to 9v, to battery without dropping power, and return to mains power from battery after a power drop.

Cheers.

Rob

 

OK I am a bit "old school" so would like to avoid specialist power detection chips, but if that is what is needed OK, I have no objection to large capacitors being housed in my casing to cover the failover/ recovery. I am running a load of data loggers across my garden to monitor the environment to improve it to boot wildlife and insects, I am a bit remote from (CMP's [Centers of Mass Population]) so power drops are fairly short but not infrequent.

Rob

 

Hi Rob,
If your system can tolerate a small voltage drop, say 9V down to 8.3v, you could do the auto selection with two Schottky diodes
E

 

Make sure the adaptor voltage is a little above 9V, then feed the power from each through a diode. When the power is on, the battery will be out of the circuit because it is reverse biased. When it goes off, the battery supplies power.

 

Also an old timer, I would put a 9.1V charger across the battery. If the power fails there is no delay as the battery is always across the project.

 

Hi Rob
Is your 9V supply a battery, say a PP3 type or some other type.?
E

 

Hi

I am a retired engineer, my question is as follows:

How do I auto switch over from mains supply, regulated down to 9v, to battery without dropping power, and return to mains power from battery after a power drop.

Cheers.

Rob

Also being old school, I follow the KISS method. Keep it simple stupid. And what could be simpler than a relay?

Mains go down, relay drops out switching to the battery backup.

 

Also being old school, I follow the KISS method. Keep it simple stupid. And what could be simpler than a relay?

Two diodes. No moving parts. Switches far faster.

 

Two diodes. No moving parts. Switches far faster.

We may be missing part of the issue.

Is he trying to run a power supply in parallel to a battery backup? Or does he want isolation also?

Plus relays are time proven and durable. Less prone to damage due to power spikes. I've replaced many fried diodes. And I have only had to replace one fried relay that wasn't burnt in a fire.

Also, I'm a former radio engineer that is aware of the harsh conditions that remote and infrequently tested gear has to go through.

 

I may not have explained my suggestion well enough for the youngsters to understand.

Use a relay with the coil powered by the mains. The basic low voltage power supply is connected to the NO contacts. The battery backup is attached to the NC contacts. With the equipment power feed on common.

Mains power keeps the coil energized. When the mains go down, the relay cuts loose and switches to the battery backup. When power is restored, the relay re-energizes.

KISS

Of course you can add on things like a capacitor bank and then an indicator light. Maybe even a remote indicator of the relay status. All just using a relay.

KISS with durability and options.

 

Is he trying to run a power supply in parallel to a battery backup? Or does he want isolation also?

Go back and read the first post. He is trying to run a 9V device from a mains to 9V supply with a battery that takes over when the mains supply is interrupted.

Yes, a relay will work but I think two diodes is simpler, smaller, faster, quieter, cheaper and more reliable. You may disagree, no problem. Nothing wrong with having options.

 

Go back and read the first post. He is trying to run a 9V device from a mains to 9V supply with a battery that takes over when the mains supply is interrupted.

Yes, a relay will work but I think two diodes is simpler, smaller, faster, quieter, cheaper and more reliable. You may disagree, no problem. Nothing wrong with having options.

True. And, for every electrical problem, there are at least 27 solutions.

We are here discussing an issue without all the facts. Such as actual layout and power conditions.

 

True. And, for every electrical problem, there are at least 27 solutions.

We are here discussing an issue without all the facts. Such as actual layout and power conditions.

Rob, the original poster, could you give us your opinion?