Switching power sources on power failure?

[steven6282]

Ok, so something else I'm trying to tackle.  How to switch the power source of a circuit if there is a power failure.  Switch between a wall wart to a battery backup if the power fails on the wall power.

I figured I could do this with a normally closed relay, but I can't seem to find one of these for sale anywhere, they must be rather uncommon to be used.

So is there another more common method for achieving this?  A logic chip that does it or something maybe?

[retrolefty]

Ok, so something else I'm trying to tackle.  How to switch the power source of a circuit if there is a power failure.  Switch between a wall wart to a battery backup if the power fails on the wall power.

I figured I could do this with a normally closed relay, but I can't seem to find one of these for sale anywhere, they must be rather uncommon to be used.

So is there another more common method for achieving this?  A logic chip that does it or something maybe?

A common method is to just use two voltage sources and two power diodes wired as a OR gate, the higher voltage will supply the load current and if either power source drops the load is still supplied by the other source, no switching delay.

Lefty

[steven6282]

I was wondering if that would work, I thought that might cause issues with the batteries still getting slowly drained over time due to some current getting pulled from them as well though.

What about a single pole double throw relay?  Do those touch one terminal when there is no power on the coil and the other terminal when there is power?  Or is it a stage latching type thing where power is dropped from the coil and then applied again to change the state of it?

If it's the first it might work as well.  Just put power to the coil from the wall power, and to one of the terminals, and power from the battery on the other terminal, with the common going out to a capacitor to handle the slight delay in switching.  The downside to that is if it rapidly switches back and forth for some reason it could still cause a power interruption, although I guess that could be solved with another capacitor in front of the coil wire from the wall power. 

This is also assuming I can share a common ground between the batteries and the wall terminal, I don't know if I can or not?

Sorry if some of this sounds idiotic lol, I still have quite a bit of learning to do about circuits

[majenko]

I'd go with the diodes, or something altogether more complex.

My preferred method is to have a LiPo battery with charge manager circuit and P-FET battery output switch.

When there is power it charges the LiPo, and turns off the P-FET that provides power from the battery to the rest of the circuit - a diode also feeds the main power into the rest of the circuit.  When power fails, the P-FET turns off, and allows power out from the battery to the rest of the circuit, giving uninterrupted service and batteries that are always fully charged.

[retrolefty]

I was wondering if that would work, I thought that might cause issues with the batteries still getting slowly drained over time due to some current getting pulled from them as well though.

No, if the fixed voltage supply is higher (by a volt or more) then the battery voltage then the battery isolation diode will be reversed biased and no current will flow from or to the battery and you would only have to deal with the battery's self discharge rate (shelf life).

What about a single pole double throw relay?  Do those touch one terminal when there is no power on the coil and the other terminal when there is power?  Or is it a stage latching type thing where power is dropped from the coil and then applied again to change the state of it?

Can be done with relays but it will be more costly and complex. You also have to deal with short switching drop outs that can effect high speed items like micro-controllers. Sometimes having a large storage capacitor properly placed can deal with the switching transients for critical high speed loads. Best to work from a known design or if self designed best to draw it up and seek advice on it.

If it's the first it might work as well.  Just put power to the coil from the wall power, and to one of the terminals, and power from the battery on the other terminal, with the common going out to a capacitor to handle the slight delay in switching.  The downside to that is if it rapidly switches back and forth for some reason it could still cause a power interruption, although I guess that could be solved with another capacitor in front of the coil wire from the wall power.

Again, best to draw something out as it's best to work with the true language of electronics, schematics, rather then words and sentences. 

This is also assuming I can share a common ground between the batteries and the wall terminal, I don't know if I can or not?

Should be able to as most DC wall power modules are transformer isolated and therefore you are free to common either polarity wire as you wish with another voltage source.

Sorry if some of this sounds idiotic lol, I still have quite a bit of learning to do about circuits

Not idiotic at all. Many projects require the ability to survive a household power outage by having a back-up voltage source. It's complexity and cost is dependent on your requirements of how long a power outage you need to be able to handle and how much current your load consumes Vs battery capacity.

Lefty