Circuit to switch between main and backup 12v power supply

Thread Starter

Les1951

Joined Apr 6, 2021
5
Hi
can anyone suggest a simple working idea to make this work
I need. A circuit perhaps using a DPDT relay which will allow a mains powered 12v transform supply to a 12v air pump to switch to a backup 12v battery supply to the air pump in the event of mains power loss, and then back to the transformer when mains power is restored.
I would appreciate any suggestion.
Thanks
Les
 

Papabravo

Joined Feb 24, 2006
15,431
Connect the two supplies to the load with appropriately sized Schottky diodes. If the supplies have output voltage adjustments you can compensate for the voltage drop in the Schottky diodes. The load will draw power from the supply with the highest potential.
 

Thread Starter

Les1951

Joined Apr 6, 2021
5
Hi
Thank you for replying to my inquiry, below is the specifications of the pump, the idea is to run the pump by 240v input 12v dc output transformer, in the event of a power cut I want a 12v leisure battery to take over running the pump until the power is restored. I thought a NC DPDT relay held open by the power supply until a power cut which would then close connecting the battery might do the trick, or any idea you may have.
Thanks

Specifications:
Voltage: DC 12V
Power: 35W
Current: 3.5A
Pressure: 0.068Mpa
Flow: 68L/min
Diameter: 10mm / 0.4inch
Size: Approx. 220 * 110 * 140mm / 8.6 * 4.3 * 5.5inch
Weight: Approx. 1660g
 

sghioto

Joined Dec 31, 2017
2,264
Relay circuit will work fine doesn't matter what the battery voltage is.
Diodes will work as long as the 12 volt supply output voltage is higher then the battery voltage.
1617752390753.png
 

Tonyr1084

Joined Sep 24, 2015
5,802
Check to see if the pump can handle a voltage of 13.8 volts. If so then you can use mains power to keep the battery charged. When power fails the battery will just take over the load. A diode to prevent back feeding the power supply would be all that is necessary. Obviously you'll want to be able to keep the battery charged and ready for use. A 12 volt lead acid battery likes a float charge of 13.8 volts.

In my garage I have a car radio hooked to a battery and a 13.8 volt charger. The charger keeps the battery topped off. The charger CAN run the radio but there's not enough power for good base tone. So the bigger supply is necessary. When I play the radio it mostly plays off the battery and the charger helps keep the charge topped off. When I shut the radio off the battery is charged back up to 13.8 volts and is ready for the next time I am in the garage working on a project or the car.
 

Tonyr1084

Joined Sep 24, 2015
5,802
Relay circuit will work fine doesn't matter what the battery voltage is.
Diodes will work as long as the 12 volt supply output voltage is higher then the battery voltage.
View attachment 234731
D2 wouldn't be necessary as the supply can keep the battery charged if the voltage is set high enough to maintain a proper float charge. 12 volt supplies sometimes run a little higher than 12 volts. Nominal voltage for a lead acid battery is 12.6 volts. A charge voltage would have to be higher than that to maintain the battery. So if the charger can be adjusted then D2 would not be necessary.
 
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