I was asked to repair a camping cooler (a box where you put water bottles and food so they stay cool). It can be powered with the 12VDC from the car's cigarette lighter plug, or from external power supply 230VAC to 12VDC 5A.
It was an easy repair because there was a cracked solder joint in the external power supply, I just resoldered it and it works.
But I want to improve it! Currently, at the end of the external power brick there is a female cigarette ligther plug, where you connect the male plug from the cooler when you want to run it from 230VAC.
There is enough free space inside the cooler's cover to put the electronic board of this external power supply, so I would like to integrate it, remove the female cigarette lighter plug and connect the 12V output directly to the "main board" of the cooler.
Of course I still want to be able to plug it into the car's cigarette lighter plug, so what do I need to add in the circuit so it doesn't backfeed the (now internal) 230V to 12V power supply ?
Is it OK to use diodes on the + of both power sources, like this:
And what kind of diodes would I need in my case ?
Or is it safe to just backfeed this power supply?