trying to build a switched power source with time delay

Thread Starter


Joined Jun 28, 2017
Hi folks,
I've been powering a car-tablet with 12Vto5V adapter fed off switched 12V (ACC). Works well for the most part with minimal drain once the tablet enters deep sleep like it should, but there are times when something keeps the tablet awake draining the battery. When I see <10% charge, I would like to add a mechanical switch to let it charge off of a constant 12V line when engine is turned off. While charging the tablet would incur minimal drain on the car battery, I am a bit worried it could still get discharged fully if left in this state (forgotten) and the car not driven for over a week. I am thinking something like a 2hr delay timer would allow the tablet charge fully then disconnect the constant 12V line. Ideally, the timer should fire again only if the mechanical switched is reset (i.e. if the mechanical switch is left in the on position without being reset, then the 12V line should only be energized just once); this is so that the mechanical switch is more of an emergency maintenance feature rather than a part of normal operation scheme.

Here is a crude circuit diagram I came up with:

I added a NC SPST relay there to safeguard against the condition where I might reconnect the car battery when the mechanical 12V line switch is engaged. That would short the ACC and 12V line, wouldn't it? Perhaps a diode each at the upstream of 12V and ACC enough to address this?