2 power sources with one output

ok so i am trying to figure out how to wire this so this works

what i want to do is have a 14 amp 12v power supply (connected to a 120v source) in the back of my truck to power my strobes for when i do events. currently the strobes are powered by the trucks battery. i want to be able to switch the power source without it leaking back into the trucks power grid.

i was thinking of using relays so when i click the power supply on it activates the relay and gives power to my strobes. and when the power supply is off the trucks battery now supplies the power to the strobes.

hope this makes sense. if someone can draw me a diagram this may make more sense to me.

Thanks

Shouldn't need a diagram...

Common the GNDs.

A 12V 30A SPDT relay.

#30 to the strobes.
#85 to GND.
#87 & #86 from the AC supplied 12V.
#87a from the battery +12V.

so the power supply can be grounded to the chassi and that wont affect anything?

Not if the chassis is floating.
Nor if the power supply -ve is floating.

If in doubt, use a DPDT relay energised by the power supply 12V.

sorry about all the questions but here is the last one.

not sure what you mean by floating and -ve
what if i just used 2 relays? one for ground and one for the pos? this way it would be isolated when in use by the power supply

Very simple, where does the ground wire on your strobes go to?

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Or use this diagram, it gives you a choice of sources, in fact depending on the load current of the strobes with a heavy (i.e. current capacity)switch you don't need a relay:-
strobes.bmp

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

You don't need a relay use Mouser SPDT switch, part no- 611-CG103J11S205QF, about CAN$2. Handles 15 amps.
Use it as your ON (Ext) - OFF - ON (Batt) switch.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

couldnt pull up the part number. i have a bunch of relays here would like to use these. but i should be fine using a common groundlike the truck itself? the other guy was saying in a previous post that it depends if the voltage is floating.

but anyways i could also use 2 relays and this way its isolated. but if you think that using the ground that is already there should work without problems i will go that route.

Thanks for the help its appreciated

Floating - as in the vehicle is not connected to AC ground or any other mains/AC etc, and the AC supply's DC output is not referenced or connected to vehicle +ve or GND etc.
All voltages are "floating" or "isolated" with respect to each other...