Certainly never been too good at electronics.
I need to hard wire a laptop charger to come on via the ignition, so this needs to go through a relay. 4 pin I assume... Power, earth, ignition live, and power out?
Now where I'm lost is the specs...
Its going between the 12-19v converter, so 12v obviously. But how many amps?
What should the coil resistance be? And coil nominal current?
What size fuse? Just to be sure... should it be wired between the battery and relay, or relay and converter?
Also needs to have an off over ride switch, does that go before or after fuse?
how many amps does the charger draw? it should say a number-- something like this: "4.62A" usually near the voltage of the adapter as well. that would be a good start for making sure most relay's will work.
but i am sure that it falls below the max rating of a standard bosch automotive relay available at most auto parts stores.
terminal 85 gets connected to the ignition wire
terminal 86 gets connected to ground
terminal 30 gets connected to +12v wire
terminal 87 gets connected to laptop charger
the override switch should be wired before terminal 30, but after the fuse.
for the fuse size, go with the closest rating near to that "A" reading the charger says. i believe it will be either a 5 amp fuse, or a 10 amp fuse.
A relay may not be needed.
And fuses are always at the start of what they are protecting - in this case the wiring and relay etc.
Equipment protection fuses are a different issue and are usually at the equipment end, though they can be the first fuse provided it's rated lower than the downstream cabling, relays, etc.