If you use a relay with the ignition, you can ensure that you never forget to turn the switch off and come back to a flat battery.
I've got a relay that controls a fuse box in the trunk wired directly to the battery (with a fuse as close to the battery as possible). From the fuse box, I power a Kensington 120W laptop adapter (which was spliced into to get a 5V line for my external soundcard), another 5V power adapter for my USB hub (from a 2W USB cigarette lighter power source), my amp for the front and rear speakers, and my subwoofer.
the point of the relay (as thefdog mentioned) is to ensure that power [to your pc, monitor, cig lighter, etc] is cut when car is off. you can still use a switch with a relay to only apply power when you want it. the relay simply ensures that you'll never have a dead battery because you forgot to switch power off to all your devices.
your plain old on/off switch will work fine (as long as it can handle the current that all those devices will pull from it), but you'll have to remember to switch it off when you leave the car. otherwise it may drain your battery. with a relay you'll never have to worry about that.
from this thread and several others you started (they are basically all about the same topic) you seem very reluctant to add a relay to your setup and I'm not really sure why. it will only make your life easier. they only cost $5 and it's only 2 extra wires
As a low voltage electrician i agree totally. Use a relay. This way there is no OOPS factor. Will also help prolong the life of you battery from excessive deep discharges of you battery.......when you forget to turn off the switch.......that's my 2 cents.
what extra wires would i have to run for a relay? You said there are two extra wires, my guess would be Acc and Ign? My main concern about a relay is where i'd put it.... Also with a relay when i turn my car off it sounds like the pc would turn off, unless i put an addition switch infront of the relay which would bypass the relay and direct signal directly to the pc.
You're going to want to run external power to the hub if you want to prevent potential issues, especially if you're going to have a slim DVD player hooked up to it. Also, in your diagram you're missing a data line (USB) from the touchscreen to the laptop.
what extra wires would i have to run for a relay? You said there are two extra wires, my guess would be Acc and Ign?
close ... one wire is a plain old ground (for the relay coil), the other is a switched power input (for the relay coil). the relay power input should be wired to the ACC or the IGN circuit. the ACC circuit (first key position) is live when the key is in the 'ACC' or 'run' position. the IGN circuit is live only when the key is in the 'run' position.
a standard bosch-style relay has 4 or 5 leads. 86 is the coil power (from ACC or IGN line), 85 is coil ground, 30 is your PSU's power source (car battery), 87 is the switched output (87 and 30 are connected when the relay coil is powered). on 5-lead relays, 87a is the other side of the 'switch' (87a and 30 are connected when the coil is NOT powered. for your setup you have no need for 87a, so just leave it disconnected).
I think I remember from one of your other posts that you wanted to use a lighted switch. if that is the case, then it would only be 1 extra wire (since you would be wiring a ground wire anyways )
Also with a relay when i turn my car off it sounds like the pc would turn off, unless i put an addition switch infront of the relay which would bypass the relay and direct signal directly to the pc.
with a desktop-type setup that would be true. but since you're using a laptop, it can run off it's own battery. when the relay switches off, the laptop will simply stop receiving power from the car's battery. it would continue to operate with it's own battery until you turn it off (or it's battery dies). if you want to continue powering it from the car's battery without the engine running, then you simply leave the key in the 'ACC' position
I would suggest that you power your LCD Monitor from the 12VDC output of your power supply. The raw 12VDC from the car's system usually runs about 14.8 with spikes and sags on the line from starting and as other loads are switched on and off. The thought is that the regulated output of your PSU will give you generally better prefromance and life on one of the most expensive components in you system.