3 way switch for inverter

I want to start out by saying thanks, and I am pretty handy in a lot of areas, but automotive electronics is NOT even remotely one of them. Not even a little bit. Now to the question .. I have just mounted my laptop in my car that I will be using for work. I currently have an inverter hooked up to it. 100 / 200 watt, depending on whether it's plugged into the cigarette lighter or directly to the battery. I basically want to have a 3 way switch mounted and controlling the inverter. Position 1 = Entirely off Position 2 = On when car is on Position 3 = On all of the time (even if car is off and doors locked) Reason for the 3rd option is there will definitely be times when I need my laptop charging or dvd playing for kids and we are not rolling down the road. (no i do not shut my kids in my car unattended!) Thank you again for your assistance!

A single pole double throw switch, AKA SPDT, centre off will do the trick but note the inverter's current draw say 15 amps, you will need to allow for that. Alternative would be to power up the unit permanently with the ignition on and just a switch for when engine isn't on. I'll draw a diagram.

This will give you what you want just use a standard Bosch/Tycho relay, two 1N4004 diodes and a standard illuminated switch. Power handling for the switch isn't important because the relay handles your load which will incidentally be the full 200 watts because if you use thick enough cables and go to the battery you won't loose much. Having said that this set-up will drain you battery in about 2-3 hours! It's unsafe I would run that relay off ignition or ACC only, I can't imagine ANY circumstance in normal vehicle where you would want something with this amount of current draw in a vehicle that was left alone. Only wire 86 from ignition or ACC. inverter.bmp

It is only running my laptop on the inverter, maybe a dvd player at the same time too, but not when my car is off. It would only be charging my laptop if left unattended. Everything you are saying is kind of greek to me. Questions 1) is my entire idea unsafe? 2) i don't know what the numbers 86 87 87a 85 and 30 refer to 3) what gauge wire should i use? 4) the power will always be set to on if car is on unless the switch is turned on, going with your diagram? Thank you very much for your assistance.

The main point is that you don't leave it charging without the engine running or last unable to charge without you being there. The numbers refer to the ISO markings on an automotive relay. If it's all geek to you why even bother to post?

jmcknight wrote: the power will always be set to on if car is on unless the switch is turned on

And with a 100W inverter drawing more than most high beams, you'd want to make sure it isn't for long if the car isn't charging. And 200W output can push some alternators at idle... That is Howie's point. And yes, Howie's method allows you to select which input mode you want. (Fuse & relay near battery. The inverter could also be there.) And with his method, it is easy to add a low voltage cut-out later (a $20 MW728 etc).