Graynomad: Load dump is something that should be considered, > 100V I think. Can the TVS etc handle that?
That is the question. It can handle 1.5kW for a short time (in ms). My goal is to shunt any short bursts to ground, and blow the fuse on longer anomalies. In all cases, I want the components to survive. At the very least, the PSU should sacrifice itself while protecting the load.
Since I have no desire to put my car through the wringer in the name of experimentation, I had considered testing failure modes by feeding the input from a 120v wall outlet for 100ms or so. (Maybe I should try 500ms to be safe.) I'm thinking this could be accomplished with a reasonably fast relay and, of course, an Arduino to sequence it. A 50ft extension cord and a concrete driveway being the test bench, of course. ;-)
spirilis: The way I see it, if the accessory is on then you probably expect your gadget to be available.
This is the gist of my remaining concern with this feature. I can't think of any case where the difference between "accessory on" and "engine running" is of any real importance, but end users are funny like that. There is often an ignition switch as well, where the dash lights come on and the fuel pump engages and all that, but I don't think it will be accessible to most people without tearing into the steering column. I'm sure it's usually not going to be worth the trouble.