OK, just a few stupid questions.... 1: If I switch round the 12V and GND on a fan, it'll go backwards right? 2: Why don't computers use a similar sytem as cars, ie, using the chassis as GND? Given that there is probably a good reson why not to do it, would it still be ok to try with things like fans or LED's, That way I'd only need one wire running to something, and then a ground wire attached to the case at some point...
I don't think the fan will go backwards, because the brushless motors use transistors and other components which probably won't work when the polarity is reversed.
Wrong. PC brushless fans contain a semiconductor switching system instead of carbon brushes, power can only be one way round or they won't work. That would mean every component needing a good connection to chassis through its mounting method, far more reliable to have a plug-in ground and simpler to do in a single multi-way plug. You've still got 2 wires at the fan or led.
A PC uses the chassis as ground for allmost everything with a metal chassi, like harddrives and CD-roms. But since computers have highly sensitive electronics, the ground wires have been added as extra safety. It allso makes it cheaper to make the chassi since less care has to be taken in fitting the pieces together. You can usually make a fan go backwards by swapping 2 wires to the coil inside the fan, but most fans have shaped blades that work really bad if run backwards.
Cpemma's right fans won't work when voltage is reversed, they just sit there. Besides the blades would be very inefficient since they're only designed to go one way. In cars the chassis is grounded and is used for simple things like small lights and position sensors i.e. if the door is open or closed. However, it is a very poor ground and would never be used for signal paths. Its also considered bad practise since the current has to go through many mechanical connections, rivets and spot welds, which is what makes it a poor ground. If it was one big piece of continuous copper (or a ground plane as its called), then it would be the ideal place to attach to ground, but your chassis isn't. If you're asking if it will work... yes it will. Only do this for LED's and fans and such since they wont notice a bad ground, but do not do this for HDDs and the mobo as you will surely get errors.
I hate to think how much noise a cheesy fan would put into the ground plane if he actually did that, though... might create some untraceable havoc, there.
