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you can get 12V directly from the battery, use some caps for filter....
but for 5V and -12, -5 and 3.3 or 2.5 you need some more complicated psu. you should use a good switching...
but about the transformer, i dont know......i dont think that it will work...
i mean get 12v from battery, convert to AC 12 or whatever the output of the transformer was then connect to where the transformer was in the power supply, doing it this way gets the -12, -5 and all the other voltages you mentioned
why wouldn't it work ? (i don't want to waste my time on a dud project)
It is my understanding that in some car electrical systems that disconnection (intentional or the cable breaks) of the battery while the car is running will send voltages to very high levels momentarily and that electronics for those applications are designed with this in mind. The 12 volts is also incredibly noisy. I've veiwed the voltage spikes from the fuel injectors as they appear at the cigarette lighter -they are significant. I'd avoid direct use of the 12 volts unless you've addressed the overvoltage and noise.
i am an automotive tech. i will explain what happens, when you disconnect the battery it is like taking the capacitor out of a circuit that requires a constant 12v. the alternator supplys the power to your vehicle while it is running, this is true. the battery acts as a capacitor. thats basically it. it wont neccesarily send a voltage spike but if it does the tronics have a thing called a zener diode to prevent damage to the major comp. such as the PCM and the BCM and others( car pcu) the diode is there to protect against volt spike from the injectors and the um somthing else also. you would prolly not be able to see them from the cig lighter because that is connected to the battery. the control from the pcm is where you would see it. the noise you are seeing is the alternator ac noise through what is called electromagnetic interference(emi) and you can sometimes here it in the speakers as well.
i apologize for sounding stuck up or rude but now that i know this information it just bugs me a little when i hear people stating incorrect facts. again i apologize if anyone took offense to the language.
edit: the alt. technically puts out 13.5 to 14.4 volts somewhere in that range. plugging directly into the battery would not be noisy the alt is the thing that is noisy, the battery acts as a stabilizer ie capacitor