33 replies to this topic

[mattbeddow]

Fair enough, shouldnt be too hard to knock up a circuit to provide a nice stable 12v supply. Ive done it 100s of times for 5v and 3.3v supplies, just need to find an appropriate regulator

[tiger99]

It isn't easy in a car. Most of the available regulators will fail the first time you use the starter, or switch off headlights while the engine is at high revs, due to the spikes of 45 to 55V which tend to occur. Nor will you necessarily get a good regulated 12V all the time, because when the input goes below about 13.5V, the regulator will not regulate any more. You do get low dropout regulators, but they tend to be even less intolerant of transient overvoltage.

I will come back with a proper circuit, given time. The only validated designs I have right now are for aircraft 28V systems, and it is not just a case of scaling them, besides which I need to find components which are readily available.

[mattbeddow]

The LM7812 would be a good place to start, it can take a constant 35v and still deliver a stable 12v output at 1A. you should also be using a 1000uF capacitor rated to at least 63v (200v's are redily available) before the regulator to smooth off any short term fluctuations for switching off lights etc. Also, if you were happy enough to turn off the fans before you started the engine then you wouldnt have to worry about the starter motor. and a zenir diode and transistor would make a very effective over voltage protection on the output, assuming you can find a 12v diode. Total cost of parts would be no more than £2

Also, dont alternators provide a much more constant voltage than that?

[dklawson]

Disclaimer: I am not an electrical engineer.
Statement: You can use chips from the 78XX series as fixed output regulators in negative earth cars without too much worry about voltage spikes destroying them. I have used the 7810 chip for year (in gauge systems) on several applications without loosing one. This has been in spite of the fact that I did not install anything (not even the datasheet recommended caps) on the input to the chip. That is not to say that the installations would not have been better with the cap installed. Putting a cap before the chip will certainly not hurt and should protect it. My experience using these has only been in cars with alternators.