Regulated 12VDC from car

[MikeMl]

Here is a "protected" LDO regulator.

For reverse polarity input, I added a 2A fuse and a 3A silicon diode. In the simulation, you can see what happens to the output voltage V(out) just before the fuse blows. btw- almost all avionics and ham radio transceivers (Yaesu, Icom, Kenwood) all use this method of reverse polarity protection. If you are stupid enough to connect the car battery backwards, you deserve having to replace a fuse.

For positive excess voltage inputs, I added a zener diode and a signal diode. As the input voltage increases past the reverse conduction voltage of the Zener, current through it tricks the regulator into shutting down. Note that in the simulation, the input voltage is swept from 10V to 40V. You can see the saturation behavior from 10V to 12V, at which point V(vss) keeps climbing but V(out) stays at 12V (the regulation region). When V(Vss) exceeds about 15V, the Zener conducts, and the regulator shuts down, causing V(out) to go to zero. To show how that protects the PFET, I also plotted its power dissipation (black trace, in Watts).

The upper voltage limit in the positive direction is limited by the breakdown voltage on the 2N3904 and the PFET. If you think you are going to disconnect the car battery with the engine running, then you might want to get a higher voltage NPN in place of the 2n3904, and get a 100V PFEt. The 2n3904 is good to 40V.

 

[MikeMl]

The only problem with a 5V 4A linear regulator that starts from 15V is that it needs to dissipate (15-5)*4= 40W in heat. Good news is that you can bolt it to a lot of metal (the car), but the mounting must have a low thermal resistance.

At this power level, a switching regulator is a lot more desirable. Typical efficiencies run 90%, so to deliver 5V @ 4A (20Watts), a switching regulator would only dissipate 2W or less. Unfortunately, switching regulators require some specialized components, like an inductor, fast diode, fast fet, and a moderately expensive IC.

Here is a variant of my circuit using an NFET. This is not a LDO, but it doesn't have to be to make 5V from 12-15V in. The NFET has to be rated for 50W or more.

 

[fernando_g]

I'm not belittling your circuit, Mike, not at all. It actually will work very well.

I'm just pointing out that the SAE has some stringent requirements for automotive electronics, because as you rightly put it, stupidity happens.