Car battery input protection

[mayor]

Hello, I'm working on a CAN bus project, and started to look more closely at input protection for the car's 12V. I don't know if there is protection along the way where I will be tapping in, but assuming not to be on the safe side.

I happened on this TI schematic, and am not sure how the Zener (D10) wouldn't burn itself out in case of excess voltage? Shouldn't there be some current limiting, say with a resistor?

I'm looking for any practical advice to protect the input to the regulator.

[TimNJ]

Whether the zener will burn out depends on the nature of the transients you expect on the supply rail, and the effective source impedance. Harsh transients like 'load dump' are very hard to protect against, if you need an economical, reasonably sized solution. Some people may say "don't care" about load dump, because of a statistically low likelihood (?), and just make sure that the device will fail safe if hit with a load dump surge (properly fused). Other transients can be reasonably suppressed with a medium sized TVS in an SMA or SMB package, e.g. 400-1000W rated range, without additional series impedance.

Yes, adding impedance will limit the zener/TVS current/dissipation. If your device has a low current draw, say <100mA, then a resistor may be suitable. You still need to mind peak power dissipation during transients, and may want to use a surge/pulse rated resistor if necessary. Add as much series resistance as you can reasonably tolerate.

[mayor]

Whether the zener will burn out depends on the nature of the transients you expect on the supply rail, and the effective source impedance. Harsh transients like 'load dump' are very hard to protect against, if you need an economical, reasonably sized solution. Some people may say "don't care" about load dump, because of a statistically low likelihood (?), and just make sure that the device will fail safe if hit with a load dump surge (properly fused). Other transients can be reasonably suppressed with a medium sized TVS in an SMA or SMB package, e.g. 400-1000W rated range, without additional series impedance.

Yes, adding impedance will limit the zener/TVS current/dissipation. If your device has a low current draw, say <100mA, then a resistor may be suitable. You still need to mind peak power dissipation during transients, and may want to use a surge/pulse rated resistor if necessary. Add as much series resistance as you can reasonably tolerate.

Thanks,

the device can use up to ~400 mA, so I don't really want to add much impedance at all. I think I may opt for a TVS as you suggest, along with a resettable fuse if I can find one. In the event of a load dump, I am ok for the device to shut down.