How to protect from damaging circuit during engine start ?

Thread Starter

fsq

Joined Jul 1, 2018
8
Hi, I assembled indicator beacon using led, ne555 and cd4017. It is propelled by 12V from a cigarette socket in a car. ICs sometimes get damaged, when beacon is plugged during engine start. I assume that problem is either spikes or reverse voltage. How to build simple protection ?
 

Hymie

Joined Mar 30, 2018
836
Hi, I assembled indicator beacon using led, ne555 and cd4017. It is propelled by 12V from a cigarette socket in a car. ICs sometimes get damaged, when beacon is plugged during engine start. I assume that problem is either spikes or reverse voltage. How to build simple protection ?
Gone are the days when I added all sorts of electronics to vehicles I owned – but that was in the days of no lights on reminder, no intermittent wipers, no alarm, no electronic ignition system etc.

Most of the time I would protect the added circuit feeding the +12V through a diode with a large electrolytic capacitor providing smoothing and minimising the voltage drop in the circuit, while engine cranking.
 

danadak

Joined Mar 10, 2018
4,057
https://www.mouser.com/applications/automotive-circuit-protection/

Load dump power surges are unique to automotive applications. Load dump pulses are momentary voltage spikes on the car’s 12V power bus caused by the rather slow regulation of the alternator output voltage. The sudden removal of a large load on the power bus (such as a disconnected battery, a blown power fuse, etc.) will cause the alternator output to suddenly jump to 60 volts or more. It can take as long as 400 milliseconds for the voltage to recover to normal tolerances.
Regards, Dana.
 

DickCappels

Joined Aug 21, 2008
6,459
Other techniques include using parts rated at higher than usual voltages, such as regulators that can handle large spikes and varistors across the power supply. TVS diodes are probably a more modern solution.
 

Hymie

Joined Mar 30, 2018
836
Many ICs will die if the voltage on certain pins exceeds that of the supply.

Engine cranking may result in a sudden drop in the supply voltage to the IC, but other IC pins may be held up by charged capacitors within the circuit – destroying the IC.
 
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