automotiveover-voltage-protectionprotection
I need to connect some things (LEDs and other misc.) to the 12V rail in a car. I understand that this rail gets a beefy ~100V spike on ignition. I have also heard that the battery soaks this spike (like a big capacitor). In any case, should I add circuitry to prevent the spike from hitting my LEDs?
Let me know if you need clarification. Thanks in advance.
Edit:
I am also interested in keeping MOSFETs and microcontrollers safe. Would hiding it behind an LM7812/LM317 offer protection?
I'm assuming you don't want to use a microcontroller for this, so here's a 555/logic based idea:
Disclaimer: I threw this together in 5 mins, and haven't used a 555 timer for ages, so I may have made a mistake (or three)
Anyway, the basic idea is simple, set the 555 up in monostable "one-shot" mode, and use an inverter gate and a couple of caps/diodes to produce negative pulses on ignition on/off. The inverter gate can be anything that has a reasonably low output impedance.
The inverter power pins are not shown, and ignore R3 (it's just there for SPICE simulation purposes)
Here's simulation of the circuit (on/off time of ignition shortened from "normal" use)
The ignition starts the simulation on, turns off at 1 second, and turns on again at 11 seconds, each time producing a 3 second pulse at the 555 output (this can be used to drive a transistor to switch your GoPro):
in most cars the cigarette lighter socket is on the accessory circuit and is only active when the key is in the accessory or run positions.
some cars have it always live.
Best Answer
It is not uncommon indeed to see voltage spikes as high as 100V or even 120V. Generally spikes are clamped. To what voltage first depends on country/local regulations and standards (when there are). The lowest clamp I've read about (sorry, I cannot get my hands on that document anymore) is about 40 volts, which is still too high in almost all cases. You will need some form of protection as soon as non-passive, non-resilient circuitry is involved. But don't take my word for it as it's a condensed version of what I remember. This is just an introduction to what the automotive environment can be.
You can search for terms "engine crank and load dump" to get an idea. This is what I did when I started my automotive projects. You might be interested as well in a dedicated application note from Texas Instruments. There's also a "simple" protection against load dump overvoltage from the same manufacturer.
As a final note be sure to select automotive components when designing your projects. Not that it won't work with ordinary parts but the automotive qualification guarantees that those parts are designed for the harsh automotive environment, which is not limited to just electrical "goodies".