I'm making a control board with an ATMega328P-PU to control bunch of LEDS (making them blink and such) that will be used in a lightbar, such as ones used on emergency vehicles. Since this is hooked to a running vehicle. Should I have some type of protection for my circuit in case the alternator has a spike in voltage? Or stuff like that not very common and shouldn't be a problem?
The schematic below in Blue is the part of the circuit I'm working on.
Such protection circuitry is absolutely essential in automotive applications, and you need it on the I/O connections as well as the power supply connections.
TVS diodes (quite cheap!) are commonly used in the front end of automotive protective circuits. There is some useful info in this PDF document.
It is not going to work and survive. There is such a thing as a double battery jump (some tow companies use 24 Volt batteries to jump cars). Go further to the "right" in your drawing, there are automotive regulators that will do this for you as well as protect for reverse battery. Load dump is normally generated when a battery with a low charge is being charged with the alternator and the battery line opens, the alternator regulator cannot shut it down fast enough so the spike goes out with lots of amps, be prepared. Some automotive regulators also have the reset circuitry, which you missed. The current limit looks OK but you are way short on drive for the MOSFET especially if you are doing PWM. You can also use a much smaller mosfet, preferable one that is avalanche rated. I assume C3 and C4 will be mounted right at the regulator. This should get you started. You should do some research on automotive electronics and the OEM requirements.
Side question can you use mov and tvs diode together in same circuit to help fight this problem?
jremington:
and you need it on the I/O connections as well
Before R1 and ground or after R1 and ground?
Also shouldn’t I put one on each LED panel since I’m jumping to the 12v rail?
gilshultz:
You should do some research on automotive electronics and the OEM requirements.
Thank you for your reply, I’m still looking it over and searching some of the things you mentioned.
The problem with that large C1 is that it might prevent you using a fast-blow fuse on the input, which
lessens the protection the fuse affords. Fuses are basically to stop wires catching fire, they are not fast
enough to guarantee protecting semiconductors.
A choke on the input will block high frequency voltage spikes, coupled with the MOV / zener / capacitor
would be useful.
Often its simpler just to buy an automotive rated buck converter to get 5V from a vehicle - some are
rated for input voltage range of 9V to 36V IIRC. Add a simple filter on the front for high frequency
noise (in both directions ideally). Note that quality in-car phone-chargers are 5V supplies rated for
automotive use, and commonly available (as are the cheaper, rubbishy ones, alas).
