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~5mH for my application. I used ~40 turns of #18 on a 5/8" ID ferrite toroid, an FT-82-77, I think - but it is not critical. All it needs to do is slow down the current rise of sharp spikes to prevent blowing up the Zeners (actually 600W TVSs, SM6T22As - small despite the brief large dissipation) and, if there is a substantial resistance (a few Ohms) in the 12V supply, an inductor is probably not required.
In any case, the conditioning must be able to supply the operating current continuously. If each of those lamps draws 5A or so and they are all on, that is a considerable load, much larger than my single 6.5A lamp and motor load - hence my suggestion to switch unconditioned 12V to the lamps, but the lamp-switching Mosfets also then need to handle the spikes - so 100V or larger parts are wise.
Awesome thanks for this info Tom. I will be filtering the power supply with your method and then run it to a 5V regulator to power the micro. The FETS will be running directly off 12V from the battery, no filtering (as of right now). The FET's I am using are IRF3711.
I don't think that's your best choice; its Vds of only 20v is pretty low for a vehicular application. An IRF3710 (100V) or IRFZ44 (60V) is likely safer.
Pay attention to the Rds(on) values when shopping the FETs. Lower is better and reduces the power the part will dissipate when the lamp is on. I used a pair of paralleled IRFZ44s and use no heatsink for my ~5A load; they run only warm - but, to be honest, I lost both FETs one time in about four years of operation. It appeared that one failed and the other cooked - in fact, unsoldered - itself trying to take the load.
Since you are planning on switching near the load you might consider using an optoisolator to prevent shared ground concerns; that also makes driving the FET gate well easy.
If you're looking at needing a heatsink, consider using a relay instead (standard Bosch-type relays are cheap, can handle large loads, are easy to mount and can be driven with just a transistor and anti-kick diode), although you can't PWM with a relay.
Good luck. Looking forward to a photo of bright lights.
This board used IR3310 High Side drivers, with an additional transistor needed between the micro and the IR3310's to switch them on and off. It was the power controller unit for an emergency vehicle, and switched the power for the various lights and siren.
The IR3310 is designed for vehicle applications, Ishutdown is <= 100 A, and it includes over current and over temperature protection, reverse polarity protection, etc.