Alternator Noise Filter / Power Distribution

[Rick Finsta]

Okay, let me start off by saying that I am required to have an engine kill switch in my car, and NHRA/IHRA require this to be on the positive side of the system. So the alternator charge wire goes directly to the battery, and then the main 12V feed (independent of the starter feed as I run a remote solenoid and a separate system for the starter) goes through the cutoff switch to a distribution block on the firewall, where the remote sensing wire is located as well. This means that my alternator regulates at the actual distribution point rather than at the battery (negating voltage loss over the long wire runs from the trunk and back) and it also means that after the engine is killed by the switch, there is still current flow into the battery as the alternator spins down.

I have been dealing with noise issues and thought it might be nice to install a filter on the actual alternator charge line to the battery. Something like this:

http://www.powerwerx.com/emergency-vehi ... 0-amp.html

I figure that this likely isn't anything more than a choke and a cap in a shiny box, though. So what I'd like to know is what components and schematic would you gents use to accomplish the same thing as this filter? I am not familiar with the operation of chokes, though I realize this could like be done with the same capacitor to ground scheme as used to noise filter on ignition coils, I have generally seen chokes on higher current feeds like this. It would need to handle the output of a 140A alternator.

Thanks!

[elaw]

Honestly as a pretty experienced hobbyist and sometime electrical engineer, I'd say buy the unit you linked!

If you go look at digikey.com, you'll see they sell thousands of chokes. But none are rated for the current you need! And the ones that are anywhere near that rating have so little inductance they wouldn't do anything.

Most likely the "choke" in that box consists of a few turns of heavy-gauge wire wound through a toroid core - or perhaps several cores. Although that sounds simple, designing your own choke of that nature, and in particular picking the cores, is not trivial. You have to know the core permeability, make sure it's not going to magnetically saturate, and various other things that can fry your brain.

[Rick Finsta]

Thanks for that!