capacitor across alternator terminals

[msmith10]

The PO of my boat was an electrical engineer-- not an electrician judging from the makeshift wiring jobs all over the boat. He wired a large capacitor (about the size of the ignition coil) across the (+) and (-) terminals of the alternator. Does anybody know the possible function of this capacitor? I'm guessing it serves as a filter to eliminate noise in the instruments or radio.

[Administrator]

You got it right! This is a common way of getting the proverbial alternator whine out of VHF radios which is well known and widely practiced in the ham radio world. I don't know what it would do for GPS's, etc., but it couldn't hurt.

The size you describe goes well beyond what is required for this application, so one may assume that the selection was made on the basis of price, or lack of it.

Bill

[marthur]

What size capacitor (in Farads) would one use for this application?

[Administrator]

A quick Google search suggests capacitance values all over the place.

Here's a link to a design which uses both a capacitor and a choke coil to kill the whine. Most of the commercial products intended for this purpose are of a similar construction. Using only the capacitor portion of the design would be less effective, according to an email I got from the author, but more easily accomplished.

Note the polarity of the capacitor. It must be installed in the proper orientation.

I may have been a bit harsh in my comments about the part described in the original posting. These components are so cheap that there's little point in being especially crisp about selection. Overkill is not overly expensive.

Bill

[marthur]

Thanks for the thought. As someone who can't leave a good project alone, I'm sure I'll be building one of these soon.

ma