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Light load, bulb or resistor?

Nov 9, 2007

To the editor: I enjoyed Eric Forsyth’s plain, simple and highly understandable article on battery charging using a freewheeling prop while sailing (Battery charging under sail part II – Regulation, Sept. 2007). I would take issue with one of his suggestions, however.

In the paragraph headed “Operation under power,” Mr. Forsyth observes that “... I used an old wire-wound resistor I had lying around but a 25 watt, 120-volt incandescent lamp would be quite suitable to lightly load the generator when under power...”

While I’m no electrical engineer (you might have to check with Nigel Calder on this one), I have changed a lot of light bulbs in my life, and I would like to make the observation that failure of the filament of that 25-watt bulb will result in no load at all on the generator while under power. Not a good thing. The pounding motion of the boat and vibration while under power would likely result in very early failure of this bulb. That’s why they make “special” ones for ceiling fans and garage door openers, but I wouldn’t use one of those, either.

Breakage of that same bulb (likely mounted in the engine compartment, where its previously mentioned filament failure might go unnoticed for a significant time) would result in an unpleasant mess in the bilge.
Better to find a wire-wound resistor somewhere.

– John R Brougher III, a CPA in private practice, sails his Freedom 40 Cat Ketch Katorpus from Corpus Christi, Texas.

Eric Forsyth responds: No question a wire-wound resistor is the best choice, but nowadays these are not so easy to find; don’t try Radio Shack. I have used incandescent bulbs for years on the boat for just this kind of application, so long as they are well under the rating, i.e., don’t glow white hot, more like red to yellow, they seem to last indefinitely.

Editor’s note: In the schematic accompanying the article, there was an omission and an error. A 10-amp circuit breaker should be included in the line from the generator to the relay. And the converter box should be labeled “DC to DC converter,” not “AC to DC converter.”