I have a 4.7V/10W. How can I use this bulb without burning out the

filament?

The effective voltage is the RMS voltage, and in a variable duty cycle

application of full voltage the RMS voltage is the peak applied voltage

times the square root of the duty cycle.

If you want PWM "effective voltage reduction", then the duty cycle

should be the square of the ratio of desired effective voltage to peak

applied voltage.

Examples:

4.7 volts desired effective voltage from 6 volts: Take the square of

(4.7/6), and that is .614, or 61.4%

4.7 volts from 6.3 volts, higher side of lead-acid "6-volts":

Square of (4.7/6.3) is .556, or 55.6% - a 3/5 duty ratio is probably close

enough to run a 4.7V bulb from "6V"

4.7 volts from 12V: Square of (4.7/12) is approx. 15.3% - a 1/6 duty

cycle should be close enough for "4-cell" flashlight bulbs to be

powered from 12-12.6V

The main "gotcha": Slight chance the PWM frequency or one of its

harmonics will excite a resonance of the filament seriously enough to make

it vibrate enough to suffer damage. Try projecting an image of it onto a

wall with a smaller magnifying lens of shorter focal length - if the ends

appear skinier than the middle region (even considering that the ends may

not be coiled while the middle is usually coiled), then try a different

frequency. Try variable frequency to see what is best and worst for

shaking up the filament. Then again, most lightbulbs last a little longer

on 60 Hz AC than on DC among the ones where a difference is measurable.

- Don Klipstein (

[email protected])