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])