nuckollsr(at)cox.net
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 Posted: Fri Aug 04, 2006 1:56 pm Post subject: Aero-Electric List: MPJA Speed |
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At 09:31 AM 8/4/2006 -0400, you wrote:
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Ernest,
Apparently this speed control/dimmer uses a PWM (pulse width modulation)
technique to dim the LEDs. LEDs are very fast responding unlike an
incandescent bulb which stays hot between pulses. You can get some weird
effects with this kind of dimming. When you glance across the panel you
can probably see the lights as a series of bright spots rather than solid
lights. In older calculators with 7 segment LED readouts you could move
the calculator downward across your field of vision and get a stairstep
effect due to multiplexing of the display. In other words, only one of
the numbers is on at any given time. This requires less circuitry to
drive all of the display segments. In a panel dimmer it conserves power,
and the dimmer dissipates far less power.
I have yet to see an aircraft radio with LED displays that uses
multiplexing for the display, probably to avoid the effect described
above. I would think that a linear dimmer circuit would be a better
choice for an aircraft panel. I am very happy with the A and T Labs K11
Panel Dimmer although I substituted a less massive heat convector. Just
my opinion, YMMV.
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The few cases where I've selected duty-cycle dimming
parameters or multiplexing for LED displays, the switching
rate was ALWAYS in the hundreds of Hz. The PWM modulator
under discussion can no doubt be modified . . .find the
frequency selection capacitor and cut it to say 1/10th
the present value. The probable outcome is equivalent
functionality with no visible flicker or "beading" at
any intensity.
Bob . . .
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