nuckollsr(at)cox.net
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 Posted: Thu Dec 07, 2006 7:10 pm Post subject: Pitot Current Draw (Duty Cycle Control Case) |
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So, to keep the same heat delivered to the pitot tube (USING A RESISTOR)
we can only drop the current from 15A down to 10A. Total system draw
will be
140 watts instead of the original 210 watts with 100 watts in the
tube and 40 watts in the resistor.
The foregoing assumption of equivalency holds only if the two
tubes distribute their energy over the same number of square inches
of tube surface. Equivalency for ice-melting abilities is measured
in watts per square inch of power density.
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Let us consider the duty cycle controller. If you put a transistor
in series with the pitot tube heater and drive it with a 50% duty
cycle square wave, an interesting thing happens . . .
The PEAK power is still 14v x 15A or 210 watts. But since
the heater is operating only 1/2 the time, the AVERAGE power
is now down to 1/2 of peak or 105 watts. Interestingly enough,
the AVERAGE current will also be down to 7A. The conservation
of energy rules are not violated. In the resistor case, there
are LOSSES in the current dropping device while a duty-cycle
driven transistor has minimal losses (as low as 1 watt!).
As others have mentioned, the down side of this scheme is that
your bus gets whacked with a 15A pulse for each operating cycle
of the heater system. One can explore relatively high frequency
switching to avoid seeing the panel lights flicker (and believe
me, they WILL) but this might cause audible noises elsewhere.
Suggest about 30 Hz for minimum observability and below the
audio frequencies of interest in the rest of your electro-whizzies.
Bob . . .
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