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Ed -
So something like a Pulse Width Modulator for a motor would work very well here?
Thanks for the welcome expansion of knowledge on LED's here on the LML.
John
On Wed, 30 Nov 2005 16:52:30 -0500, Ed McCauley <ed.mccauley@bltinc.com> wrote:
Using a transistor as a simple ON/OFF switch is an
effective way to address controlling the power of LEDS. With this
approach there is no series element (resistor or transistor) that's
dropping voltage/power and wasting energy in the form of heat.
PWM circuits can be used make very efficient sources for either motors or LEDs, but
there is an important difference. Motors have substantial inductance and LEDs do not.
First the motor.
You can put the full battery volatge, 13 volts, across a motor for a 100 microsecond pulse
and all that will happen is the current will climb postive a bit. Then you can switch the sign of the
voltage and the current will come back down. If you keep the duty cyle 50/50, very little current flows and the
motor does not turn, even with a 26 Volt peak to peak square wave
across the motor. (The physics equation in operation here is V=L
dI/dt.) If you want the motor to turn, adjust
the duty cyle to say, 60 percent + and 40 percent -. Then a DC
current flows. The reason to do all this switching is that the
transistors that do the switching are always either fully on or off,
and don't get hot.
The LED is different. It is a diode, so current flows only one way. It has very little inductance, so a short pulse
of voltage causes an almost instantaneous matching pulse of current.
Also the current climbs very rapidly as the voltage is increased.
It is still useful to pulse the current to the LED, however, since the
people perceive peak intensity.
Most bicycle lamps use pulsed LEDs these days. The circuit
turns a transistor fully on for short pulses and the battery internal
resistance limits the current. Then the current is off, but the human
eye remembers the bright
flash. This increases battery life.
I dont think turing such a transistor, fully on, would be a good idea
with lamina LED and a battery capable of cranking an engine.
You could probably make this work if you put an inductor in series with
the LED, but I have not thought through the details of such a plan.
--
Jeff Peterson
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