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From: stevek@locus.com (Steve Krattiger)
Newsgroups: sci.electronics
Subject: Re: motor speed controller, transistor question
Keywords: using this for R/C car
Message-ID: <23218@oolong.la.locus.com>
Date: 4 Apr 91 20:31:58 GMT
References: <68137@eerie.acsu.Buffalo.EDU>
Distribution: usa
Organization: Locus Computing Corp, Los Angeles
Lines: 79

In article <68137@eerie.acsu.Buffalo.EDU> v129raaq@ubvmsd.cc.buffalo.edu writes:
>
>	Hi everybody. I am trying to build an electronic speed controller
>for a radio controlled car's motor. The radio receiver that I have has output
>plugs with three wires that normally go to a servo. I would like to use the
>output pulses converted from the radio signal to control a 555 timer, which
>would in turn switch on and off transistors to control the motor. I want the

It would be much better to plug in a servo, and hook the output to a slide
potentiometer for variability of the speed.  Wouldn't want to blow out the
receiver module, they can be pretty expensive...

>timer IC to send out a variable high frequency determined by the output from
>the radio control system. This high frequency would switch the transistors on
>and off so fast that the motor will keep turning, with faster switching making
>the motor go faster. I would like to have the transistors bypassed when the
>motor travels at full speed.

Increasing the switching frequency won't increase the speed of the motor.
What you want to use is 'pulse width modulation'.  If you look at a standard
square wave, half the time the power is on, half the time it is off.

     _____     _____     _____     _____     _____
     |    |    |    |    |    |    |    |    |    |
     |    |____|    |____|    |____|    |____|    |____

At a high enough frequency (1KHz - 2KHz) you can apply this to a motor, and
it will run smooth.  Play with the frequency a bit, it will affect your 
power output, I've used frequencies all the way down to 200Hz, it depends
primarily on the motor speeds you are trying to achieve.  Anyways, looking
back at the square wave, the period that the wave is high is called the duty
cycle.  For this wave, the duty cycle is 50%.  The average power applied
to the motor is approx Vin/2.

        __        __        __        __        __
        | |       | |       | |       | |       | |
        | |_______| |_______| |_______| |_______| |_______

With the above duty cycle, the power output is approx. Vin/5 so obviously
the motor will run signifigantly slower.  If you adjust the duty cycle
from 0% up to and including 100%, you have complete control over the
speed of the motor, and by linking a slide potentiometer to a servo, you
can control the motor speed directly from the remote control.

Using a very simple circuit with a single LM339 OP-amp, I was able to 
construct exactly what you are looking for.  However I was running a 
motorized boat towbar I made so it was drawing between 30 to 50 amps,
which kinda leads to your next question:

>	Also, I am using transistors rated at 15 amps each. If I wire them up
>   in parallel...

With regular transistors, this will not work well.  But I found that it does
work using FET's, however, you must use an individual gate resistor on each
FET.  You can just hook all the source leads together, and all the drain
leads together to increase the amperage.

>
>Rich Ciesla
>v129raaq@ubvmsd.cc.buffalo.edu

I don't think that I could properly draw the schematic using ascii characters
however I'll give it a try if you or anyone else would like to see how I 
built this circuit.  Basically, all you need is a triangle or sawtooth wave-
form fed into one input of an OP-amp, and a slide-potentiometer tied between
Vcc and Gnd with the wiper attached to the other OP-amp input.  Set the gain
of the amp at 1000 or so, then adjust the potentiometer somewhere between
Vcc and Gnd, and as the triangle waveform crosses the steady voltage line,
it will produce a square waveform at the output of the amp.  Vary the pot
and you vary the duty cycle of the output waveform.  Feed this into the
gate of a high power MOSFET, and run the high power stuff of the source
and drain of the MOSFET.  Voila!  Heavy duty motor speed control! simple.

e-mail me if you have any more questions.

+-----/\/\/\/--------||--+---->|---|---/\/\/\/--------/\/\/\/-------|<--------+
|    Steve Krattiger     |-/\/\/\--|        Locus Computing Corp.       ..    |
|    stevek@locus.com             ===       Los Angeles, California     \/    |
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