Posted - 09/14/2006 : 10:13:55
| Pulse Width Modulation
A regulation method based on Duty Cycle. At full power, a pulse-width-modulated circuit provides
electricity 100 percent of the time. At half power, the PWM is on half the time and off half the time.
The speed of this alternation is generally very fast, typically the alternation or frequency can be in the
range of 10 thousand to 20 thousand cycles per second. (0.0001 to 0.00005 seconds per cycle) and
power is only delivered for a percentage of those cycles.
The ratio of "on" time to "off" time for an electronic component or signal. In a power system duty cycle
is typically used in reference to the semiconductor switch (in PWM controlled systems) or clock signal.
What does this mean to me?
Compared to a car, when you start from a standing stop you (generally) apply gradually increasing
pressure to the gas pedal and receive gradually increasing acceleration / power / speed in return.
The alternative would be instant full power from the motor, regardless of how much pressure is applied
to the gas pedal. NOT a good thing for every day driving, due to the physical shock to the components
of the vehicle, and the control demands placed on the driver. PWM can provide the same type of
gradual acceleration to a electric motor(s).
Do I Want / Need PWM?
If you are using 6 volts ONLY on a ‘PW” type vehicle I would say “don’t bother”.
At 12 volts the question (in my view) becomes debatable because PWM systems can be designed to provide a ‘Soft Start’ function and with a micro processor or selector switch an adjustable “Top End Governor“ could be applied, taking into account the skill set of the driver. Anything over 12 volts should definitely use one.
Where do I find it?
It’s not something you’re going to find at RadioShack or the Auto-Parts Store. Good ‘Scooter Controllers’ have PWM built in, a couple of links have been posted on modifiedpowerwheels.com
Some electric wheelchair controllers can be had surplus at reasonable prices.
The DIY Route, Can I Build My Own?
If you know enough about electronics to build your own then you can find examples of schematics on
the web, search for “PWM + Motor Controller” or “PWM + H Bridge”. Check the voltage and current
ratings of any schematics you find, determine if it can handle your requirements or if the circuit can be
modified to do so. ‘Hobby Robotics’ related websites would be a good place to look for circuits.
If going the DIY route, I would consider “(MOSFET) FET” based designs before “Transistor” based,
as transistors will consume a volt or two each, that is less power to the motors. Also, consider that
FET (s) can be paralleled to handle more current whereas you should not attempt to parallel
The absolute simplest thing you could build would be a ‘Relay H Bridge’ (voltage polarity reversing) configuration with a FET in the ground line, and a PWM signal to the gate of the FET.
Although this circuit uses TRANSISTORS and will NOT handle the current of a ‘PW’ type vehicle
it might give you a place to start from. http://www.bobblick.com/techref/projects/hbridge/hbridge.html
Edited by - Scratch_6057 on 09/14/2006 10:33:25