Turning functions on and off
Generating lighting features
Generating sounds
Monitoring for safe operations
Motor Driver and Motor Controls
The motor driver circuitry is basically a switch that connects
the motor to the full power- supply voltage whenever the
microprocessor tells it to do so. The microprocessor also tells
the driver which wire to connect to which polarity, to achieve
forward or reverse movement.
The brain says “on forward” and the driver connects the orange
wire to the power supply positive and the gray wire to the neg-
ative. “On backward” will have the motor driver connect the
wires to the opposite polarity from the power supply.
Okay, that will let the loco run forward or backward, but it will be
at full speed. How does the decoder run the loco at slower speed?
Motor control uses Pulse Width Modulation (PWM) to control
motor speed. The microprocessor is programmed to turn the
motor on every few microseconds for some time. The longer
the motor is turned on (the wider the pulses), the faster the
loco will go (7).
Think about driving your car by selecting drive and pushing the
gas pedal all the way to the floor for ½ second and then letting
off completely for ½ second and then repeating the cycle. That
might result in the car going 30 miles per hour.
If you increased the time you hold the pedal down to ¾ second
out of every second, the pulse rate would be the same, but the
car would run faster, due to the throttle being held down lon-
ger (wider pulses of power – 8, 9).
DCC Impulses Column - 5
7
7: Motor Driver equivalent circuitry – a multiple transis-
tor switch connects the motor to the power supply in one
direction or the other or not at all.
Modern decoder designs have the pulse rate in the 15,000
to 30,000 times per second range. Why? This puts the drive
pulses outside the range of human hearing. Even though the
motor will “sing” just a bit, it will be beyond our ability to hear
it. Manufactures call this feature things like “silent drive” or
“supersonic drive”.
BEMF (motor control)
Most decoders today offer BEMF motor control. As I discussed
in my December 2012 column on Basic Electronics
, the motor will generate a reverse voltage
(Back EMF) that is directly proportional to its speed. A detector
across the orange and gray wires can sense this voltage dur-
ing the time that the motor driver is switched off. Providing
MRH-Feb 2013
