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by
Dennis Clark
Since I like to group similar topics with the questions in this
column, the theme this month seems to be motor controls.
Q. Hey, could something like a relay board be used
for driving really big honking motors with PWM?
I have eight relays, and (unless I’m confused) you
can make an H-bridge out of four, so it seems like you could
use this as a dual H-bridge motor driver if you wanted, and
have your motor power completely separated from your logic
circuitry. Or is this nonsense?
- Joe in Fort Collins
A. In reality, you don’t need so many relays to make a
reversible motor controller. If you design a motor
controller that is just all relays, you would only need
two relays if one of them is a DPDT (double dole, double
throw). One example of this type of driver is shown in Figure
1. There are only two speeds with this type of a driver: ON
and OFF. Diodes D1 and D2 shunt the CEMF, often called
Back EMF to the relay coil and not to the transistors. Diodes
D3 and D4 try to keep those voltage spikes out of your
Figure 1. Relay only motor driver.
power supply. The odd looking diodes D5 and D6 are
transient voltage suppressors (TVS) that protect the
transistors from excessive voltage spikes between the
collector and emitter. The values of all of these diodes and
transistors depend upon the current that you are running
through your relays; more current = bigger relays = more
drive current to turn on their coils. This type of motor
controller is pretty common in older “BattleBot” style
vehicles. It is simple, rugged, and high powered. We call it
the “Bang! Bang!” motor controller because it has little
finesse. It literally bangs the motor on and off. It works, but
it ain’t pretty. With a little more effort, we can create a more
useful motor controller that can handle very large loads and
have variable speeds, as well. Usually, our robot motor
controllers are H-bridges that use transistors to choose the
direction of current flow through our motors, and from that
the motor direction. However, high powered H-bridges are
expensive and need proper care and feeding. If we use a
MOSFET and single DPDT relay, we can create very high
current motor drivers for very little cost. Figure 2 shows just
such a motor controller. This particular motor driver is limited
to eight amps because that
is the current limit of the
transformer. The MOSFET is
capable of handling over 30
amps. Note that this design
uses opto-isolators to protect
the rest of our robot from
unfriendly voltage and
current spikes from the
motor controller. The relay
has the usual CEMF diode
and the MOSFET is protected
by a TVS, as well. Another
notable design characteristic
is the zener diode on the
gate of the MOSFET. This is
to protect the MOSFET from
having the gate ring at a
14 SERVO 11.2008
