Build Your Own Combat Robot

FIGURE 7-3
Starter solenoid
type of relay.
Chapter 7: Controlling Your Motors 131
coil with a movable metal rod down the middle—to pull a shorting bar across a
pair of contacts. Commonly known as starter solenoids, they are used for
high-current, intermittent-duty applications such as running the starter motor on
an internal combustion engine. Industrial starter solenoids are available for power
levels of up to 400 amps. Some solenoids have one side of the coil internally connected
to one of the internal contacts. These are designed for automotive use, in
which the motor circuit and the coil circuit have a common return line to the battery.
These solenoids can be used for robot combat applications, provided that the common
line is taken into account when designing the electrical system.
One thing you cannot do is connect multiple relays in parallel to get a higher current
capacity. The closing of a relay contact is a slow event, as compared to the time
it takes for current to start flowing through the motor. Because of manufacturing
differences, all the relays would not close at the same time, so the first relay to make
contact—or the last relay to break contact when opening the circuit—would take
the entire motor load by itself. So a bank of relays wired in parallel can still safely
switch only as much current as any single relay acting alone could.
The coil of a relay should be operated at the voltage for which it was designed.
Running the coil of a relay on less than its design voltage can result in insufficient
pressure on the contacts, reducing the area of metal through which current is flowing
and increasing the chances of welding. Running a relay coil on more than its intended
voltage can result in the coil burning up and overheating, especially on relays
designed for intermittent use. Running the relay coil on more than its intended
voltage doesn’t offer any advantage in reliability or performance, although it may
make the robot’s wiring simpler if the motors are being run off a different voltage
than the relay was designed for. For the duration of a typical combat match, most
relays can survive twice their intended operating voltage, although this should be
tested prior to a match. The voltage polarity applied to the relay coil itself usually
doesn’t matter, but some relays have diodes internally wired across the coil connections
and must be connected with the appropriate polarity.