Hardware Reference
In-Depth Information
Figure 10-9.
Driver for inductive load
Since there is no current-limiting resistor used in series with
L
1
, whether motor
or relay, make sure that no more current than
I
Cmax
will flow. For relays, you need a coil
resistance greater than or equal to 50 Ω, when driven from approximately 5 V. Otherwise,
you risk burning out driver
Q
1
(assuming the 2N2222A with its power limit of 0.5 watts
at 5 V). You can drive lower resistance coils, if you designed your driver to handle the
additional current. For example, a 500 mA driver can handle coil resistances as low as 10
ohms (at 5 V).
The 2N2222A transistor is probably suitable for only the smallest of electric motors.
Depending on how it is used, a motor can stall and thus greatly increase its current
demands. Motors also have high startup currents. If the motor is started and stopped
frequently, the driving transistor may be overworked.
Driver Summary
This section on the transistor driver should not be thought of as your only choice in
driver solutions. It was presented because it is simple and cheap and can fill the need for
many small loads, like high-current LEDs or panel lightbulbs. Simple and cheap may be
essential for robot building when many drivers are required.
While students may use the Gertboard for labs, we still need to provide a substitute
when the Raspberry Pi is integrated into something that was built (like a robot). It might
be wise to stock up on a few good transistor types for this purpose.
