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MOTOR CONTROL BY POWER MOSFET TRANSISTOR 241

diodes, and a bunch of others. They all mean the same thing; the different terms describe their
function, not their type.
You need two battery sources to provide the split voltage necessary for this circuit to run.
Use a pair of two- cell battery holders, and wire them as shown.

FULL- BRIDGE TRANSISTOR CONTROL
A more elaborate form of transistor motor control uses the full- bridge, also called an H- bridge.
The “H” comes from the way the motor is connected to its control circuitry, looking a wee bit
like the letter H.
H- bridges require four transistors, along with associated components— resistors, flyback
diodes, and perhaps more, depending on the design. They’re harder to build, and harder to
get right. Given the availability of other motor control circuitry, it makes more sense to use
these other techniques, which tend to be easier and cheaper.
Instead, you can use MOSFET transistors, as described in the following section, or rely on
one of the many low- cost motor bridge ICs or modules. Those are covered later in this chap-
ter. Both provide more flexibility, and they are often cheaper and easier to build.

Motor Control by Power MOSFET Transistor

MOSFET stands for “metal oxide semiconductor field effect transistor.” Metal oxide indicates
the process used to manufacture them, and field effect refers to the way the transistor con-
ducts current. Power MOSFET is a further classification that indicates the device is intended
to drive some kind of load, like a motor.
MOSFETs look like bipolar transistors (they come in the same type of packages), but inter-
nally there are some important differences. First, due to their MOS construction, they are
more susceptible to being damaged by static electricity. Always keep the protective foam
around the terminals of the device until you’re ready to use it.
Second, the names of the terminals are different from what you find on bipolar transistors.
These variations are discussed in more depth in Chapter 31, “Common Electronic Compo-
nents for Robotics,” but for now just know that if you connect a MOSFET to your circuit
incorrectly, odds are great that it’ll be destroyed the instant you turn on the power. Bipolar
transistors aren’t usually so sensitive.

BASIC MOSFET MOTOR CONTROL SWITCH
A commonly available power MOSFET is the IRF5xx series, such as the IRF510, IRF520, and
so on. These are available in the popular TO- 220- style transistor case. These devices can
control several amps of current, when on a suitable metal heat sink. The heat sink is a piece
of metal that provides a large surface area to draw heat away from the MOSFET.
A basic circuit that uses a commonly available IRF510 power MOSFET is shown in Figure
22- 10. It’s a simple on/off switch to control a motor. Apply a 5- volt signal to the gate con-
nection of the transistor, and the motor is turned on. Resistors are used for additional protec-
tion of the circuit controlling the transistor.
Notice the Motor voltage label. The voltage to your motor can be different from the volt-
age that controls the MOSFET transistor. Often, the control electronics in your robot are
powered by 5 volts; the voltage to your motors can be 5 volts or over, up to the specified limit

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