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DIRECTION CONTROL 259
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Motor supply
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On/Off control signal On 1 Off 0
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Direction control signal CW CCW 1 0
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FIGURE 18.4 Both on/off and direction relay controls in one.
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Direction control
b Q1 TIP31
R2 1-3K b Q2 TIP32
D2 1N4002
FIGURE 18.5 Using a complementary pair of transistors to control the direction of a motor. Note the doubleended ( and ) power supply.
260 WORKING WITH DC MOTORS
TABLE 18.3 PARTS LIST FOR TWO-TRANSISTOR MOTOR DIRECTION CONTROL.
Q1 Q2 R1, R2 D1, D2 Misc.
TIP31 NPN power transistor TIP32 PNP power transistor 1 3K resistor 1N4002 diode Heat sinks for transistors
All resistors have 5 or 10 percent tolerance, 1/4-watt.
+V Forward control Forward 1 Off 0 R1 1-3K b e c Q1 TIP41 D1 D3 !N4002 Q2 TIP41 e c b R2 1-3K
D2 !N4002 c b e Q3 TIP41 Q4 TIP41 e D4 !N4002 c b R4 1-3K
Reverse control Reverse 1 Off 0
R3 1-3K
FIGURE 18.6 Four NPN transistors connected in an H pattern can be used to control the direction of a motor. The power supply is single ended.
TABLE 18.4 PARTS LIST FOR H-BRIDGE BIPOLAR TRANSISTOR MOTOR DIRECTION CONTROL.
Q1 Q4 R1 R4 D1 D4 Misc.
TIP41 NPN power transistor 1 3K resistor 1N4002 diode Heat sinks for transistors
All resistors have 5 or 10 percent tolerance, 1/4-watt.
DIRECTION CONTROL 261
The transistors you choose should comply with some general guidelines. First, they must be capable of handing the current draw demanded by the motors, but which specific transistor you finally choose will largely depend on your application and your design preference. Most large drive motors draw about one to three amps continuously, so the transistors you choose should be able to handle this. This immediately rules out the smallsignal transistors, which are rated for no more than a few hundred milliamps. A good NPN transistor for medium-duty applications is the TIP31, which comes in a TO-220 style case. Its PNP counterpart is the TIP32. Both of these transistors are universally available. Use them with suitable heat sinks. For high-power jobs, the NPN transistor that s almost universally used is the 2N3055 (get the version in the TO-3 case; it handles more power). Its close PNP counterpart is the MJ2955 (or 2N2955). Both transistors can handle up to 10 amps (115 watts), when used with a heat sink, such as the one in Fig. 18.7. Another popular transistor to use in H-bridges is the TIP120, which is known as a Darlington transistor. Internally, it s actually two transistors: a smaller booster transistor and a larger power transistor. The TIP120 is preferred because it s often easier to interface it with control electronics. Some transistors, like the 2N3055, require a hefty amount of current in order to switch, and not all computer ports can supply this current. If you re not using a Darlington like the TIP120, it s sometimes necessary to use small-signal transistors (the 2N2222 is common) between the computer port and the power transistor. The driving transistors should be located off the main circuit board ideally directly on a large heat sink or at least on a heavy board with clip-on or bolt-on heat sinks attached to
FIGURE 18.7 Power transistors mounted on a heat sink.
262 WORKING WITH DC MOTORS
the transistors. Use the proper mounting hardware when attaching transistors to heat sinks. Remember that with most power transistors, the case is the collector terminal. This is particularly important when there is more than one transistor on a common heat sink and they aren t supposed to have their collectors connected together. It s also important when that heat sink is connected to the grounded metal frame of the robot. You can avoid any extra hassle by using the insulating washer provided in most transistor mounting kits. The power leads from the battery and to the motor should be 12- to 16-gauge wire. Use solder lugs or crimp-on connectors to attach the wire to the terminals of T0-3-style transistors. Don t tap off power from the electronics for the driver transistors; get it directly from the battery or main power distribution rail. See 15, All about Batteries and Robot Power Supplies, for more details about robot power distribution systems.
POWER MOSFET CONTROL
Wouldn t it be nice if you could use a transistor without bothering with bias resistors Well, you can as long as you use a special brand of transistor, the power MOSFET. The MOSFET part stands for metal oxide semiconductor field effect transistor. The power part means you can use them for motor control without worrying about them or the controlling circuitry going up in smoke. Physically, MOSFETs look a lot like transistors, but there are a few important differences. First, like CMOS ICs, it is entirely possible to damage a MOSFET device by zapping it with static electricity. When handling it, always keep the protective foam around the terminals. Further, the names of the terminals are different than transistors. Instead of base, emitter, and collector, MOSFETs have a gate, source, and drain. You can easily damage a MOSFET by connecting it in the circuit improperly. Always refer to the pin-out diagram before wiring the circuit, and double-check your work. A commonly available power MOSFET is the IRF-5xx series (such as the IRF-520, IRF-530, etc.), from International Rectifier, one of the world s leading manufacturers of power MOSFET components. These N-channel MOSFETs come in a T0-220-style transistor case and can control several amps of current (when on a suitable heat sink). A basic, semi-useful circuit that uses MOSFETs is shown in Fig. 18.8 (see the parts list in Table 18.5). Note the similarity between this design and the transistor design on Fig. 18.6. An even better H-bridge with power MOSFETs uses two N-channel MOSFETs for the low side of the bridge and two complementary P-channel MOSFETs for the high side. I won t get into the details about why this is better (the subject is adequately addressed in many books and Web sites). Suffice it to say that the use of complementary MOSFETs allows all four transistors in the H-bridge to turn completely on, thereby supplying the motor with full voltage. Fig. 18.9 shows a revised schematic (refer to the parts list in Table 18.6). In both circuits, logic gates provide positive-action control. When the control signal is LOW, the motor turns clockwise. When the control signal is HIGH, the motor turns counterclockwise.
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