Section 3 Motors and BJT's Section 3: Motors and BJT's Section Overview Semiconductors are amazing. This section covers the functionality of two common types of semiconductors, diodes and Bipolar Junction Transistors (BJTs). Diodes allow current to flow one direction, but not the other. BJT's can either act as switches, or as signal amplifiers. Two diode experiments will be done in this lab, and then one transistor experiment. The motor board will then be examined to see how transistors are used to create an H-bridge to allow the motors to go forward and backward. Lastly there is a challenge that uses a BJT as an amplifier. Procedure 1. Build the circuit in figure 28. The 100 Ohm resistor with an arrow pointing towards it is a potentiometer. Turning the knob on the potentiometer will adjust the voltage across the diode. 2. Adjust the voltage from the smallest value possible (it should be negative) to the largest value possible. Plot enough points on figure 29, so that the one way nature of the diode is evident. Diode I-V characteristics Figure 28: Schematic for testing the I-V characteristic of a diode. ECE112 Lab Book Oregon State University Page 37 Section 3: Motors and BJT's Volts Figure 29: Graph for plotting the I-V characteristic of a diode. Zener Diode Voltage Characteristics 1. Build the circuit in figure 30. The 1K Ohm resistor with an arrow pointing towards it is a potentiometer. Turning the knob on the potentiometer will adjust the voltage at Vin. 2. Adjust the voltage from the smallest value possible (it should be close to 3.6) to the largest value possible. Plot enough points on figure 31, so that conductive properties of a zener diode are understood. ECE112 Lab Book Oregon State University Page 38 Section 3: Motors and BJT's Figure 30: Schematic for testing the zener diode. Figure 31: Axis for plotting the voltage characteristics of a voltage regulator. ECE112 Lab Book Oregon State University Page 39 Section 3: Motors and BJT's The BJT as a saturated switch The simplest way to use a BJT is as a switch. The contacts of an NPN BJT switch are closed by injecting base current and opened by removing base current. When sufficient base current is present and collector current is flowing, the collector and emitter terminals are essentially connected together with a voltage source of approximately 0.2V. The voltage source does not actually exist inside the transistor, but to there is a .2 volt difference from C to E. See figure 32. Figure 32: Key for using BJT's as switches. 1. With the wheels removed from the robot, build the circuit, in figure 33, on the protoboard. Create the switch using a movable wire. Figure 33: A switch using a NPN transistor and the pins of the 2N4401. ECE112 Lab Book Oregon State University Page 40 Section 3: Motors and BJT's 2. With the switch open, measure the transistor's Vbe and Vce. Vbe________V; Vce:________V What are the values of Ib and Ic? (You shouldn't have to measure these to know.) Ib:________mA; Ic:________mA What is the power dissipation of the transistor (motor off) Power dissipated is:________mW 3. Now close the switch contacts and the motor should begin to run. Measure Vbe ,Vce, Ib and Ic. Vbe:________V; Vce:________V; Ib:________mA; Ic:________mA Using these measurements just taken, compute the beta of the transistor, where I C. Beta:________ IB Neglecting the base current (considering only Ic and Vce) compute the power dissipation of the transistor. Power dissipated by the transistor is:________(mW) 4. Place another 2.2k ohm resistor in parallel with the one connected to the base lead and make the measurements from step three again and record them below Vbe:________V; Vce:________V; Ib:________mA; Ic:________mA Does the motor run any faster?_____________________________ You should see that Ib increased considerably and Ic increased slightly relative to its previous value in step three. Note the difference in Vce. It should change only slightly. ECE112 Lab Book Oregon State University Page 41 Section 3: Motors and BJT's What's the LED for? Internally, the motor consists of many windings of wire. When current flows through the windings, a magnetic field is produced in the motor that causes it to turn. But when the motor drive current is removed, the collapsing magnetic field produces a very high voltage. So high in fact (~200V) that the transistor could be destroyed. The high voltage created when the field collapses is oriented in the opposite direction to the original applied voltage. Therefore the diode passes this &quot;flyback&quot; current and recirculates it back through motor windings protecting the transistor. The current spike causes the brief flash of the LED when the power is removed. Study Questions 1. Use the results of step three and four write out your own description of the condition of saturation. It should include the relationship of Ib and Vce. 2. Is the BJT an effective switch as far as its own power dissipation is concerned? In other words, considering the power dissipated by the transistor when on or off, is it effective in ensuring that most of the power is delivered to the load? Why? What characteristic about being saturated makes the BJT an effective switch? ECE112 Lab Book Oregon State University Page 42 Section 3: Motors and BJT's The <a href="/keyword/motor-control/" >motor control</a> board The <a href="/keyword/motor-control/" >motor control</a> board needs to be able to spin either motor in either direction to allow the robot to back up and turn. The way we turn the motors on or off is with a high or low voltage level applied to the motor drive circuits. These signals come from digital logic gates. This arrangement necessitates having two identical sets of motor drive circuits. The arrangement of the transistors gives this circuit the name of &quot;H-bridge&quot;. Can you see the &quot;H&quot; in Figure 34? Figure 34: <a href="/keyword/motor-control/" >motor control</a> board H-bridge. Shown above is the H-bridge that controls the direction of current flowing through the motors and therefore the direction of rotation. This circuit is actually a combination of the NPN and PNP switch circuits we just built. The current flows from left to right through the motor when Q1 and Q4 are turned on. The current flows from right to left through the motor when Q2 and Q3 are turned on. This action can be seen in Figure 34. Q5, Q6, and the various resistors are used to correctly bias the Hbridge transistors to work with logic-high &quot;on&quot; input signals. Testing the <a href="/keyword/motor-control/" >motor control</a> board Power the <a href="/keyword/motor-control/" >motor control</a> ler board with a 4 pin keyed power wire from the power distribution area of the charger board. Use all four pins so that if you put the plug in backwards you will only have NC (no connection) plugged into + and GND. The <a href="/keyword/motor-control/" >motor control</a> ler is active low. When GND is connected to EN, the channel is sending current. When DIR is also connected to GND then the channel is sending current in the reverse direction. Use some stripped wires to connect the motor board like the one shown in figure 35. ECE112 Lab Book Oregon State University Page 43 Section 3: Motors and BJT's Figure 35: Diagram of the <a href="/keyword/motor-control/" >motor control</a> board. The <a href="/keyword/motor-control/" >motor control</a> ler has digital gates, and it isn't convenient to talk about EN being grounded or at 0 volts potential. The term for grounded is 0. When the inputs to the <a href="/keyword/motor-control/" >motor control</a> ler aren't grounded they are called 1. It is just 0 or 1. Fill in the table below with the correct motor actions and which LED is on, red or green. Enable 1 1 0 0 Direction 1 0 1 0 Motor State LED color Figure 36: Control table for the TekBots motor board. Use the <a href="/keyword/motor-control/" >motor control</a> ler schematic and fill in the tables below. This will verify that the <a href="/keyword/motor-control/" >motor control</a> ler is operating correctly. Use Ohms Law to calculate the current through resistors R12 and R26. ECE112 Lab Book Oregon State University Page 44 Section 3: Motors and BJT's Left motor driver: Vce Q2 Vce Q2 Vce Q3 Vce Q3 Vce Q4 Vce Q4 Vce Q5 Vce Q5 IR12 IR12 Vmotor Vmotor Motor direction forward Motor direction reverse When going forward.....: Which transistors are off?________________ Which transistors are in saturation?________________ When going backwards.....: Which transistors are off?________________ Which transistors are in saturation?________________ Study Questions 1. Explain in words, how you know when transistors on the <a href="/keyword/motor-control/" >motor control</a> ler board are on or when they are off? ECE112 Lab Book Oregon State University Page 45 Section 3: Motors and BJT's Challenge Audio Amplifier An NPN BJT acting as a linear amplifier is more like a dimmer switch than an on-off switch. In this mode, a small base current is able to control a much larger current flowing from collector to emitter. The big difference with the BJT amplifier is that we use a large collector resistor to convert the varying collector current to a varying voltage at the output. Shown in figure 37 is the schematic for a s...