Physics 222. Lab 5: Characterizing a transistor, and using it to control motor speeds. Objectives:

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1 Fresh page; your name, your partners full names, date, title. You may copy the objectives, introduction, equipment, safety and procedure sections, or you may print this handout and neatly tape in these sections into the appropriate pages in your lab notebook. Note that there are three sections you need to prepare: The introduction/relevant calculation/code section has a schematic, the voltage divider calculation and the pseudocode that you will prepare ahead of time. Leave a page blank for the circuit diagram section Set up a data table as prescribed in the data section part A, then leave enough space to include your working Arduino code for part B. The writeup to this lab is due 4 p.m., Friday, February 23, in my mailbox, though you may submit it earlier in class. The writeup consists of a photocopy of your lab notebook pages for Lab 5, from the title page to the conclusion section. Physics 222 Lab 5: Characterizing a transistor, and using it to control motor speeds Objectives: Determining the type of transistor from multimeter information Building a circuit with a voltage divider Building a circuit with a transistor controlling the speed of a motor Determining b and operation mode for a transistor Introduction and relevant calculations/code: The junction transistor is closely related to the semiconductor junction diode whose I-V characteristics you have already studied. A bipolar junction transistor consists of a three-layer "sandwich" of doped semiconductor materials, either P-N- P (positively doped silicon negatively doped silicon positively doped silicon) or N- P-N. Each layer forming the transistor has a specific name, and each layer is provided with a wire contact for connection to a circuit. Shown here are schematic symbols and physical diagrams of these two transistor types: PNP transistor NPN transistor P N P N P N

2 The only functional difference between a PNP transistor and an NPN transistor is the proper biasing (polarity) of the junctions when operating. For any given state of operation, the current directions and voltage polarities for each type of transistor are exactly opposite each other. Bipolar transistors work as current-controlled current regulators. In other words, they restrict the amount of current that can go through them according to a smaller, controlling current. The main current that is controlled goes from to for a NPN transistor; the small current that controls the main current goes from to. The arrow in the schematic above points in the direction of the current remember that electrons are really flowing the opposite way: B C E = small controlling current = large controlled current Bipolar transistors are called bipolar because the main flow of electrons through them takes place in two types of semiconductor material: P and N, as the main current goes from to. In other words, two types of charge carriers electrons and holes comprise this main current through the transistor. As you can see, the controlling current and the controlled current are always together in the wire, and their electrons always flow against the direction of the transistor's arrow. This is the first and foremost rule in the use of transistors: all currents must be going in the proper directions for the device to work as a current regulator. The small, controlling current is usually referred to simply as the current because it is the only current that goes through the wire of the transistor. Conversely, the large, controlled current is referred to as the current because it is the only current that goes through the wire. The current is the sum of the and currents, in compliance with Kirchhoff's Current Law (KCL). If there is no current through the of the transistor, it shuts off like an open switch and prevents current through the. If there is a current, then the transistor turns on like a closed switch and allows a proportional amount of current through the. For a typical NPN transistor like the 2N2222, the ratio of the current to the current in the active mode is roughly (this ratio is called b or hfe ). Collector current is primarily limited by the current, regardless of the amount of voltage available to push it. Another way of describing how this type of transistor works is to say that it turns on (allows a to current) when the voltage of the with respect to ground is about 0.6 V (assuming the is grounded). Note that you can damage a transistor like this if you give it too large of a current or have too large of a

3 to current. Refer to the manufacturer s data sheet to see what these maximum currents are. Identifying wires on a transistor Before you can use a transistor you need to decide which wire lead is the, which is the and which is the. This is important because transistor packaging, unfortunately, is not standardized. All bipolar transistors have three wires, of course, but the positions of the three wires on the actual physical package are not arranged in any universal, standardized order. Bipolar transistors are constructed of a three-layer semiconductor "sandwich," either PNP or NPN. As such, they register as two diodes connected back-to-back when tested with a multimeter's "resistance" or "diode check" functions ( ). Use a meter has a designated "diode check" function; the meter will display the actual forward voltage of the PN junction and not just whether or not it conducts current. The Fluke meters seem to work better for this. Note that the actual forward voltage will be shown only when the + lead on the multimeter is on a P wire, and the (ground) lead is on an N wire. The other direction (the leads reversed) will not conduct a current and thus will read OL for overload. If a multimeter with a "diode check" function is used in this test, it will be found that the - junction possesses a slightly greater forward voltage drop than the - junction. This forward voltage difference is due to the disparity in doping concentration between the and regions of the transistor: the is a much more heavily doped piece of semiconductor material than the, causing its junction with the to produce a higher forward voltage drop. This voltage drop can be seen when the meter is placed in the diode check mode this is what allows you to tell apart the from the. Controlling a DC motor with software The DC motor requires a transistor which acts like a solid-state switch, because the Arduino digital and analog out pins produce current strong enough to light small LEDs (up to ma, or so), but they're not strong enough to run motors and other power-hungry parts (this motor needs ma). Because the motor needs more current than an Arduino pin can provide, a transistor like the 2N2222 can switch up to 200 ma, when we give it a small amount of current. The DC motors in this lab can handle 4.5 V at most so this circuit will need a voltage divider to ensure 5 V (the + in the Arduino) is not sent to the motor, which could fry it. Use 10.0 W and W resistors to construct a voltage divider so that the voltage across one of the resistors is about 4.5 V with no load (no circuit attached to the

4 voltage divider). Show the voltage divider schematic and voltage divider calculation below. In actuality, when the motor is connected, the voltage going through it will likely be around 3.5 V since the load resistance is in parallel with one of your voltage divider resistors and that lowers the resistance which lowers the output voltage under load ideally, you should pick resistors that are much lower than the load resistance. However, since the voltage across the motor is low enough, go with your schematic. One other point about motors: When a motor stops, a small amount of current might be generated as the shaft continues spinning. A diode placed in parallel with the motor leads will keep any generated electricity from damaging your circuit. Of course, place it in such a way as to prevent current from going backwards through the circuit. On the Arduino software side, you will need to write code to control the flow of current from either a digital power pin or an analog power pin to the of the transistor. The goal is to get the motor to spin at two different speeds for three seconds each, then stop. These two different speeds may be accomplished using HIGH and LOW from the digital pin, or two suitable numbers between 0 and 1023 from the analog pin. The analogwrite command will be invaluable here; this command requires a number between 0 and 255, and outputs an appropriate PWM signal to an analog pin (like pin 9, just saying). For instance, analogwrite(255) outputs 100% of the PWM voltage (i.e., always on) and analogwrite(127) outputs 50% of the PWM voltage (i.e., half the time on). Below, write code or pseudo-code for the key parts of running the motor. Equipment 2N2222 bipolar junction transistor Fluke multimeter Arduino kit and laptop 4.5V motor Circuit diagrams

5 Part A Rather than a circuit diagram, draw the transistor as distinctly as you can, identifying the, and wires. Part B Draw a schematic of the transistor/motor/arduino circuit. Use the pin numbers on the Arduino and use the transistor symbol above (including the little arrow). Safety The 2N2222 transistor is low-power, so it is extremely sensitive to short-circuiting due to static charge. Please take the appropriate precautions. Procedure Part A (Identifying the wires on a transistor) 1. Obtain the transistor and place it upright on the breadboard, taking care not to stress the wires too much. 2. Using the Fluke multimeter, perform the tests suggested in the introduction above. Record six measurements in the data section. 3. Deduce the identity of each wire and record it in Part A of the circuit diagram section; it will be critical for Part B to know this correctly (check with the instructor before proceeding). Part B (Controlling a motor using a transistor) 4a. Write a simple program that will make the motor spin at two different speeds for three seconds each, and then stop. Make sure the two speeds are such that the casual observer can easily tell that the motor is spinning at two different rates. 4b. On the breadboard, connect the transistor s pin through a 470 W resistor to a PWM digital pin (like 9 ) or analog pin (like A0 ) on your microcontroller, depending on how you wrote the Arduino code. Connect the transistor s pin to GND (0 V). 5b. Connect the black wire on the motor to the transistor s pin. Connect the red wire on the motor to 4.5 V from your voltage divider. 6b. You will also need a flyback diode: When the motor is spinning and then is suddenly turned off, the magnetic field inside it collapses, generating a voltage spike according to Faraday s Law, which you won t see until Chapter 30 in the text. Faraday s law says that a voltage will be created whenever the magnetism changes in a circuit, like when a motor is turned off. This voltage spike can damage the

6 transistor and possibly the microcontroller. To prevent this from happening, we use a "flyback diode", which diverts the voltage spike around the motor. Connect the side of the diode with the band (cathode) to 4.5V and connect the other side of the diode (anode) to the black wire on the motor: 470 W 7b. Place a small piece of tape on your motor s shaft so you can see it spin. 8. Show your working circuit to the instructor and include your entire code in the data section. Part C (Measuring b) 9. Modify the Arduino code slightly to have the motor run continually at a single speed. 10. With the power off, put the current meter in series with the motor and the of the transistor to measure the current (IC). Make sure you have it hooked up correctly before connecting the power! Record IC, and estimate the uncertainty in IC. 11. To measure the current (IB), either: connect your meter in series with the 470 W resistor or measure the voltage across the resistor and use Ohm s law to calculate the current Record your result (IB) and show work, if needed. Estimate the uncertainty in IB. 12. While the motor is running, measure the potential difference from the to the (VBC) and from the to the (VBE) on the transistor, along with their uncertainties. These measurements will help determine what operation mode this transistor is in.

7 Data and Analysis Part A Label the transistor wires 1, 2 and 3, arbitrarily; then 1+ will mean the + (red) lead on the multimeter is placed on the 1 pin of the transistor. Make a table that shows the six possible permutations of the leads, and their results: use OL to indicate overload. Explain, using the data above, which numbered wire is the, then do the same for the and. Make sure to draw the transistor with the properlylabeled wires in the circuit diagram section. Part B Don t forget to draw a schematic of the entire circuit, including the voltage divider, in the circuit diagram section. Include the working Arduino code here. Make sure the key steps are fully documented (it s also good to have a comment at the top of the code that identifies the author(s) of the code and the date). Part C Measured IC ± u(ic) (units?) Measured IB ± u(ib) (units?) Calculate b (= IC/IB), and u(b), using GUM. Note this is a unitless number. Measured VBC ± u(vbc) (units?) Measured VBE ± u(vbe) (units?) Discussion and conclusion Clearly state b ± u(b). Go to and determine which operation mode the transistor was running on, when the motor was on. Use your data to explain your determination. In the active mode, b is between 100 and 300. When saturated, b is about 10. Does your calculation support your conclusion about the transistor s operation mode? If there is a discrepancy, give a plausible explanation.