Lab 7 transistors 2024-03-15

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Centennial College *

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EETQ114

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Electrical Engineering

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Apr 3, 2024

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CENTENNIA - COLLEGH Centennial College Electrical Engineering Techniclan / Technology EETQ-114 Electronics 1 and Instrumentation 1 Name: /(/«l-mr—v Atsrdrend, == Student#: 3O /02 Section: Date: /(8 -D7—2024- LAB #7 Transistors Objectives: At the end of this lab the student will be able to: « Recognize different regions of the transistor. « Describe the transistor as a switch, LED drivers. « Demonstrate basic transistor circuit troubleshooting. Theory: Some manufacturers, such as Motorola and RCA, use their own numbering system for some devices. Motorola often uses MJ, MJE, or SDT. RCA uses an SK number to identify some of their transistors. Some equipment manufacturers use their own special numbers to identify components. These numbers mean nothing to anyone except the manufacturer, and in most cases cannot be identified. Transistor Schematics Figure A shows the symbol for the NPN and PNP type of transistor and the polarity markings for each. Notice that the transistors have the base connected to the same polarity as the collector. The arrow on the emitter of the transistor points in the direction of conventional current flow, +to -. C E B B E Cc NPN PNP Figure A Transistors can be tested with an ohmmeter. The test will indicate if the transistor is good or bad. If the polarity of the leads of the ohmmeter is known, it will indicate if the transistor is NPN or PNP. To an ohmmeter, a transistor appears to be two diodes with their anodes (NPN) or their cathodes (PNP) connected. If the positive lead of the ohmmeter Is connected to the base of the transistor, it shows continuity to both the collector and the emitter. If the negative lead of the meter is connected to the base of the transistor, it does not show continuity between the base and the collector or the base and the emitter. Page 1of 5 Scanned with CamScanner

The PNP transistor can be tested by reversing the polarity and connecting the negative lead of the ohmmeter to the base. The meter indicates continuity between the base and the collector and the base and emitter. If the positive lead of the ohmmeter is connected to the base of the PNP transistor it shows no continuity to the collector or the emitter. The ohmmeter test is considered to be about 95% accurate, but there are some conditions under which the test would not be valid. If the transistor is breaking down under a high voltage, the ohmmeter will not be able to supply enough voltage to show the transistor defective. If the transistor is being broken down by heat, the ohmmeter will not supply enough power to show the transistor is bad. Identifying the Leads The leads of transistors packaged in standard T03, T05, or T018 cases are relatively simple to identify. Transistors packaged in the T092, 70220, or T0218 cases can sometimes be difficult because of the position of their leads, when testing a transistor in the TO5 or TO18 case, hold the transistor upside down. The three leads form a triangle on one side of the case. The little tab on the case is always closest to the emitter lead of the transistor, Figure B. The T03 case transistor leads can be identified by holding the transistor with the leads facing you and down. To5;1018 /—TM ¢ € o Figure B Index Tab The lead to the left is the emitter and the lead to the right is the base. The collector is the case of the transistor. Refer to Figure B. Plastic case transistors such as the TO92, the T0220, or the T0218 can have their leads at any position. The following procedure can be used to identify these transistors: 1. Assume the transistor is an NPN. 2. Assume that one lead of the transistor is the base. Connect the positive lead of the ohmmeter to it. 3. Touch each of the other two leads one at a time. If there is continuity to the other two leads it is the base of the transistor and it is an NPN. If there is no continuity, assume another lead to be the base and repeat the test. 4. If none of the three transistor leads prove to be the base pin, assume the transistor to be a PNP. Repeat the procedure using the negative lead of the ohmmeter connected to the base of the transistor. 5. If no lead can be identified as the base of the transistor, the component is probably not a transistor, or it is defective. Page 2of 5 Scanned with CamScanner

6. Once the base lead is identified and it is determined whether the transistor is NPN or PNP, the two remaining leads will have to be Identified as to which is the collector and which is emitter. (Note: The base of the transistor must have the same polarity as the collector in order for the transistor to operate, Figure A). 7. Assume that the transistor is an NPN and the base lead has been identified. Assume that one of the two remaining leads is the collector. Connect the positive ohmmeter lead to it. Connect the negative lead to the other pin. The ohmmeter should not indicate continuity. 8. Using a resistor of any value from 10 to 5000 ohms, touch one lead of the resistor to the positive ohmmeter lead and the base of the transistor to the other resistor lead. If the ohmmeter shows conduction, the assumption Is correct, and the positive ohmmeter lead is connected to the collector of the transistor. If the ohmmeter does not show continuity, reverse the leads and assume the other transistor pin to be the collector. 9. APNP transistor can be tested in the same manner. Connect the negative ohmmeter lead to the collector and touch the resistor between the negative lead of the ohmmeter and the base pin. Equipment: © Breadboard Variable DC Power Supply (BK Precision 1672) Alligator test leads * DMM (Keithley 2110) * Resistors: 1.0 kQ, 10 kQ, 10 kQ pot Oscilloscope e LED © NPN Transistor (2N3904) Procedure: 1. Measure and record the values of the resistors listed in Table 1. Ry is a 10 kQ potentiometer and is not listed in the table. Resistor Listed Value Measured Value Re 10ko D.25 K.OQ R 1ka 2.99 K o Table 1 2. Ideally, a transistor switching circuit should operate either in cutoff in saturation. The circuit shown in Figure 1is a basic transistor amplifier. It can easily be set for operation at these extremes by varying the potentiometer (R). However, it can also operate between cutoff and saturation, a condition not desirable in a switching circuit. Compute the collector-emitter voltage (Vce) at cutoff and saturation and the voltage across the collector resistor at saturation (Vkc). Compute ls; by assuming a 2.0 V drop across the LED and 0.1 V across the transistor. computed values in Table 2. Resistor Listed Value Measured Value Ve (cutoff) 40 Vee (sat) A.2 VRe (st 00 M Isat A0 A Table 2 Page30f5 Scanned with CamScanner

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3. Construct the circuit in Figure 1 and observe the effect of varying the potentiometer. Set the potentiometer to the minimum value and measure Ve at cutoff (the LED should be off). Then set the potentiometer for maximum value (LED on), and measure Ve (saturation) and the voltage across the 1 kQ collector resistor, Vrc (saturation). The transistor is saturated since it can supply no more current in the collector circuit. Record measurements in Table 2 and compare to the computed values of step 2. Figure 1 Questions: 1. For a given collector voltage, what happens to the collector current as the base current increase? It Increases/ c) Itdecreases b) It remains constant d) Inapproaches cutoffs 2. Can collector current be cut off (reduced to nearly zero) by reducing base current? a) sometimes ) It has almost no effect b) Never @ Yes/ 3, Which of the following is a true statement? a) An NPN transistor can control power to a load by its collector circuit. - @A PNP transistor can be used a switching device. c) An NPN transistor can be used to control current from zero to some predetermined value. d) All of the above. 4, The Beta of a transistor is the: Lo r0diC GF He bramsislar currenl 4o He Lo Crirleyid—. 5. How many layers of semiconductor material are required to make a transistor? e Mtzf/<( & Hrarsicly N2, nfied 2 Jalors o~ s condocden e *t"l“: - 6. What are the two major types of transistors? 1) B per u/.uhcl/—/’c}n fran=,< Fork. Ao fees/d GJC‘CF Lrons,'s5lors . Page4of 5 Scanned with CamScanner

7. Why is silicon used more often than germanium in the production of transistors? / / ) / gl - due to iF having a. higher Lomoera dure . - v 8. What is the most common system used in the identification of transistors? The mps % COoOmron 53 5 7L<°-I'7’] [J-J‘(-’L/ S //7(“' Ah reg./'s ter sosdoms 9. When an NPN transistor is to be tested with an ohmmeter, does the transistor appear to be two diodes with their anodes connected together or their cathodes connected together? N ' = an [ / yes ¢ docs Jndeed appear fhat L 4 10. When a transistor is in a standard T05 package, which lead is located closest to the metal tab on the case of the transistor? The cem/Fda '/fi‘[tfi/- 11. When a transistor is in a standard T3 case, which lead is connected to the case of the transistor? The CON/ECtor /F‘/d 12. What controls the amount of current flow through the collector-emitter section of a transistor? The /3. 20104 Hrapm'st-er - Conclusion: State briefly, but clearly, what you have gained from this assignment. L fewrped hnas do crendS Clrown'tS tudd transistors, ore fenauledae ma b/ Fthe Y LIprk, Lo 2 ron Lo/ hot/ and lhcrh 40 vee frawns /< Lab Submission: - Please submit this lab report at the beginning of the next lab class for a full grade. Criteria Max Grade Materials, Tools and Equipment Set-up 15 Following Procedure 2 Data Collection and Analysis 2 Evaluation of Experiment, Review Questions 2 Conclusion 1.5 Spelling, Grammar, Sentence Structure 1 TOTAL Page 50f 5 Scanned with CamScanner