Exercise 1: EXERCISE OBJECTIVE When you have completed this exercise, you will be able to test a transistor by forward biasing and reverse biasing the junctions. You will verify your results with an ohmmeter. DISCUSSION The best way to test a transistor is to connect it in a circuit that uses the transistor. voltages, and if the leakage current is within tolerance. Tests can be made with commercial transistor testers; however, it is also possible to test a transistor with just an ohmmeter. Simple ohmmeter tests will show if the transistor is open or shorted. is a PNP or NPN type. Testing a transistor with an ohmmeter can show if it a. is open or shorted. b. is a PNP or NPN type. c. d. All of the above For testing, a transistor can be thought of as two diodes. Festo Didactic 91564-P0 203
The base-emitter PN junction can be tested as if it is one diode, and the base-collector PN junction can be tested as if it is another diode. However, two diodes connected back-to-back do not function like a transistor does. You learned that a PN junction conducts when forward biased and does not conduct when reverse biased. By making base-emitter and base-collector measurements, you can determine: the type of transistor (PNP or NPN). if an open or short exists. if the leakage current is excessive. 204 Festo Didactic 91564-P0
Each of the two junctions of a transistor can be tested as if it were a a. resistor. b. diode. NPN transistor The base-emitter junction of a good NPN transistor conducts when forward biased and does not conduct when reverse biased. The same is true for the base-collector junction of a good NPN transistor. when tested with an ohmmeter. Festo Didactic 91564-P0 205
The base-emitter junction of an NPN transistor conducts when forward biased. The transistor is a. good. b. bad. The base-collector junction of an NPN transistor conducts when reverse biased. The transistor is a. good. b. bad. PNP transistor The base-emitter junction of a good PNP transistor conducts when forward biased and does not conduct when reverse biased. The same is true for the base-collector junction of a good PNP transistor. 206 Festo Didactic 91564-P0
when tested with an ohmmeter. The base-collector junction of a PNP transistor conducts when forward biased. The transistor is a. good. b. bad. A transistor is in its off state. An ohmmeter reading connected The transistor is a. good. b. bad. When the ohmmeter is connected to forward bias a good transistor junction, it should show a number (reading) that indicates conduction. junction. The magnitude of the meter reading depends on the type of meter and the selected meter scale. It can vary widely for different meters and scales. When the ohmmeter is connected to reverse bias a good transistor junction, it shows an off-scale (overload) reading, which indicates no conduction. To forward bias an NPN transistor base-emitter junction with an ohmmeter, connect the positive lead (usually the red lead) to the base (B) and the negative lead (usually the black common lead) to emitter (E). Festo Didactic 91564-P0 207
To reverse bias an NPN transistor base-emitter junction with an ohmmeter, connect the positive lead (usually the red lead) to the emitter (E) and the negative lead (usually the black common lead) to the base (B). To forward bias an NPN transistor base-collector junction with an ohmmeter, connect the positive lead to the base (B) and the negative lead to the collector (C). To reverse bias an NPN transistor base-collector junction with an ohmmeter, connect the positive lead to the collector (C) and the negative lead to the base (B). The ohmmeter shown is connected to a. forward bias the base-emitter junction. b. reverse bias the base-emitter junction. 208 Festo Didactic 91564-P0
To forward bias a PNP transistor base-emitter junction with an ohmmeter, connect the positive lead to the emitter (E) and the negative lead to the base (B). To reverse bias a PNP transistor base-emitter junction with an ohmmeter, connect the positive lead to the base (B) and the negative lead to the emitter (E). To forward bias a PNP transistor base-collector junction with an ohmmeter, connect the positive lead to the collector (C) and the negative lead to the base (B). Festo Didactic 91564-P0 209
To reverse bias a PNP transistor base-collector junction with an ohmmeter, connect the positive lead to the base (B) and the negative lead to the collector (C). To test a transistor (PNP or NPN) between the collector and emitter, connect one lead (red or black) to the collector and the other to the emitter. The ohmmeter shown is connected to a. forward bias the base-collector junction. b. reverse bias the base-collector junction. If an ohmmeter indicates an overload (off-scale) when a transistor junction is forward biased, the transistor junction is open; the conduction path is broken. If an ohmmeter indicates zero or a very low reading when a transistor junction is reverse biased or forward biased, the transistor junction is shorted. transistor has high leakage current and is defective. If an ohmmeter indicates zero or a very low reading when connected across the collector and emitter, there is a short or high leakage current. 210 Festo Didactic 91564-P0
If the ohmmeter shown indicates a zero reading, a. the base-collector junction is shorted. b. there is a short between the collector and emitter. c. the transistor is not defective. The following table provides setup information to use the following meters when you test a transistor junction: 1. MODEL 1247-10 DIGITAL MULTIMETER 2. MODEL 1247-00, 1246-00, or 1242-00 DIGITAL MULTIMETERS 3. DIGITAL MULTIMETER WITH A DIODE TEST FUNCTION To test semiconductor PN junctions, apply power to your meter, and refer to this table to set up your meter. Meter Type Multimeter Multimeter (includes 1242 & Diode Test Function 1246 models) Input Jacks OHMS ( ) & COM VOLTS (V) & COM ) & COM Set FUNCTION to, Set MANUAL RANGE to Select the DIODE symbol RANGE to AUTO, and select the DIODE symbol 2 V DC and place all DMM pushbuttons outward (reverse-biased or open-circuit) Overload (.OL) Overload (2.000 2.999 V), indicates 2.6 V Overload (1 or OL) (forward-biased) (short-circuit).20 1.99 V.200 1.999 V 0.5 1.2 V or 500 1200 mv (.60 V typical) (.600 V typical) (0.6 V or 600 mv typical).00 V.000 V 0 V Festo Didactic 91564-P0 211
PROCEDURE Locate transistor Q1 on the TRANSISTOR JUNCTION circuit block on the circuit board. Connect the red (positive) meter probe to the test point at the base (B) of transistor Q1 and the black (negative-common) meter probe at the emitter (E) of transistor Q1. Your ohmmeter reading indicates that the base-emitter junction is a. not conducting. b. conducting. Your measurement indicates that the Q1 base-emitter junction is a. forward biased. b. reverse biased. Connect the black (negative-common) meter probe at the collector (C) of transistor Q1. 212 Festo Didactic 91564-P0
Your meter reading indicates that the base-collector junction is a. not conducting. b. conducting. Your measurement indicates that the Q1 base-collector junction is a. forward biased. b. reverse biased. Because your measurements showed that the Q1 base-emitter and base-collector junctions are forward biased when the base is more positive than the emitter or collector, Q1 is a(n) a. PNP transistor. b. NPN transistor. Observe diodes CR1 and CR2 connected at the base of transistor Q1 on the TRANSISTOR JUNCTION circuit block. If the black (negative-common) meter probe was connected at the cathode of diode CR1 or diode CR2 while the red (positive) lead was connected to the Q1 base, the diodes would be a. forward biased. b. reverse biased. Can you conclude that the Q1 base-emitter junction behaves like diode CR2 and that the Q1 base-collector junction behaves like diode CR1? a. no b. yes With a good transistor, the ohmmeter as connected would indicate a. no conduction between the collector and emitter. b. conduction between the collector and emitter. Place CM switch 2 in the ON position to introduce a fault in Q1. Use the ohmmeter to test the junctions. The fault is a. in the Q1 base-collector junction. b. between the Q1 collector and emitter. c. in the Q1 base-emitter junction. When CM 2 is on, the Q1 base-emitter junction is a. open. b. shorted. Festo Didactic 91564-P0 213
Connect the red (positive) meter probe to the test point at the base (B) of transistor Q2 and the black (negative-common) meter probe at the emitter (E) of transistor Q2. Your meter reading indicates that the base-emitter junction is a. not conducting. b. conducting. Your measurement indicates that the Q2 base-emitter junction is a. forward biased. b. reverse biased. Connect the black (negative-common) meter probe at the collector (C) of transistor Q2. Your meter reading indicates that the Q2 base-collector junction is a. not conducting. b. conducting. Your measurement indicates that the Q2 base-collector junction is a. forward biased. b. reverse biased. Because your measurements showed that the Q2 base-emitter and base-collector junctions are reverse biased when the base is more positive than the emitter or collector, Q2 is a(n) a. PNP transistor. b. NPN transistor. Observe the TRANSISTOR JUNCTION circuit block. If the black (negative-common) meter probe were placed at the anode of diode CR3 or diode CR4, the diodes would be a. forward biased. b. reverse biased. Can you conclude that the Q2 base-emitter junction behaves like diode CR4 and that the Q2 base-collector junction behaves like diode CR3? a. no b. yes With a good Q2 transistor, the ohmmeter as connected would indicate a. no conduction between the collector and emitter. b. conduction between the collector and emitter. Make sure all CMs are cleared (turned off) before proceeding to the next section. 214 Festo Didactic 91564-P0
CONCLUSION An ohmmeter can be used to determine if a transistor is an NPN or PNP, is shorted or open, and has When the NPN transistor base is more positive than the emitter or collector, the junctions are forward biased. When the PNP transistor base is more negative than the emitter or collector, the junctions are forward biased. The PN junctions of a transistor are similar to the PN junction of a diode because when forward biased, the junction conducts, and when reverse biased, the junction does not conduct. There is no conduction between the collector and emitter when the transistor is in the off state. REVIEW QUESTIONS 1. Locate Q2 on the TRANSISTOR JUNCTION circuit block. Place CM switch 3 in the ON position to activate it. With an ohmmeter, test the junctions of transistor Q2. Your tests show that a. a base-emitter junction is shorted. b. the base connection is open. c. a short exists between the collector and emitter. d. the collector connection is open. 2. When the base-emitter junction of a PNP transistor is forward biased, the a. base is more negative than the emitter. b. base is more positive than the emitter. c. base is more positive than the emitter and collector. d. collector is more positive than the base. 3. The base-collector junction of an NPN transistor is reverse biased when the a. collector is more negative than the base. b. base is more positive than the collector. c. emitter is more negative than the base. d. base is more negative than the collector. Festo Didactic 91564-P0 215
4. An ohmmeter reads zero when the base-emitter junction of a PNP transistor is forward biased or reverse biased. The junction is a. functioning properly. b. open. c. shorted. d. reverse biased. 5. The section that is sandwiched by the two outer sections of a transistor is the a. base. b. emitter. c. collector. d. CE junction. NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section. 216 Festo Didactic 91564-P0