Testing Transistors:
1. Testing Small Power Transistors
A. Identifying Positive and Negative Electrodes
(a) Observe the symbol markings on the casing. Typically, the casing of the diode is marked with the diode symbol, where the end with the triangular arrow is the positive electrode, and the other end is the negative electrode.
(b) Observe the color dot on the casing. On the casing of the point-contact diode, there is usually a colored dot (white or red) indicating the polarity. Generally, the end marked with a color dot is the positive electrode. Some diodes have color bands, where the end with the color band is the negative electrode.
(c) Use the lower resistance measurement as the standard; the end connected to the black probe is the positive electrode, and the end connected to the red probe is the negative electrode.
B. Testing the Maximum Operating Frequency fM. The operating frequency of the transistor can be checked in the relevant specification table, but in practice, it is often distinguished by observing the internal filament of the diode. For example, point-contact diodes are high-frequency devices, while surface-contact diodes are mostly low-frequency devices. Additionally, a multimeter set to R×1k can be used for testing, where generally, a forward resistance of less than 1k indicates a high-frequency device.
C. Testing the Maximum Reverse Breakdown Voltage VRM. For AC, since it is constantly changing, the maximum reverse operating voltage is the peak AC voltage that the diode can withstand. It should be noted that the maximum reverse operating voltage is not the breakdown voltage of the diode. Generally, the breakdown voltage is much higher than the maximum reverse operating voltage (approximately double).
2. Testing Glass-Sealed Silicon High-Speed Switching Diodes
The method for testing silicon high-speed switching diodes is the same as that for testing ordinary diodes. The difference is that these types of diodes have a higher forward resistance. When measured with an R×1k resistance setting, the typical forward resistance value is between 5k and 10k, while the reverse resistance is infinite.
3. Testing Fast Recovery and Ultra-Fast Recovery Diodes
The method for testing fast recovery and ultra-fast recovery diodes is basically the same as that for testing plastic-sealed silicon rectifier diodes. First, use the R×1k setting to check their unidirectional conductivity, where the typical forward resistance is around 4.5k, and the reverse resistance is infinite; then retest using the R×1 setting, where the typical forward resistance is a few ohms, and the reverse resistance remains infinite.
4. Testing Bidirectional Trigger Diodes
A. Set the multimeter to the R×1k setting; both the forward and reverse resistance values of the bidirectional trigger diode should be infinite. If the probes are swapped for measurement and the multimeter needle swings to the right, it indicates that the tested diode has leakage failure.
Set the multimeter to the appropriate DC voltage range. The test voltage is provided by a megohmmeter. During testing, the voltage indicated by the multimeter is the VBO value of the tested diode. Then, swap the two leads of the tested diode and measure the VBR value in the same manner. Finally, compare VBO and VBR; the smaller the absolute difference between the two values, the better the symmetry of the tested bidirectional trigger diode.
5. Testing Transient Voltage Suppressor Diodes (TVS)
A. Use the multimeter on the R×1k setting to measure the quality of the diode.
For unidirectional TVS, the method is the same as that for measuring ordinary diodes, where the forward resistance is typically around 4kΩ, and the reverse resistance is infinite.
For bidirectional TVS, regardless of how the red and black probes are swapped, the resistance between the two leads should be infinite; otherwise, it indicates that the diode’s performance is poor or has been damaged.
6. Testing High-Frequency Varistors
A. Identifying Positive and Negative Electrodes
The visual difference between high-frequency varistors and ordinary diodes is the color of the color code; ordinary diodes typically have black color codes, while high-frequency varistors have lighter colors. The polarity follows a similar rule to ordinary diodes, where the end with the green band is the negative electrode, and the end without the green band is the positive electrode.
B. Measure Forward and Reverse Resistance to Determine Quality
The method is the same as for measuring the forward and reverse resistance of ordinary diodes. When using a 500-type multimeter on the R×1k setting, the normal forward resistance of a high-frequency varistor is between 5k and 5.5k, while the reverse resistance is infinite.
7. Testing Varactor Diodes
Set the multimeter to the R×10k setting; regardless of how the red and black probes are swapped, the resistance between the two leads of the varactor diode should be infinite. If during measurement, the multimeter needle shows a slight swing to the right or the resistance is zero, it indicates that the tested varactor diode has leakage failure or has been damaged. For varactor diodes with lost capacitance or internal open circuit faults, a multimeter cannot be used to detect or determine. If necessary, a substitution method can be used for inspection and judgment.
8. Testing Monochrome Light Emitting Diodes
Connect a 1.5V dry battery externally to the multimeter and set it to R×10 or R×100. This connection effectively adds 1.5V to the multimeter, increasing the testing voltage to 3V (the forward voltage of the light-emitting diode is 2V). During testing, alternate contact of the multimeter probes with the two leads of the light-emitting diode. If the diode is functioning well, it will light up at least once; at this point, the black probe is connected to the positive electrode, and the red probe is connected to the negative electrode.
9. Testing Infrared Light Emitting Diodes
A. Identifying the Positive and Negative Electrodes of Infrared Light Emitting Diodes. Infrared light emitting diodes have two leads; usually, the longer lead is the positive electrode, and the shorter lead is the negative electrode. Since infrared light emitting diodes are transparent, the electrodes inside the casing are clearly visible; the larger and wider internal electrode is the negative electrode, while the narrower and smaller one is the positive electrode.
B. Set the multimeter to the R×1k setting and measure the forward and reverse resistance of the infrared light emitting diode. Typically, the forward resistance should be around 30k, and the reverse resistance should be above 500k for the diode to be considered usable. A higher reverse resistance is preferable.
10. Testing Infrared Receiving Diodes
A. Identifying the Lead Polarity
(a) Identify by appearance. Common infrared receiving diodes are black. When identifying the leads, facing the light-receiving window, the left lead is the positive electrode and the right lead is the negative electrode. Additionally, there is a small beveled plane at the top of the infrared receiving diode casing, typically the lead with this beveled plane is the negative electrode, and the other end is the positive electrode.
(b) Set the multimeter to the R×1k setting and use the method for identifying the positive and negative electrodes of ordinary diodes, that is, swap the red and black probes and measure the resistance between the two leads of the diode. Normally, one measurement should yield a high resistance and the other a low resistance. The smaller resistance measurement indicates the negative electrode, while the larger indicates the positive electrode.
B. Testing Performance. Use the multimeter resistance setting to measure the forward and reverse resistance of the infrared receiving diode. Based on the values of the forward and reverse resistances, a preliminary judgment can be made about the quality of the infrared receiving diode.
11. Testing Laser Diodes
A. Set the multimeter to the R×1k setting. Use the method for measuring the forward and reverse resistance of ordinary diodes to determine the lead arrangement of the laser diode. However, during testing, it is important to note that the forward voltage drop of laser diodes is greater than that of ordinary diodes, so when measuring forward resistance, the multimeter needle will only slightly deflect to the right, while the reverse resistance will be infinite.
Methods for Testing Transistors
1. Testing Medium and Small Power Transistors
A. For known models and lead arrangements of transistors, the following methods can be used to assess their performance.
(a) Measure the inter-electrode resistance. Set the multimeter to R×100 or R×1k, and test using the six different connections of the red and black probes. Among them, the forward resistance values of the emitter junction and collector junction are relatively low, while the resistance values of the other four connections are very high, typically ranging from several hundred kilo-ohms to infinite. Regardless of whether the resistance is low or high, the inter-electrode resistance of silicon transistors is much greater than that of germanium transistors.
(b) The leakage current ICEO of the transistor is approximately equal to the product of the transistor’s current gain β and the reverse current ICBO of the collector junction. ICBO increases rapidly with rising ambient temperature, and the increase in ICBO will inevitably lead to an increase in ICEO. The increase in ICEO will directly affect the stability of the transistor’s operation, so it is advisable to choose transistors with a smaller ICEO during use.
By directly measuring the resistance between the e-c electrodes of the transistor with a multimeter, the size of ICEO can be indirectly estimated. The specific method is as follows:
The multimeter resistance range is generally set to R×100 or R×1k. For PNP transistors, connect the black probe to the e lead and the red probe to the c lead. For NPN transistors, connect the black probe to the c lead and the red probe to the e lead. The larger the measured resistance, the better. A larger e-c resistance indicates a smaller ICEO; conversely, a smaller measured resistance indicates a larger ICEO. Generally, for medium and small power silicon transistors, the resistance values should be several hundred kilo-ohms, while for germanium low-frequency transistors, they should be above several tens of kilo-ohms. If the resistance is very small or the multimeter needle fluctuates during testing, it indicates a large ICEO, and the transistor’s performance is unstable.
(c) Measure the amplification factor (β). Currently, some models of multimeters have scales for measuring the hFE of transistors and testing sockets, making it convenient to measure the transistor’s amplification factor. First, set the multimeter function switch to the hFE position, and set the range switch to the ADJ position. Short the red and black probes, and adjust the zero knob until the multimeter needle indicates zero. Then, switch the range switch to the hFE position, and insert the transistor into the test socket to read the amplification factor from the hFE scale.
Additionally, for certain models of medium and small power transistors, manufacturers directly indicate the amplification factor β value at the top of the transistor casing using different color dots. The correspondence between colors and β values is shown in the table, but it should be noted that the color codes used by different manufacturers may not be completely the same.
B. Identifying Electrodes
(a) Determine the base. Use the multimeter R×100 or R×1k setting to measure the forward and reverse resistance values between each pair of the three electrodes of the transistor. If the first probe connects to one electrode and the second probe measures low resistance with both of the other two electrodes, then the electrode connected by the first probe is the base b. At this point, it is important to note the polarity of the multimeter probes; if the red probe connects to the base b, and the black probe connects to the other two electrodes, the measured resistance values will be small, indicating that the tested transistor is a PNP type; if the black probe connects to the base b, and the red probe connects to the other two electrodes, the measured resistance values will be small, indicating that the tested transistor is an NPN type.
(b) Determine the collector c and emitter e. (Taking PNP as an example) Set the multimeter to R×100 or R×1k; connect the red probe to the base b, and use the black probe to measure the other two leads. The measured resistance values will show one larger and one smaller. In the measurement with the smaller resistance, the lead connected by the black probe is the collector; in the measurement with the larger resistance, the lead connected by the black probe is the emitter.
C. Distinguishing High-Frequency and Low-Frequency Transistors
High-frequency transistors have a cutoff frequency greater than 3MHz, while low-frequency transistors have a cutoff frequency less than 3MHz. Generally, they cannot be interchanged.
D. Voltage Detection Judgment Method
In actual applications, medium and small power transistors are often directly soldered onto printed circuit boards. Due to the high density of component installation, disassembly can be troublesome, so during testing, the voltage values at the leads of the tested transistor are often measured using the multimeter’s DC voltage setting to infer whether it is functioning normally, and thereby assess its quality.
2. Testing Large Power Transistors
The methods used to test the polarity, type, and performance of medium and small power transistors with a multimeter are generally applicable to testing large power transistors as well. However, since large power transistors carry larger operating currents, the area of their PN junctions is also larger. With a larger PN junction, the reverse saturation current will inevitably increase. Therefore, if the multimeter’s R×1k setting is used to measure the inter-electrode resistance as with medium and small power transistors, the measured resistance value will be very small, appearing as a short circuit between the electrodes. Thus, it is common to use the R×10 or R×1 setting to test large power transistors.
3. Testing Ordinary Darlington Transistors
The testing of ordinary Darlington transistors with a multimeter includes identifying electrodes, distinguishing between PNP and NPN types, and estimating amplification capability. Since Darlington transistors have multiple emitter junctions between the E and B leads, the multimeter should be set to the R×10k setting, which can provide a higher voltage for measurement.
4. Testing Large Power Darlington Transistors
The methods for testing large power Darlington transistors are basically the same as those for ordinary Darlington transistors. However, since large power Darlington transistors are equipped with protective and discharge leakage current components such as R1 and R2, it is important to distinguish the influence of these components on the measurement data to avoid misjudgment. The specific steps are as follows:
A. Use the multimeter R×10k setting to measure the PN junction resistance between B and C; a significant unidirectional conductive performance should be clearly measurable. The forward and reverse resistance values should show a large difference.
B. There are two PN junctions between B and E in large power Darlington transistors, connected with resistors R1 and R2. When testing with the multimeter resistance setting, the value measured during forward measurement is the result of the forward resistance of the B-E junction in parallel with R1 and R2. During reverse measurement, the emitter junction is cutoff, and the measured value is the sum of the resistances of R1 and R2, which is approximately several hundred ohms and remains fixed regardless of the resistance setting. However, it is important to note that some large power Darlington transistors may also have diodes connected in parallel with R1 and R2, in which case the measured value will not be the sum of R1 and R2 but rather the parallel resistance of (R1 + R2) and the forward resistances of the two diodes.
5. Testing Damped Output Transistors
Set the multimeter to the R×1 setting and individually measure the resistance values between the electrodes of the damped output transistor to determine its normality. The specific testing principles, methods, and steps are as follows:
A. Connect the red probe to E and the black probe to B; this measurement effectively assesses the resistance value of the equivalent diode of the B-E junction of the large power transistor in parallel with the protective resistor R. Due to the small forward resistance of the equivalent diode and the general resistance of the protective resistor R being only 20Ω to 50Ω, the parallel resistance will also be small. Conversely, swapping the probes, connecting the red probe to B and the black probe to E, will measure the reverse resistance of the equivalent diode of the B-E junction, which will yield a larger value, indicating the value of the protective resistor R, which remains small.
B. Connect the red probe to C and the black probe to B; this measures the forward resistance of the equivalent diode of the B-C junction of the large power transistor, which generally yields a small measured value. When the probes are swapped, connecting the red probe to B and the black probe to C, this measures the reverse resistance of the equivalent diode of the B-C junction, which typically yields an infinite resistance.
C. Connect the red probe to E and the black probe to C; this measures the reverse resistance of the damping diode within the transistor, which typically yields a large value, around 300Ω to infinity. When the probes are swapped, connecting the red probe to C and the black probe to E, this measures the forward resistance of the damping diode within the transistor, which typically yields a small value, ranging from several ohms to several tens of ohms.
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