Introduction to transistor detection experience

(I) Discrimination of transistor materials and polarity
1. Identify the material and polarity of transistors from their model names The second part of the domestic transistor model names uses the English letters A to D to represent the transistor's material and polarity. Among them, "A" represents a PNP transistor made of germanium material, "B" represents a NPN transistor made of germanium material, "C" represents a PNP transistor made of silicon material, and "D" represents a NPN transistor made of silicon material.

The third part of the name of the model of the *** produced transistor uses letters A~D to indicate the material and type of the transistor (not the polarity). Among them, "A" and "B" are PNP type transistors, and "C" and "D" are NPN type transistors. Usually, "A" and "C" are high-frequency transistors, and "B" and "D" are low-frequency transistors.

The first part of the European transistor model name uses the letters "A" and "B" to indicate the material of the transistor (not indicating NPN or PNP polarity). Among them, "A" indicates germanium material and "B" indicates silicon material.

2. Identify the pins of the transistor from the package shape Before using the transistor, you must first identify the polarity of each pin of the transistor.

Transistors of different types, models, and functions have different pin arrangements. By reading the above "Transistor Package Outlines", you can quickly identify the polarity of the pins of commonly used transistors.

3. Use a multimeter to determine the polarity and material of the transistor. For transistors whose model numbers are unclear or whose pins cannot be identified even though they have models, a multimeter can be used to determine the polarity, pins and materials of the transistor.

For general low-power transistors, you can use the multimeter R×100Ω or R×1k range, and use two test leads to measure the forward and reverse resistance values ​​between any two pins of the transistor.

During the measurement, you will find that when the black test lead (or red test lead) is connected to a pin of the transistor, and the red test lead (or black test lead) is used to touch the other two pins, the multimeter indicates low resistance. At this time, the pin of the transistor being measured connected to the black test lead (or red test lead) is the base B, and the other two pins are the collector C and the emitter E. If the base is connected to the red test lead, the tube is a PNP tube; if the base is connected to the black test lead, the tube is an NPN tube.

You can also first assume that any pin of the transistor is the base, touch it with the red or black test lead, and then use the other test lead to touch the other two pins respectively. If two smaller resistance values ​​are measured, the pin connected to the fixed test lead is the base B, and the other two pins are the emitter E and the collector C.

After finding the base B, compare the forward resistance values ​​between the base B and the other two pins. Usually, the electrode with a larger forward resistance value is the emitter E, and the electrode with a smaller forward resistance value is the collector C.

For a PNP transistor, you can connect the red test lead to the base B and the black test lead to the other two pins, and you will get two slightly different resistance values. In the measurement with a smaller resistance value, the pin connected to the black test lead is the collector C; in the measurement with a larger resistance value, the pin connected to the black test lead is the emitter E.

For NPN transistors, connect the black test lead to the base B. Use the red test lead to touch the other two pins. In a measurement with a smaller resistance value, the pin connected to the red test lead is the collector C; in a measurement with a larger resistance value, the pin connected to the red test lead is the emitter E.

By measuring the forward and reverse resistance values ​​of the transistor PN junction, the material of the transistor (to distinguish whether it is a silicon tube or a germanium tube) and its quality can also be determined. Generally, the forward resistance value of the germanium tube PN junction (between the B and E poles or between the B and C poles) is 200~500Ω, and the reverse resistance value is greater than 100kΩ; the forward resistance value of the silicon tube PN junction is 3~15kΩ, and the reverse resistance value is greater than 500kΩ. If the forward and reverse resistance values ​​of a certain PN junction of the transistor are measured to be 0 or infinite, it can be determined that the tube has been broken down or open-circuited.

(II) Testing of transistor performance
1. Testing of reverse breakdown current The reverse breakdown current (also called reverse leakage current or penetration current) of ordinary transistors can be estimated by measuring the resistance between the emitter E and the collector C of the transistor. When measuring, set the multimeter to the R×1k position, connect the collector C of the NPN tube to the black test lead, and the emitter E to the red test lead; connect the collector C of the PNP tube to the red test lead, and the emitter E to the black test lead.

Normally, the resistance value of low-power and medium-power transistors made of germanium is generally greater than 10KΩ (measured with R×100, the resistance value is greater than 2kΩ), and the resistance value of high-power germanium transistors is above 1.5kΩ (measured with R×10). The resistance value of silicon transistors should be greater than 100kΩ (measured with R×10k), and the actual measured value is generally above 500kΩ.

If the resistance between the C and E poles of the transistor is too small, it means that the leakage current of the transistor is large; if the resistance between the C and E poles is close to 0, it means that the C and E poles have been broken down and damaged. If the resistance between the C and E poles of the transistor becomes much smaller as the temperature of the tube case increases, it means that the thermal stability of the tube is poor.

You can also use the ICEO position of the transistor DC parameter test table to measure the reverse breakdown current of the transistor. When testing, first set the hFE/ICEO selection switch to the ICEO position, select the polarity of the transistor, insert the three pins of the transistor to be tested into the test hole, then press the ICEO key and read the reverse breakdown current value from the table.

2. Detection of amplification capacity The amplification capacity of the transistor can be measured using the hFE position of the multimeter. When measuring, the multimeter should be set to the ADJ position for zeroing, and then to the hFE position, and the C, B, and E pins of the transistor to be tested should be inserted into the corresponding test jacks (for high-power transistors in TO-3 package, the three electrodes can be connected to three leads and then connected to the three jacks respectively), and the multimeter will indicate the amplification factor of the tube.

If the multimeter does not have the hFE range, the R×1k range of the multimeter can also be used to estimate the transistor's amplification capacity. When measuring a PNP tube, the black probe of the multimeter should be connected to the emitter E of the transistor, the red probe to the collector C of the transistor, and then a resistor (100 kΩ for silicon tubes and 20 kΩ for germanium tubes) should be connected in parallel to the collector junction of the transistor (between the B and C poles), and then the resistance value of the multimeter is observed. If the multimeter pointer swings a large amplitude, it means that the transistor has a strong amplification capacity. If the multimeter pointer does not change or swings a small amplitude, it means that the transistor has no amplification capacity or has a poor amplification capacity.

When measuring an NPN tube, connect the black probe of the multimeter to the collector C of the transistor, the red probe to the emitter E of the transistor, connect a resistor to the collector junction, and then observe the change in resistance of the multimeter. The greater the swing of the multimeter pointer, the stronger the amplification ability of the transistor.

You can also use the hFE/test function of the transistor DC parameter tester to measure the amplification capability. When measuring, first set the hFE/ICEO range of the tester to hFE-100 or hFE-300, select the polarity of the transistor, insert the transistor into the test hole, press the corresponding hFE key, and then read the hFE value from the table.

3. Detection of reverse breakdown voltage The reverse breakdown voltage of the transistor can be measured using the V (BR) test function of the transistor DC parameter test table. When measuring, first select the polarity of the transistor to be tested, then insert the transistor into the test hole, press the corresponding V (BR) key, and then read the reverse breakdown voltage value from the table.

For transistors with a reverse breakdown voltage lower than 50V, the circuit shown in Figure 5-58 can also be used for testing. After connecting the collector C and emitter E of the transistor to be tested VT to the A and B terminals of the test circuit (the E terminal of the PNP tube is connected to point A, and the C terminal is connected to point B; the E terminal of the NPN tube is connected to point B, and the C terminal is connected to point A), adjust the power supply voltage. When the light-emitting diode LED lights up, the voltage value between the A and B terminals is the reverse breakdown voltage of the transistor.

(III) Detection of special transistors
1. Detection of damped row output tubes Use a multimeter with R×1 gear to measure the forward and reverse resistance values ​​of the emitter junction (between base B and emitter E). For normal row output tubes, the forward and reverse resistance values ​​of the emitter junction are both small, only 20~50Ω.
Use a multimeter with R×1k gear to measure the forward and reverse resistance values ​​of the collector junction (between base B and collector C) of the row output tube. Under normal circumstances, the forward resistance value (black test lead connected to base B, red test lead connected to collector C) is 3~10kΩ, and the reverse resistance value is infinite. If the measured forward and reverse resistance values ​​are both 0 or infinite, it means that the collector junction of the tube has been broken down or damaged by an open circuit.