The Principle of the Transistor and the Method of Using a Multimeter to Measure and Determine the Pin and the Model of the Transistor
Step 1: Determine the base and the tube type (NPN or PNP)
As shown in the figure above, the base of a PNP tube is the common point of the two negative poles, and the base of an NPN tube is the common point of the two positive poles. At this time, we can use the diode file of the digital multimeter to measure the base. For a PNP tube, when the black test lead (connected to the negative pole of the internal battery of the meter) is on the base, and the red test lead is used to measure the other two poles, the readings are generally small and not much different (usually between 0.5 and 0.8). If the test leads are reversed, the reading will be a relatively large value (usually 1). For an NPN tube, the red test lead (connected to the positive pole of the internal battery of the meter) is connected to the base.
Step 2: Judge the emitter and the collector
If a pointer multimeter is used at this step, two hands may be needed, and some friends may even use their mouth and tongue, which is quite troublesome. It is much more convenient to use the hfe file (used to measure the DC current gain of the transistor) of the digital multimeter for measurement. Set the multimeter to the hfe file, insert the transistor into the NPN hole, with the b pole corresponding to the b letter on the multimeter, and take the reading. Then reverse the other two pins and take the reading again. The polarity corresponding to the letter on the multimeter is the one with the larger reading, and the same applies to other transistors!
II. The method of using a digital multimeter to detect the quality of a transistor is as follows:
Find the base: Set the digital multimeter to the diode file. Connect the red test lead to any pin, and use the black test lead to successively touch the other 2 pins. If the values displayed twice are both less than 1V or both display the overflow symbol 1, the pin connected by the red test lead is the base b. If, during the two tests, one display value is less than 1V and the other displays the overflow symbol 1, it indicates that the pin connected by the red test lead is not the base. Then, use other pins to measure again to find the base.
Determine the tube type: Set the digital multimeter to the diode file. Connect the red test lead to the base, and use the black test lead to successively touch the other 2 pins. If both display values are between 0.5V and 0.8V, the measured tube belongs to the NPN type. If both display the overflow symbol 1 twice, it indicates that the measured tube belongs to the PNP type.
Distinguish the collector C and the emitter e: Take the NPN tube as an example. Set the digital multimeter to the HFE file and use the PNP jack. Insert the base B into the B hole, and insert the remaining 2 pins into the C hole and the E hole respectively. If the measured HFE value is between dozens and hundreds, it means that the tube is connected correctly and has strong amplification ability. At this time, the pin inserted into the C hole is the collector C, and the pin inserted into the E hole is the emitter E. If the measured HFE value is only a few or a dozen, it indicates that the collector c and the emitter e of the measured tube are inserted reversely. At this time, the pin in the C hole is the emitter e, and the pin in the E hole is the collector c. To make the test results more reliable, fix the base b inserted in the B hole, exchange the collector c and the emitter e and repeat the test twice. Take the test result with the larger display value as the standard. The pin inserted into the C hole is the collector c, and the pin inserted into the E hole is the emitter e.
Test the quality: Still take the NPN type as an example. Open the base b and measure the resistance between the c and e poles. Connect the red test lead of the multimeter to the emitter and the black test lead to the collector. If the resistance value is above tens of thousands of ohms, it indicates that the penetration current is small and the tube can work normally. If the resistance between the c and e poles is small, the tube works unstably and cannot be used in circuits with high technical index requirements. If the measured resistance value is approximately 0, the tube has been broken down. If the resistance value is infinite, it indicates that the inside of the tube has been open-circuited.
