Transistor detection method and experience

The resistance range of the multimeter is generally R×100 or R×1K. For PnP transistors, the black meter pen is connected to the e pole and the red meter pen is connected to the c pole. For NPN transistors, the black meter pen is connected to the c pole and the red meter pen is connected to the e pole. The larger the resistance measured, the better. The larger the resistance between e and c, the smaller the ICEO of the tube; conversely, the smaller the measured resistance, the larger the ICEO of the tube being tested.

1 Detection of medium and low power transistors

A For transistors with known models and pin arrangements, their performance can be judged by the following methods

(a) Measure inter-electrode resistance. Set the multimeter to R×100 or R×1K and test according to six different connection methods of the red and black meter pen. Among them, the forward resistance values ​​of the emitter junction and the collector junction are relatively low, and the resistance values ​​measured by the other four connection methods are all very high, ranging from several hundred kilo-ohms to infinity. But no matter it is low resistance or high resistance, the inter-electrode resistance of silicon transistor is much larger than that of germanium transistor.

(b) The value of the transistor's penetration current ICEO is approximately equal to the product of the tube's multiple β and the reverse current ICBO of the collector junction. ICBO increases rapidly with the increase of ambient temperature, and the increase of ICBO will inevitably cause the increase of ICEO. The increase of ICEO will directly affect the stability of the tube's operation, so tubes with small ICEO should be used as much as possible.

By directly measuring the resistance between the e-c pole of the transistor with a multimeter resistor, the size of ICEO can be indirectly estimated. The specific method is as follows: The

multimeter resistance range is generally selected as R×100 or R×1K. For PnP tubes, the black tube is connected to the e pole and the red pen is connected to the c pole. For NPN transistors, the black pen is connected to the c pole and the red pen is connected to the e pole. The larger the measured resistance, the better. The larger the resistance between e and c, the smaller the ICEO of the tube; conversely, the smaller the measured resistance, the larger the ICEO of the tube being tested. Generally speaking, the resistance of medium and low power silicon tubes and low frequency tubes made of germanium materials should be hundreds of kilo-ohms, tens of kilo-ohms and more than ten kilo-ohms respectively. If the resistance is very small or the multimeter pointer shakes back and forth during the test, it means that the ICEO is very large and the performance of the tube is unstable.

(c) Measuring the amplification capacity (β). At present, some models of multimeters have scale lines for measuring the hFE of transistors and their test sockets, which can easily measure the amplification of transistors. First, turn the multimeter function switch to the ? position, turn the range switch to the ADJ position, short-circuit the red and black test leads, adjust the zero adjustment knob, make the multimeter pointer indicate zero, then turn the range switch to the hFE position, and separate the two short-circuited test leads, insert the transistor being tested into the test socket, and read the tube amplification from the hFE scale line.

In addition: For medium and low power transistors of this type, the manufacturer directly marks different color dots on the top of the tube shell to indicate the amplification factor β value of the tube. The corresponding relationship between the color and β value is shown in the table, but it should be noted that the color codes used by different manufacturers are not necessarily exactly the same.

B Detection and discrimination electrode

(a) Determine the base. Use the multimeter R×100 or R×1k block to measure the forward and reverse resistance values ​​between each two electrodes of the three electrodes of the transistor. When the first probe is connected to a certain electrode, and the second probe successively touches the other two electrodes and measures a low resistance value, the electrode connected to the first probe is the base b. At this time, pay attention to the polarity of the multimeter probe. If the red probe is connected to the base b. When the black probe is connected to the other two poles, the measured resistance values ​​are all small, then it can be determined that the transistor being tested is a PnP type tube; if the black probe is connected to the base b, and the red probe touches the other two poles, the measured resistance values ​​are small, then the transistor being tested is an NPN type tube.

(b) Determine the collector c and emitter e. (Take PnP as an example) Set the multimeter to R×100 or R×1K, with the red probe on the base b and the black probe on the other two pins. The two resistance values ​​measured will be one larger and one smaller. In a measurement with a small resistance value, the pin connected to the black probe is the collector; in a measurement with a large resistance value, the pin connected to the black probe is the emitter.

C. Distinguishing high-frequency tubes from low-frequency tubes

The cutoff frequency of high-frequency tubes is greater than 3MHz, while the cutoff frequency of low-frequency tubes is less than 3MHz. Generally, the two cannot be interchanged.

D. In-circuit voltage detection judgment method

In practical applications, small-power transistors are mostly directly soldered on printed circuit boards. Due to the high installation density of components, disassembly is more troublesome. Therefore, during detection, the DC voltage block of the multimeter is often used to measure the voltage value of each pin of the transistor being tested to infer whether it is working normally, and then judge whether it is good or bad.

2. Testing of high-power transistors

The various methods of using a multimeter to test the polarity, tube type and performance of medium and low-power transistors are basically applicable to testing high-power transistors. However, since the working current of high-power transistors is relatively large, the area of ​​their PN junctions is also relatively large. The larger the PN junction, the larger the reverse saturation current. Therefore, if the multimeter's R×1k block is used to measure the inter-electrode resistance of medium and low-power transistors, the measured resistance value will inevitably be very small, as if the inter-electrode short circuit occurs. Therefore, the R×10 or R×1 block is usually used to test high-power transistors.

3. Testing of ordinary Darlington tubes

The testing of ordinary Darlington tubes with a multimeter includes identifying electrodes, distinguishing between PnP and NPN types, and estimating amplification capabilities. Because there are multiple emitter junctions between the E-B poles of the Darlington tube, the R×10K range of the multimeter that can provide a higher voltage should be used for measurement.

4 Detection of high-power Darlington tubes

The method of detecting high-power Darlington tubes is basically the same as that of detecting ordinary Darlington tubes. However, since the high-power Darlington tubes are equipped with protection and leakage current discharge components such as V3, R1, and R2, the impact of these components on the measured data should be distinguished in the detection quantity to avoid misjudgment. Specifically, the following steps can be followed:

A Use the multimeter R×10K range to measure the PN junction resistance between B and C, and it should be clearly measured that it has unidirectional conductivity. There should be a large difference between the forward and reverse resistance values.

B There are two PN junctions between B and E of the high-power Darlington tube, and resistors R1 and R2 are connected. When using the multimeter to detect the resistance block, when measuring in the forward direction, the measured resistance is the result of the parallel connection of the forward resistance of the B-E junction and the resistance of R1 and R2; when measuring in the reverse direction, the emitter junction is cut off, and the measured resistance is the sum of (R1 + R2), which is about several hundred ohms, and the resistance is fixed and does not change with the change of the resistance block position. However, it should be noted that some high-power Darlington tubes also have diodes on R1 and R2. At this time, the measured resistance is not the sum of (R1 + R2), but the parallel resistance of (R1 + R2) and the sum of the forward resistances of the two diodes.

5 Detection of damped output transistors

Set the multimeter to the R×1 position, and judge whether it is normal by measuring the resistance between the electrodes of the damped output transistor separately. The specific test principles, methods and steps are as follows:

A. Connect the red test lead to E and the black test lead to B. This is equivalent to measuring the resistance of the equivalent diode of the B-E junction of the high-power tube and the protection resistor R in parallel. Since the forward resistance of the equivalent diode is small and the resistance of the protection resistor R is generally only 20~50?, the resistance of the two in parallel is also small; conversely, swap the test leads, that is, connect the red test lead to B and the black test lead to E, then what is measured is the reverse resistance value of the equivalent diode of the B-E junction of the high-power tube and the parallel resistance value of the protection resistor R. Since the reverse resistance value of the equivalent diode is large, the resistance measured at this time is the value of the protection resistor R, which is still small.

B Connect the red test lead to C and the black test lead to B. This is equivalent to measuring the forward resistance of the B-C junction equivalent diode of the high-power tube in the tube. Generally, the measured resistance value is also small. Swap the red and black test leads, that is, connect the red test lead to B and the black test lead to C. This is equivalent to measuring the reverse resistance of the B-C junction equivalent diode of the high-power tube in the tube. The measured resistance value is usually infinite.
                                
C Connect the red test lead to E and the black test lead to C, which is equivalent to measuring the reverse resistance of the damping diode in the tube. The measured resistance is generally large, about 300 to ∞; swap the red and black test leads, that is, connect the red test lead to C and the black test lead to E, which is equivalent to measuring the forward resistance of the damping diode in the tube. The measured resistance is generally small, about a few ohms to tens of ohms.