How to detect high power transistors

The various methods of using a multimeter to detect the polarity, tube type and performance of medium and low power transistors are basically applicable to the detection of high power transistors. However, since the working current of high power transistors is relatively large, the area of ​​their PN junction is also large. The larger the PN junction, the greater 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 detect high power transistors.

(1) Testing of ordinary Darlington tubes

The detection of ordinary Darlington tubes with a multimeter includes identifying electrodes, distinguishing between PNP and NPN types, estimating amplification capabilities, etc. Because the Darlington tube contains multiple emitter junctions between the E-B poles, the multimeter should be used to provide a higher voltage R×10K block for measurement.

(2) Detection of high-power Darlington tubes

The method of testing high-power Darlington tubes is basically the same as that of testing ordinary Darlington tubes. However, since 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 during the test to avoid misjudgment. The specific steps can be as follows:

1) Use the multimeter R×10K to measure the PN junction resistance between B and C. It should be obvious that it has unidirectional conductivity. There should be a large difference between the forward and reverse resistance values.

2) There are two PN junctions between B and E in the high-power Darlington tube, and resistors R1 and R2 are connected. When using the multimeter resistance block for detection, when measuring in the forward direction, the measured resistance is the result of the forward resistance of the B-E junction in parallel with the resistance values ​​of R1 and R2; when measuring in the reverse direction, the emitter junction is cut off, and the resistance measured is the sum of the resistances (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.

(3) Detection of damped output transistors

Set the multimeter to R×1 and measure the resistance between the electrodes of the damped output transistor to determine whether it is normal. The specific test principle, method and steps are as follows:

1) 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 20~50Ω, 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. What is measured is the parallel resistance of the reverse resistance of the equivalent diode of the B-E junction of the high-power tube and the protection resistor R. Since the reverse resistance of the equivalent diode is large, the resistance measured at this time is the value of the protection resistor R, which is still small.

2) Connect the red test lead to C and the black test lead to B. This is equivalent to measuring the forward resistance of the equivalent diode of the B-C junction of the high-power tube in the tube. The measured resistance is generally 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 equivalent diode of the B-C junction of the high-power tube in the tube. The measured resistance is usually infinite.

3) 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~∞; swap the red and black test leads, that is, connect the red test lead to D 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.