Detection of medium and small power transistors

Detection of medium and small power transistors

A. The performance of a transistor with known model and pin arrangement can be judged by the following method

(a) Measure the interpole resistance. Place the multimeter in R × 100 or R × 1k gear, test according to six different connections of red and black pens. Among them, the forward resistance values of the emission junction and the collector junction are relatively low, while the resistance values measured by the other four connection methods are all very high, ranging from several hundred kiloohms to infinity. But whether it is low resistance or high resistance, the inter electrode resistance of silicon based transistors is much higher than that of germanium based transistors.

(b) The value of the penetration current ICEO of the transistor is approximately equal to a multiple of the transistor β The product of the reverse current ICBO of the collector junction. ICBO grows rapidly with the increase of environmental temperature, and the increase in ICBO inevitably leads to an increase in ICEO. The increase in ICEO will directly affect the stability of pipe operation, so pipes with small ICEO should be selected as much as possible in use.

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

The resistance range of a multimeter is generally selected as R × 100 or R × 1k gear, for PNP s, the black meter  is connected to the e-pole, and the red meter pen is connected to the c-pole. For NPN type triodes, the black meter pen is connected to the c-pole, and the red meter pen is connected to the e-pole. The greater the resistance required to be measured, the better. The larger the resistance value between e-c, the smaller the ICEO of the pipe; On the contrary, the smaller the measured resistance value, the greater the ICEO of the tested . Generally speaking, the resistance values of medium and small power silicon s and germanium low-frequency s should be in the hundreds, tens, and tens of thousands of ohms, respectively. If the resistance value is very small or the multimeter pointer shakes back and forth during testing, it indicates that the ICEO is large and the performance of the  is unstable.

(c) Measurement amplification capability（ β)。 At present, some models of multimeter have scale lines and test sockets for measuring the hFE of the transistor, which can easily measure the amplification of the transistor. First, turn the function switch of the multimeter to the gear position, and then turn the range switch to the ADJ position. Short circuit the red and black probes, adjust the zero adjustment knob to make the multimeter pointer indicate zero. Then, turn the range switch to the hFE position and separate the two short connected probes. Insert the tested transistor into the test socket to read the magnification of the  from the hFE scale line.

B. Detection and discrimination electrode

(a) Determine the base. Using a multimeter R × 100 or R × Measure the forward and reverse resistance values between each of the three electrodes of a crystal transistor at 1k gear. When the first probe is connected to an electrode, and the second probe contacts the other two electrodes in sequence, both of which measure low resistance, the electrode to which the first probe is connected is the base electrode b. At this point, pay attention to the polarity of the multimeter probe, if the red probe is connected to the base b. When the black probes are connected to the other two poles and the measured resistance values are small, it can be determined that the tested transistor is a PNP type transistor; If the black probe is connected to the base electrode b, and the red probe contacts the other two electrodes respectively, and the measured resistance value is small, then the tested transistor is an NPN type transistor.

(b) Determine the collector c and emitter e. (Using PNP as an example) Place the multimeter in R × 100 or R × At 1k gear, with the red probe base b and the black probe touching the other two pins separately, the two measured resistance values will be larger and smaller. In a measurement with a low resistance value, the connecting pin of the black probe is the collector electrode; In a measurement with a high resistance value, the pin taken over by the black probe is the emitter.

C. Distinguishing between high-frequency and low-frequency s

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

D. On line voltage detection and judgment method

In practical applications, small power transistors are mostly directly welded to the printed circuit board. Because of the high installation density of components and the troublesome disassembly, it is often used to measure the voltage value of each pin of the tested triode by using the DC voltage block of a multimeter to infer whether its work is normal and then judge whether it is good or bad.