Detailed explanation of measurement methods for transistors

Firstly, connect the multimeter to the test diode terminal. Use the red probe of the multimeter to touch one of the pins of the transistor, and use the other probe of the multimeter to test the remaining pins until the following result is obtained:

1. If the black probe of a transistor is connected to one of its pins, and the other two pins are both conducting with a voltage display when measured with a red probe, then this transistor is a PNP transistor, and the pin connected to the black probe is the base B of the transistor. When tested using the above method, if the voltage of the red probe of a multimeter connected to one of its pins is slightly higher, then this pin is the emitter E of the transistor, and the remaining pin with a lower voltage is the collector C.

2. If the red probe of a transistor is connected to one of its pins and the other two pins are both conducting with a voltage display when measured with a black probe, then this transistor is an NPN transistor, and the pin connected to the red probe is the base B of the transistor. When tested using the above method, if the voltage of the black probe of a multimeter connected to one of its pins is slightly higher, then this pin is the emitter E of the transistor, and the remaining pin with a lower voltage is the collector C.

Another method is to use the hFE gear for judgment. After determining the base and transistor type of the transistor, insert the base of the transistor into the Lu value measurement hole according to the position of the base and the transistor type, and insert the other two pins into any two of the remaining three measurement holes. Observe the size of the data on the display screen, find the collector and emitter of the transistor, exchange positions, and then measure again. Observe the value on the display screen, repeat the measurement four times, and compare and observe. The maximum value measured is the current amplification factor of the transistor, which corresponds to the collector and emitter electrodes of the transistor.

The discrimination of transistor type and pins is a basic skill for beginners in electronic technology. In order to help readers quickly master the measurement and judgment methods, the author summarizes four mnemonics: "Reverse three, find the base; PN junction, determine the transistor type; Following the arrow, there is a large deviation; I'm not sure, move your mouth Let's explain sentence by sentence below.

1、 Reverse three, find the base

As we all know, a transistor is a semiconductor device that contains two PN junctions. According to the different connection methods of the two PN junctions, they can be divided into two different conductivity types of transistors: NPN type and PNP type.

To test the transistor, use the ohm range of a multimeter and select the R × 100 or R × 1k range. The red probe is connected to the negative terminal of the battery inside the meter, while the black probe is connected to the positive terminal of the battery inside the meter.

Assuming we do not know whether the transistor being tested is NPN or PNP type, and cannot distinguish which electrode each pin is. The first step of testing is to determine which pin is the base. At this point, we take two electrodes and measure their forward and reverse resistance by reversing the two probes of a multimeter, observing the deflection angle of the meter pins; Next, take electrodes 1 and 3, as well as electrodes 2 and 3, and measure their forward and reverse resistance by reversing them respectively. Observe the deflection angle of the pointer. In these three inverted measurements, there must be two measurement results that are similar: that is, in the inverted measurement, the pointer deviates greatly in one measurement and slightly in the other; The remaining measurement must be reversed, and the deflection angle of the pointer before and after the measurement is very small. The foot that was not measured this time is the base we are looking for.

II PN junction, fixed diode type

After identifying the base of the transistor, we can determine the conductivity type of the transistor based on the direction of the PN junction between the base and the other two electrodes. Connect the black probe of the multimeter to the base and the red probe to either of the other two electrodes. If the pointer of the multimeter head deviates greatly, it indicates that the tested transistor is an NPN transistor; If the deflection angle of the meter pointer is very small, the tested diode is PNP type.

3、 Following the arrow, there is a large deviation

Found the base b, which of the other two electrodes is the collector c and which is the emitter e? At this point, we can use the method of measuring the penetration current ICEO to determine the collector c and emitter e.

(1) For NPN type transistors, according to this principle, the positive and negative resistances Rce and Rec between the two poles are measured by reversing the black and red probes of a multimeter. Although the deflection angle of the multimeter pointer is small in both measurements, careful observation shows that there will always be one deflection angle that is slightly larger. At this time, the direction of the current must be: black probe → c pole → b pole → e pole → red probe, and the current direction is exactly the same as the arrow direction in the transistor symbol ("forward arrow"). Therefore, at this time, the black probe must be connected to the collector c, and the red probe must be connected to the emitter e.

(2) For PNP type transistors, the principle is similar to NPN type. The current flow must be: black probe → e pole → b pole → c pole → red probe. The current flow direction is also consistent with the arrow direction in the transistor symbol. Therefore, at this time, the black probe must be connected to the emitter e, and the red probe must be connected to the collector c.

4、 Cannot detect, move your mouth

If during the measurement process of "following the arrow, with a large deviation", it is difficult to distinguish due to the small deviation of the measuring pointer before and after reversal, then it is necessary to "move the mouth". The specific method is to use two hands to pinch the junction between the two probes and the pins in the two measurements of "forward arrow, large deflection", and use the mouth to hold (or use the tongue to hold) the base electrode b. The discrimination method of "forward arrow, large deflection" can still be used to distinguish between the collector electrode c and the emitter electrode e. The human body acts as a DC bias resistor to make the effect more pronounced.