How to use a multimeter to judge the performance of medium and small power transistors

To know the performance of transistors and quantitatively analyze their parameters, you need a special measuring instrument, such as a JT-1 transistor characteristic plotter. When there is no special measuring instrument, a multimeter can be used to roughly judge the quality of transistors. The following introduces the method of using a multimeter to judge the performance of medium and low power (below 1W) transistors.

(1) Testing the performance of transistors

1) Measurement of transistor inter-electrode resistance By measuring the inter-electrode resistance of the transistor, it is possible to determine whether the tube is short-circuited or open-circuited. The method is: use the Rx1k or Rx1O0 block of the multimeter to measure the forward resistance and reverse resistance between the base and collector, and the forward resistance and reverse resistance between the base and emitter of the tube.

For normal medium and low power transistors, the forward resistance is several hundred ohms to several thousand ohms, and the reverse resistance is several hundred kiloohms or more. Regardless of the forward resistance or reverse resistance, the inter-electrode resistance of silicon transistors is higher than that of germanium transistors.

When the measured forward resistance is close to infinity, it indicates that the tube is broken. If the measured reverse resistance is very small or zero, it means that the tube has been broken down or short-circuited. When testing low-power transistors, the multimeter's Rx100 or Rx1k block should be used. Never use the Rx1 or Rx1Ok block, because the former has a large current and the latter has a high voltage, both of which may cause damage to the transistor.

Table 1 lists the measured forward and reverse resistance values ​​between electrodes of four commonly used transistors for reference during measurement.

Table 1 Commonly used transistor inter-electrode forward and reverse resistance

2) Measurement of transistor penetration current When testing PNP tubes, the red test lead is connected to the collector, the black test lead is connected to the emitter, and the multimeter range is Rx1k. The resistance value measured should be above 50kΩ. The larger the resistance value, the smaller the penetration current of the tube, and the better the performance of the tube. If the resistance value is less than 25kΩ, it means that the penetration current of the tube is large, the operation is unstable and there is a lot of noise, so it is not suitable for selection. When testing NPN tubes, the test leads should be adjusted to measure the resistance value between the collector and the emitter. The resistance value should be much larger than that of PNP tubes, generally more than several hundred kilo-ohms.

3) Estimation of current amplification factor When measuring PNP transistors, set the multimeter to Rx1k or Rx100, connect the red test lead to the collector and the black test lead to the emitter, measure the resistance and make a record, then connect the 100kΩ resistor to the circuit, as shown in Figure 1. The resistance after connecting the resistor should be smaller than when not connected to the resistor, that is, the swing of the needle becomes larger, the larger the swing, the better the amplification ability of the tube, if the needle remains in place after connecting the resistor, it indicates that the amplification ability of the tube is very poor.

Figure 1 Estimation of transistor β value

(a) Measurement principle; (b) Measurement method

When measuring NPN transistors, the method is exactly the same as that for PNP, except that the red and black test leads are swapped.

(2) The pins of the transistor can be determined by measuring the inter-electrode resistance using the ohmmeter of a multimeter.

1) Determination of the base: Set the multimeter to the Rx1k position, connect the black test lead to any one of the transistors, and then use the red test lead to touch the other two electrodes to measure the forward and reverse resistances, until both resistances are very large. (During the measurement process, if one resistance is very large and the other is very small, you need to change the black test lead to another electrode and measure again). At this time, the electrode connected to the black test lead is the base b of the transistor, and it is a PNP type tube. When the two resistances measured are very small, the black test lead is connected to the base, and it is an NPN type tube.

2) Identification of collector and emitter For PNP and NPN tubes made of germanium, the base b of the tube can be determined by the above method, and then the multimeter is set to Rx1lk, and then the resistance of the remaining two electrodes is measured. The test leads are swapped and measured once. In the measurement with the smaller resistance, the red test lead of the PNP tube is connected to the collector, and the black test lead is connected to the emitter. For the NPN tube, the red test lead is connected to the emitter, and the black test lead is connected to the collector. As shown in Figure 2.

Figure 2 Identification of Germanium Transistor Pins

For NPN silicon tube, a 100kΩ resistor can be connected between the base and the collector. Use the same method as above to measure the resistance between the two electrodes except the base. The one with smaller resistance is connected to the collector by the black test lead and the emitter by the red test lead, as shown in Figure 3.

Figure 3 Identification of NPN silicon transistor pins

(3) Determine whether the transistor is a silicon tube or a germanium tube. The forward voltage drop of the silicon tube is larger than that of the germanium tube. The method is shown in Figure 4. Connect a 1.5V battery and a 10kΩ resistor to the base and emitter circuits, and then use the 2.5V range of the multimeter to measure the forward voltage drop of the emitter. If it is 0.2-0.3V, it is a germanium tube, and if it is 0.6-0.8, it is a silicon tube.

Figure 4: Identification of silicon tube and germanium tube iron