How to use a multimeter to determine the base and type of a transistor

How to use a multimeter to determine the base and type of a transistor

For a transistor with unclear markings, you can use a multimeter to determine its polarity, determine whether it is a silicon tube or a germanium tube, and distinguish its pins at the same time. For general low-power tubes, it is generally appropriate to use the Rx1K gear when judging. The steps are as follows:
1. Positive and negative measurement Use the red and black test leads to measure the resistance of any two pins of the transistor, and then swap the red and black test leads to measure the resistance of these two pins. If the resistance readings of the two measurements are different, we call the measurement with the smaller resistance reading the positive measurement, and we call the measurement with the larger resistance reading the negative measurement.
2. Determine the base Number the three pins of the transistor 1.2.3. The multimeter makes three measurements, namely 1-2, 2-3, and 3-1, each of which is divided into positive measurement and negative measurement. Among these six measurements, three are positive measurements, and the resistance readings are different. Find the pin with the largest positive resistance, such as 1-2, and the other pin 3 is the base. This is because no matter whether it is a tube or a tube, it is made of two diodes connected in reverse (as shown in the attached figure). The forward resistance between the emitter, collector and base is the forward resistance of a general diode, which is very small. When the two test leads are connected to the collector and emitter, the resistance is much greater than the forward resistance of a general diode.
3. Determine the polarity. Connect the black test lead to the determined base and the red test lead to any other pole. If it is a positive measurement, it is an NPN tube, and if it is a reverse measurement, it is a PNP tube. This is because the black test lead is connected to the positive terminal of the battery in the multimeter. If it is a positive measurement, the black test lead is connected to the P terminal, and the transistor is an NPN type. If it is a reverse measurement, the black test lead is connected to the N terminal, and the transistor is a PNP type.
4. Determine the collector and emitter. Perform a positive measurement on the collector and emitter. When measuring positively, the black test lead is connected to the collector of the NPN tube, and the black test lead is connected to the emitter of the PNP tube. This is because no matter whether it is a positive measurement or a reverse measurement, there is a PN junction in the reverse direction, and most of the battery voltage falls on the reverse PN junction. When the emitter junction is forward biased and the collector junction is reverse biased, the current flowing through is large and the resistance presented is small. So for NPN tube, when the collector-emitter resistance is small, the collector is connected to the positive pole of the battery, that is, the black test lead is connected. For PNP tube, when the collector-emitter resistance is small, the emitter is connected to the black test lead.
5. To distinguish whether it is a silicon tube or a germanium tube, make a positive measurement on the emitter base. If the pointer deflects 1/2-3/5, it is a silicon tube. If the pointer deflects more than 4/5, it is a germanium tube. This is because when the resistor makes a positive measurement on the base-emitter, the voltage applied between the base and the emitter is Ube=(1-n/N)E, E=1.5v is the battery voltage, N is the total number of grids of a DC voltage with a linear scale, and n is the number of grids where the needle deflects on the scale line. Usually U=0.6~0.7v for silicon tubes and Ube=0.2~0.3v for germanium tubes. Therefore, when testing, for silicon tubes, n/N is about 1/2-3/5; for germanium tubes, n/N is about 4/5 or more. In addition, for the judgment of general low power, the multimeter should not use Rx10 or Rx1 gear. Take the 500-type multimeter to measure silicon tubes as an example. The internal resistance of the meter is 100 ohms in the Rx10 gear. When measuring the be pole of the silicon tube in the positive direction, the current reaches Ibe=(1.5v-0.7v)/100 ohms=8mA. When measuring germanium tubes, the current is even larger. When using the Rx1 gear, the current is even larger, which may damage the transistor. As for the Rx1k gear, the battery voltage in this gear is relatively high, and the common ones are 1v, 12v, 15v, 22.5v, etc. When measuring in the reverse direction, it may cause PN junction breakdown, so this gear should also be used with caution.

Tips for measuring and judging transistors.
Judging the tube type and pins of transistors is a basic skill for beginners of electronic technology. In order to help readers quickly master the measurement and judgment methods, the author summarizes four tips: "Three inversions, find the base; PN junction, determine the tube type; follow the arrow, large deflection; if you can't measure accurately, move your mouth." Let us explain them sentence by sentence.

1. Reverse the three and find the base

As we all know, a transistor is a semiconductor device containing two PN junctions. According to the different connection methods of the two PN junctions, transistors of two different conductivity types can be divided into NPN type and PNP type. Figure 1 shows their circuit symbols and equivalent circuits.

To test the transistor, use the ohmmeter of the multimeter and select the R×100 or R×1k position. Figure 2 shows the equivalent circuit of the ohmmeter of the multimeter. As can be seen from the figure, the red test lead is connected to the negative pole of the battery in the meter, and the black test lead is connected to the positive pole of the battery in the meter.

Assume that we do not know whether the transistor under test is NPN or PNP, and we cannot tell which electrode each pin is. The first step of the test is to determine which pin is the base. At this time, we randomly select two electrodes (such as electrodes 1 and 2), use the two probes of the multimeter to measure its forward and reverse resistance in reverse order, and observe the deflection angle of the needle; then, take electrodes 1 and 3 and electrodes 2 and 3, respectively, and measure their forward and reverse resistance in reverse order, and observe the deflection angle of the needle. In these three reverse measurements, there must be two similar measurement results: that is, the needle deflects a lot in one reverse measurement and a little in another; the remaining one must be that the deflection angle of the pointer is very small before and after the reverse measurement. The pin that is not measured this time is the base we are looking for (refer to Figures 1 and 2 for easy understanding of the reason).

 2. PN junction, fixed tube type

After finding the base of the transistor, we can determine the conductivity type of the tube according to the direction of the PN junction between the base and the other two electrodes (Figure 1). Touch the black probe of the multimeter to the base and the red probe to any of the other two electrodes. If the meter pointer deflects a large angle, it means that the transistor being tested is an NPN type tube; if the meter pointer deflects a small angle, the tube being tested is a PNP type.

3. Follow the arrow, large deflection

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

(1) For NPN transistors, the measurement circuit of the penetration current is shown in Figure 3. According to this principle, the black and red test leads of a multimeter are used to measure the forward and reverse resistances Rce and Rec between the two poles. Although the deflection angles of the multimeter pointer are very small in both measurements, careful observation will reveal that there will always be a slightly larger deflection angle. At this time, the current flow direction must be: black test lead → pole c → pole b → pole e → red test lead. The current flow direction is exactly the same as the arrow direction in the transistor symbol ("follow the arrow"), so the black test lead must be connected to the collector c, and the red test lead must be connected to the emitter e.

(2) For PNP transistors, the principle is similar to that of NPN transistors. The current flow direction must be: black test lead → e pole → b pole → c pole → red test lead. The current flow direction is also consistent with the direction of the arrow in the transistor symbol. Therefore, the black test lead must be connected to the emitter e, and the red test lead must be connected to the collector c (see Figures 1 and 3).

4. If you can’t measure it, use your mouth

If the deflection of the pointer is too small to distinguish in the two measurements before and after the reversal, you need to "use your mouth". The specific method is: in the two measurements of "follow the arrow, large deflection", pinch the joints of the two test leads and the pins with both hands, hold the base electrode b with your mouth (or press it with your tongue), and still use the judgment method of "follow the arrow, large deflection" to distinguish the collector c and the emitter e. The human body plays the role of a DC bias resistor, the purpose is to make the effect more obvious.

Judging from the pin position of the transistor, there are two packaging arrangements for the pin position of the transistor, as shown in the right figure:

The transistor is a junction resistor device. Its three pins have obvious resistance data. When testing (taking a digital multimeter as an example, red pen +, black pen -), we switch the test gear to the diode gear (buzzer gear). The symbol is as shown on the right:

The forward resistance of the base (B) to the collector (C) and emitter (E) of a normal NPN structure transistor is 430Ω-680Ω (this value varies depending on the model and the difference in magnification), and the reverse resistance is infinite; the reverse resistance of the base (B) to the collector (C) and emitter (E) of a normal PNP structure transistor is 430Ω-680Ω, and the forward resistance is infinite. The resistance of collector C to emitter E is infinite without bias current. The test resistance of base to collector is approximately equal to the test resistance of base to emitter. Under normal circumstances, the test resistance of base to collector is about 5-100Ω smaller than the test resistance of base to emitter (more obvious for high-power tubes). If this value is exceeded, the performance of this component has deteriorated, please do not use it again. If it is used in the circuit by mistake, the operating point of the entire or part of the circuit may deteriorate, and this component may also be damaged soon. High-power circuits and high-frequency circuits react more obviously to such inferior components.