Multimeter to judge the tube type and pins of the triode
A triode 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 NPN type and PNP type transistors with two different conductivity types. Figure 1 shows their circuit symbols and equivalent circuits.
To test the triode, use the ohm block of the multimeter, and select the R×100 or R×1k gear. Figure 2 depicts the equivalent circuit for the ohm block of a multimeter. It can be seen from the figure that the red test lead is connected to the negative pole of the battery in the watch, and the black test lead is connected to the positive pole of the battery in the watch.
Suppose we don't know whether the triode under test is NPN type or PNP type, and we can't tell what electrode each pin is. The first step in the test is to determine which pin is the base. At this time, we randomly take two electrodes (for example, these two electrodes are 1 and 2), use the two test leads of the multimeter to measure its forward and reverse resistance upside down, and observe the deflection angle of the needle; then, take 1 , 3 two electrodes and 2, 3 two electrodes, respectively measure their forward and reverse resistance upside down, and observe the deflection angle of the hands. Among these three upside-down measurements, there must be two measurement results that are similar: that is, in the upside-down measurement, the deflection of the hands is large at one time, and the deflection is small at the other time; The pin is the base we are looking for (see Figure 1 and Figure 2 to understand its reason).
After finding the base of the triode, 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 test lead of the multimeter to the base, and the red test lead to any one of the other two electrodes. If the deflection angle of the pointer on the meter head is large, it means that the triode under test is an NPN tube; if the deflection angle of the pointer on the meter head is small, Then the tested tube is PNP type.
After finding out the base b, which of the other two electrodes is the collector c and which is the emitter e? At this time, the collector c and emitter e can be determined by measuring the penetration current ICEO.
(1) For NPN transistors, the measurement circuit of penetration current is shown in Figure 3.
According to this principle, use the black and red test leads of the multimeter to measure the forward and reverse resistances Rce and Rec between the two poles upside down. Although the deflection angle of the pointer of the multimeter is very small in the two measurements, if you observe carefully, there will always be a deflection The angle is slightly larger. At this time, the flow direction of the current must be: black test lead→c pole→b pole→e pole→red test lead. It must be connected to the collector c, and the red pen must be connected to the emitter e.
(2) For the PNP type triode, the reason is also similar to the NPN type. The current flow direction must be: black test lead→e pole→b pole→c pole→red test lead, and the current flow direction is also consistent with the direction of the arrow in the triode symbol. So at this time, the black test lead must be connected to the emitter e, and the red test lead must be connected to the collector c.
If during the measurement process of "following the arrow, the deflection is large", if the deflection of the two measurement pointers before and after the upside down is too small to be distinguished, it is necessary to "move your mouth". The specific method is: in the two measurements of "following the arrow, the deflection is large", use two hands to pinch the junction of the two test leads and the pins, hold the base electrode b with your mouth (or use your tongue to hold it) The collector c and the emitter e can be distinguished by the judgment method of "following the arrow, large deflection". Among them, the human body plays the role of DC bias resistor, the purpose is to make the effect more obvious.
