Rata Gemmell ttec 4847: EXPERIMENT No.6 Meter check of a transistor

EXPERIMENT No. 6

Meter check of a transistor

Bipolar transistors are constructed of a three-layer semiconductor “sandwich,” either PNP or

NPN. As such, transistors register as two diodes connected back-to-back when tested with a

multimeter’s “diode check” function as illustrated in Figure 9.1. Low voltage readings on the base with the black negative (-) leads correspond to an N-type base in a PNP transistor. On the symbol, the N-type material corresponds to the “non-pointing” end of the base-emitter junction, the base. The P-type emitter corresponds to “pointing” end of the base emitter junction the emitter.

9.1: PNP transistor meter check: (a) forward B-E, B-C, voltage is low; (b) reverse

B-E, B-C, voltage is OL.

BIPOLAR JUNCTION TRANSISTORS

Here I’m assuming the use of a multimeter has a diode test function to check the PN junctions.

If your meter has a designated “diode check” function, and the meter will display the actual forward voltage of the PN junction and not just whether or not it conducts current.

Meter readings will be exactly opposite, of course, for an NPN transistor, with both PN

junctions facing the other way.

Low voltage readings with the red (+) lead on the base is the “opposite” condition for the NPN transistor. If a multimeter with a “diode check” function is used in this test, it will be found that

the emitter-base junction possesses a slightly HIGHER forward voltage drop than the collector

base junction. This forward voltage difference is due to the disparity in doping concentration

between the emitter and collector regions of the transistor; the emitter is a much more heavily

doped piece of semiconductor material than the collector, causing its junction with the base to

produce a higher forward voltage drop.

Knowing this, it becomes possible to determine which terminal is which on an unmarked transistor.

This is important because transistor packaging, unfortunately, is not standardised. All

bipolar transistors have three terminals, of course, but the positions of the three terminals on the actual physical package are not arranged in any universal, standardised order.

Suppose a technician finds a bipolar transistor and proceeds to measure voltage drop with a

multimeter set in the “diode check” mode. Measuring between pairs of terminals and recording the

values displayed by the meter, the technician obtains the data in Figure 9.2.

Figure 9.2: Unknown bipolar transistor. Which terminals are emitter, base, and collector?

-meter readings between terminals.

The only combinations of test points giving conducting meter readings are terminals 1 and 3

(red test lead on 1 and black test lead on 3), and terminals 2 and 3 (red test lead on 2 and black test lead on 3). These two readings must indicate forward biasing of the emitter-to-base junction

(0.655 volts) and the collector-to-base junction (0.621 volts).

Now we look for the one terminal common to both sets of conductive readings. It must be the

base connection of the transistor, because the base is the only layer of the three-layer device

common to both sets of PN junctions (emitter-base and collector-base). In this example, that

terminal is number 3, being common to both the 1-3 and the 2-3 test point combinations.

METER CHECK OF A TRANSISTOR

Those sets of meter readings, the black (-) meter test lead was touching terminal 3, which tells us

that the base of this transistor is made of N-type semiconductor material (black = negative).

Thus, the transistor is a PNP with base on terminal 3, emitter on terminal 1 and collector on terminal 2 as described in Figure 9.3.

· E and C reverse: 1(+) and 2(-): “OL”

· E and C reverse: 1(-) and 2(+): “OL”

· E and B forward: 1(+) and 3(-): 0.655 V

· E and B forward: 1(-) and 3(+): “OL”

· C and B forward: 2(+) and 3(-): 0.621 V

· C and B forward: 2(-) and 3(+): “OL”

Figure 9.3: BJT terminals identified by meter.

Please note that the base terminal in this example is not the middle lead of the transistor, as one

might expect from the three-layer “sandwich” model of a bipolar transistor. This is quite often

the case, and tends to confuse new students of electronics. The only way to be sure which lead

is which is by a meter check, or by referencing the manufacturer’s “data sheet” documentation

on that particular part number of transistor.

Knowing that a bipolar transistor behaves as two back-to-back diodes when tested with a

Diode test function is helpful for identifying an unknown transistor purely by meter readings.

It is also helpful for a quick functional check of the transistor. If the technician were to measure

Using the Diode test function in any more than two or any less than two of the six test lead combinations, he or she would immediately know that the transistor was defective (or else that it wasn’t a bipolar transistor but rather something else – a distinct possibility if no part numbers can be referenced for sure identification!). However, the “two diode” model of the transistor fails to

explain how or why it acts as an amplifying device.