Make Electronics

Experiment 10: Transistor Switching

80 Chapter 2

theory

See the current

If you want to get a more precise understanding of how a

transistor works, you should try this little test. It shows the

precise behavior and limits of the 2N2222 transistor that

you used in the previous experiment.

I’ve said that in an NPN transistor, the collector should

always be more positive than the emitter and that the base

should have a potential somewhere between those two

voltages. Figure 2-94 shows this rather vague relationship.

Now I want to substitute some numbers for these general

statements.

More

positive

More

negative

Somewhere

in between

C

B

E

Figure 2-94. The proper functioning of an NPN transistor re-

quires you to maintain these voltage relationships.

Take a look at the schematic in Figure 2-95, and check the

component values. Notice that the total resistance above

the transistor, from R1 + R2, is the same as the total resis-

tance below it, from R3 + R4. Therefore the potential on the

base of the transistor should be halfway between the two

extremes—until you use potentiometer P1 to adjust the

voltage of the base of the transistor up and down.

The two 180Ω resistors, R1 and R3, protect the transistor

from passing excessive current. The two 10K resistors, R2

and R4, protect the base when the potentiometer is turned

all the way up or all the way down.

I would like you to see what the transistor is doing by mea-

suring the amperage flowing into the base at the position

marked A1, and the total amperage flowing out through the

emitter at the position marked A2. To do this, it would be re-

ally helpful if you had two meters. As that may be impracti-

cal, the breadboard diagrams in Figures 2-96 and 2-97 show

how you can swap one meter between the two locations.

Remember that to measure milliamps, you have to pass

electricity through the meter. This means that the meter

must be inserted into the circuit, and whenever you remove

the meter, you have to remake the connection where the

meter was. The breadboard diagram shows how you can do

this. Fortunately, it’s very easy to remove and replace wires

in a breadboard. Where wires are connected to the potenti-

ometer, you may need to revert to using alligator clips.

Begin with the potentiometer turned about halfway

through its range. Measure at A1 and A2. Turn the potenti-

ometer up a bit, and measure current at the two locations

again. Following is a table showing some actual readings I

obtained at those two locations, using two digital meters

simultaneously.

Milliamps passing

through location A1

Milliamps passing

through location A2

0.01 1.9

0.02 4.9

0.03 7.1

0.04 9.9

0.05 12.9

0.06 15.5

0.07 17.9

0.08 19.8

0.09 22.1

0.10 24.9

0.11 26.0

0.12 28.3

There’s a very obvious relationship. The current emerging

from the emitter of the transistor, through location A2, is

about 24 times the current passing through location A1,

into the base. The ratio of current coming out from the

emitter of an NPN transistor to current going into the base

is known as the beta value for a transistor. The beta value

expresses the transistor’s amplifying power.

It’s a very constant ratio, until you push it a little too far. Above

0.12 mA, this particular transistor becomes “saturated,” mean-

ing that its internal resistance cannot go any lower.