Make Electronics

Switching Basics and More 81

Experiment 10: Transistor Switching

theory

See the current (continued)

In my little experiment, I found that the maximum cur-
rent at A2 was 33mA. A simple calculation using Ohm’s
Law showed me that this meant the transistor’s internal
resistance was near zero. This is why you should protect a
transistor with some additional resistance in the circuit. If
you don’t, its low internal resistance would allow a huge cur-
rent flow that would immediately burn it out.

What about the other end of its range? When it passes only
1.9 mA, the transistor has an internal resistance of around
6,000Ω. The conclusion is that depending how much cur-
rent you apply to this transistor, its internal resistance varies
between zero and 6,000Ω, approximately.

So much for the theory. Now what can we do with a transis-
tor that’s fun, or useful, or both? We can do Experiment 11!

Figure 2-95. This is basically the

same as the previous circuit,

with a potentiometer added and

the LED removed. Component

values:

R1: 180Ω

R2: 10K

R3: 180Ω

R4: 10K

P1: 1M linear potentiometer

Q1: 2N2222 transistor

A1

A2

9

'C

5

4

5 5

5

3

R1

Q1

12v

DC

R3

R2

R4

Amps

A1

Figure 2-96. The meter is measuring current flowing from the
potentiometer into the base of the transistor at position A1
(see Figure 2-95).

R1

Q1

12v

DC

R3

R2

R4

Amps

A2

Figure 2-97. One end of resistor R3 has been unplugged from

the breadboard so that the meter now measures current

flowing out through the emitter of the transistor, into R3, at

position A2.