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 potentiometer, 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 potentiometer 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 240 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,” meaning that its internal resistance cannot go any lower.
Theory
See the current (continued)
In my little experiment, I found that the maximum current 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 current 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 current 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 transistor 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
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).
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.
Experiment 11: A Modular Project
You will need:
AC adapter, breadboard, wire, and meter.
LED. Quantity: 1.
Resistors, various.
Capacitors, various.
Transistor, 2N2222 or similar. Quantity: 2.
2N6027 programmable unijunction transistor (PUT). Quantity: 2.
Miniature 8Ω loudspeaker. Quantity: 1.
So far, I’ve described small circuits that perform very simple functions. Now it’s time to show how modules can be combined to create a device that does a bit more.
The end product of this experiment will be a circuit that makes a noise like a small siren, which could be used in an intrusion alarm. You may or may not be interested in owning an alarm, but the four-step process of developing it is important, because it shows how individual clusters of components can be persuaded to communicate with each other.
I’ll begin by showing how to use a transistor to make a solid-state version of the oscillating circuit that you built with a relay in Experiment 8. The relay, you may remember, was wired in such a way that the coil received power through the contacts of the relay. As soon as the coil was energized, it opened the contacts, thus cutting off its own power. As soon as the contacts relaxed they restored the power, and the process repeated itself.
There’s no way to do this with a single bipolar transistor. You actually need two of them, switching each other on and off, and the way that this works is quite hard to understand. An easier option is to use a different thing known as a programmable unijunction transistor, or PUT.
Unijunction transistors were developed during the 1950s, but fell into disuse when simple silicon chips acquired the ability to perform the same kinds of functions, more accurately and more cheaply. However, the so-called programmable unijunction transistor is still widely available, often used in applications such as lamp dimmers and motor controllers. Because its primary use is in generating a stream of pulses, it’s ideal for our purposes.
If you put together the components shown in Figure 2-98, the LED should start flashing as soon as you apply power.
Figure 2-98. Assemble these components, apply power, and the LED should start flashing.
R1: 470K
R2: 15K
R3: 27K
C1: 2.2 μF electrolytic capacitor
D1: LED
Q1: 2N6027 programmable unijunction transistor
Note that this circuit will work on 6 volts. You won’t damage anything if you run it with 12 volts, but as we continue adding pieces to it, you’ll find that it actually performs better at 6 volts than at 12. If you read the next section, “Essentials: All about programmable unijunction transistors,” you’ll find out how the circuit works.
Essentials
All about programmable unijunction transistors
The schematic symbol for a programmable unijunction transistor, or PUT, looks very different from the symbol for a bipolar transistor, and its parts are named differently, too. Nevertheless, it does have a similar function as a solid-state switch. The symbol and the names of the three connections are shown in Figure 2-99.
Note that this is a rare case (maybe the only one in the whole of electronics!) in which you won’t run into confusing variations of the basic schematic symbol.
