Make Electronics

Switching Basics and More 77

Experiment 10: Transistor Switching

essentIAls

All about NPN and PNP transistors (continued)

All-transistor basics

• Never apply a power supply directly across a transistor.
  You can burn it out with too much current.


• Protect a transistor with a resistor, in the same way you
  would protect an LED.


• Avoid reversing the connection of a transistor between
  positive and negative voltages.


• Sometimes an NPN transistor is more convenient in a
  circuit; sometimes a PNP happens to fit more easily.
  They both function as switches and amplifiers, the only
  difference being that you apply a relatively positive
  voltage to the base of an NPN transistor, and a rela-
  tively negative voltage to the base of a PNP transistor.


• PNP transistors are used relatively seldom, mainly
  because they were more difficult to manufacture in the
  early days of semiconductors. People got into the habit
  of designing circuits around NPN transistors.


• Remember that bipolar transistors amplify current, not
  voltage. A small fluctuation of current through the base
  enables a large change in current between emitter and
  collector.
  
  
  • Schematics sometimes show transistors with circles
    around them, and sometimes don’t. In this book, I’ll use
    circles to draw attention to them. See Figures 2-91 and
    2-92.
  
  
  • Schematics may show the emitter at the top and the
    collector at the bottom, or vice versa. The base may be
    on the left, or on the right, depending on what was most
    convenient for the person drawing the schematic. Be
    careful to look carefully at the arrow in the transistor to
    see which way up it is, and whether it is NPN or PNP. You
    can damage a transistor by connecting it incorrectly.
  
  
  • Transistors come in various different sizes and con-
    figurations. In many of them, there is no way to tell
    which wires connect to the emitter, the collector, or the
    base, and some transistors have no part numbers on
    them. Before you throw away the packaging that came
    with a transistor, check to see whether it identifies the
    terminals.
  
  
  • If you forget which wire is which, some multimeters
    have a function that will identify emitter, collector, and
    base for you. Check your multimeter instruction book-
    let for more details.
  
  
  
  

C

E

B

Figure 2-91. The symbol for an NPN transistor always has an ar-
row pointing from its base to its emitter. Some people include
a circle around the transistor; others don’t bother. The style of
the arrow may vary. But the meaning is always the same. The
top-left version is the one I use in this book.

C

E

B

Figure 2-92. The symbol for a PNP transistor always has an ar-

row pointing from its emitter to its base. Some people include

a circle around the transistor; others don’t bother. The style of

the arrow may vary. But the meaning is always the same. The

top-left version is the one I use in this book.