Page 430 - Robot Builder's Bonanza

TRANSISTORS 399

Transistors

Transistors (see Figure 31- 17) were designed as an alternative to the old vacuum tube. They’re
used for many of the same things, either to amplify a signal or to switch a signal on and off.
At last count there were several thousand different types of transistors available.
Transistors are divided into two broad categories: signal and power. You can usually tell the
difference between the two merely by size.
• Signal. These transistors are used with relatively low current circuits, like radios, tele-
phones, and some hobby electronics projects. They are available in either plastic or
retro- looking metal cases. The plastic kind is suitable for most uses, but some precision
applications require the metal variety. No projects in this book require great precision, so
the cheap plastic signal transistor is good enough.
• Power. These transistors are used with high- current circuits, like motor drivers and power
supplies. Power transistors come in metal cases, though a portion of the case (the back or
sides) may be made of plastic.
Figure 31- 18 shows several common varieties of transistor cases and how they are referred
to, such as TO- 92 and TO- 220. The signal transistor is rarely larger than a pea and uses
slender wire leads. The power transistor has a large metal case, to help dissipate heat, and
heavy, spokelike connection leads.

HOW TO IDENTIFY A TRANSISTOR
Transistors are identified by a unique code, such as 2N2222 or MPS6519. Refer to a data
book to ascertain the characteristics and ratings of the particular transistor you are interested

TO-92 TO-5 or TO-18

Signal

2N2222
Power

TO-3 TO-220
Small signal Transistor Figure 31- 18 There are many shapes and sizes of
transistor schematic symbol
transistors, but a common trait of most of them is that
(NPN shown here)
they have three connection wires. Larger transistors are
Figure 31- 17 Component outline and generally used for applications involving high currents,
schematic symbols for a transistor. The outline such as powering motors. Shown are four popular
view is of a small signal transistor. transistor cases.

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