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Complete Electronics Self-Teaching Guide with Projects by Harry Kybett, Earl Boyson

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Why Transistors Are Used as Switches

15 You can use the transistor as a switch (as you saw in the previous problems) to perform simple operations such as turning a lamp current on and off. Although often used between a mechanical switch and a lamp, there are other uses for the transistor.

Following are a few other examples that demonstrate the advantages of using a transistor in a circuit as a switch:

  • Example 1—Suppose you must put a lamp in a dangerous environment, such as a radioactive chamber. Obviously, the switch to operate the lamp must be placed somewhere safe. You can simply use a switch outside the chamber to turn the transistor switch ON or OFF.
  • Example 2—If a switch controls equipment that requires large amounts of current, then that current must flow through the wires that run between the switch and the lamp. Because the transistor switch can be turned ON or OFF using low voltages and currents, you can connect a mechanical switch to the transistor switch using small, low-voltage wire and, thereby, control the larger current flow. If the mechanical switch is any distance from the equipment you're controlling, using low-voltage wire can save you time and money.
  • Example 3—A major problem with switching high current in wires is that the current induces interference in adjacent wires. This can be disastrous in communications equipment such as radio transceivers. To avoid this, you can use a transistor to control the larger current from a remote location, reducing the ...

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