Why Transistors Are Used as Switches

Chapter Why Transistors Are Used as Switches

Complete Electronics Self Teaching Guide with Projects
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Complete Electronics Self Teaching Guide with Projects

  • CHAPTER 4 THE TRANSISTOR SWITCH146WHY TRANSISTORS ARE USED AS SWITCHES15  You can use the transistor as a switch (as you saw in the previous problems) to per-form 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 tran-sistor 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 cur-rent, 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 commu-nications equipment such as radio transceivers. To avoid this, you can use a tran-sistor to control the larger current from a remote location, reducing the current needed at the switch located in the radio transceiver. ■ Example 4—In mobile devices (such as a radio-controlled airplane), using transis-tor switches minimizes the power, weight, and bulk required. ■ Example 5—When you use a sensor to activate devices, the sensor provides a low current to the transistor, which then acts as a switch controlling the larger cur-rent needed to power the equipment. An everyday example is a sensor that detects a light beam across a doorway. When the beam is blocked by a person or object passing through, the sensor stops generating a current, switching a transistor OFF, which activates a buzzer.Book Authorc04V107/04/2012 2:59 PM c04.indd 1467/4/2012 3:00:36 PM