Transistor as switch

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Transistors are used in one of two ways: as switches, or as amplifiers. This example shows how to hook up a transistor as a switch that turns on a high power load. In this example, an LED is used as the high-current load, in order to keep the example simple. However, heavier loads such as motors and other actuators can also be turned on in this way.

Applying a tiny current to the base of the transistor "turns on" a larger current flowing from the collector to the emitter of the transistor. Turning off the current to the base turns off the larger current.

Note: This example uses the Arduino 5V pin as the power supply and LEDs as the "load" for simplicity. The power coming out of the Arduino 5V pin (max 40mA) is generally too small to drive much more than a few LEDs or a very small motor. For larger loads, such as bigger motors or relays, or stronger lights or sound, a stronger power supply, such as from batteries or a power adaptor, would be used instead.


The circuit

Transistor as switch circuit]

This way of using a transistor is also known as a "current sink", since the emitter sinks the current of the circuit "load" directly to ground. It is also possible to use a transistor as a "current source", whereby current flows through the transistor and powers the circuit.


Questions

  1. Why is the LED turned on by the switch dimmer than the LED turned on by the transistor?
  2. Give the approximate current flowing through each leg of the circuit (ignore any affect the transistor might have on the current flow or voltage drop).


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