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Transistor Switch

 

Circuit Diagram

 

 

How a Transistor Switch Works…..

The main component of this circuit is the general purpose BC548B transistor. It has three connections as shown : the base (b), the emitter (e) and the collector (c).

The base can be thought of as its input side, the collector its output side and the emitter common to both the input and output sides.

When a voltage greater than 0.6 volts (approximately) is seen at the base, the transistor will allow current to flow from the + power rail into its collector and then out of its emitter. Our output device (e.g. buzzer) has to be connected in this collector/ emitter circuit for current to flow through it. The output device is being sinked.

The 1K resistor is used to protect the input side of the transistor by preventing too much current entering the base.

Transistor Currents

The BC548B transistor is an npn high gain low-power semiconductor. It is a low cost device used to amplify current. Its maximum output current = 100mA.

Transistor Currents Formula:-

Emitter Current (Ie) = Base Current (Ib) + Collector Current (Ic)

Transistor Amplifier Gain

Its gain (ie. how many times greater its output is to its input) is found by looking for its "hFE" characteristics. This can be looked up in an electronic components supplier’s catalogue. For the BC548B transistor its hFE value is 125-600. Many other specifications / values can be found for other transistors.            

Gain (hFE) Formula:-