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 P1
Transistor Switch

 

Circuit Diagram

 

 


How a Transistor Switch Works…..

The main component of this circuit is the 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 being common to both sides.
When a voltage greater than 0.6 volts (approximately) is applied to 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 (see OP1). The 10K resistor pulls the output up to the + power rail when the transistor has not "switched on". This is useful when the transistor is connected to a "logic" system. 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.


The maximum output current = 100mA.

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 in electronic components supplier’s websites or catalogues. For this transistor, its value is typically 125-600 and specifications / details can easily be found for other transistors.            

Gain (hFE) Formula:-              

 

Transistor Alternatives

The following table shows a small range of npn transistors that you can use:-

Transistor

Sensitivity (hFE gain)

Output Current (amps)

BC548B

220

High

0.22

Low

BC108

110

High

0.1

Low

40

Low

1

Medium

BC 637

40
Low
1
Medium

2N3055

20

Low

15

High