How a Transistor Switch Works
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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:-
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