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The
transistor
is a small electronic device that can be used |
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It
was developed in the USA in1948, |
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Before,
products using valves |
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The
bipolar transistor uses two special types of material
called n-type or p-type
semiconductors - these have
special properties of being part conductor and part
insulator.To make a transistor, three layers of n-type
and
p-type semiconductors are needed. They can
either be
sandwiched together to make npn
or pnp
type transistors.
The transistors shown below are the npn
type, which are
very common. The connections to the layers, or legs of the
transistor, are known as the emitter, base and collector.
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The current needed
to make the lamp work can be
reasonably large. When we turn a torch switch to "on" the
required current will flow from the battery, through the switch
and then make the lamp shine.The switch can do this as
its contacts inside are made of very good conducting
material; sometimes gold plated.
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Block diagram of the torch circuit |
Using the following
diagrams we can now look at how a
transistor can be put into the system to act as an electronic
switch.
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Block diagram of transistor controller |
The transistor switch is made to turn on
when a small
current (shown green) flows into the transistor via the
switch and resistor.
The output lamp
now switches on as the larger current
(shown blue) is no longer halted but allowed to flow through
the other two connections of the transistor.
The two currents
flow back together to the negative side
of the battery.
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Click
on the switch
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Circuit diagram
of |
When the switch is opened, the small input current no
longer flows into the transistor and it switches off again.
This stops the larger output current flowing through the
transistor and the lamp goes off.
Points to note:
-The resistor
in the circuit is used to reduce the current
flowing into the transistor - without it the transistor would
be damaged. This is similar to the protection resistor
needed by an L.E.D.
-Transistors can
be used in systems to make sensitive
electronic switches / sensors or as a "buffer" to provide
higher power to an output device.