Transistor Control

The transistor is a small electronic device that can be used
as a switch, or to amplify either current or voltage. In this
section we will see how it can be used as part of a
system to control an output device.

It was developed in the USA in1948,
by a small team led by William
Shockley. It replaced the valve and
was a very important invention.

Before, products using valves
had been much larger, more
expensive, less reliable and
very fragile. In the early 1950's,
small pocket-sized transistor
radios were soon developed and
launched by the Sony Corporation.
A second generation of more
compact transistor computers also
began to replace the original
valve types.


Original valve and
new transistor

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.


Q29, Open the system components link above and
find the transistor circuit symbol. Look at it carefully

and then return to this page to drag the correct names
to the red highlighted connections on the symbols
below.

(To check your answers click the button below)

 

 

 

 

 


 

Control with Switches

We have already seen how switches can be used to
mechanically control lights and turn them on and off.

Click here to take a look at this happening again (to
return: close its window).

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.


Click here
to take a different look at this happening
(to return: close its window).

 

Click on the switch
input
to see this
happening on the
block diagram.

Block diagram of the torch circuit

 

Control with transistors

Using the following diagrams we can now look at how a
transistor can be put into the system to act as an “electronic
switch”.

 

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.

Click on the switch
symbol to see the
two different
currents flowing in
the circuit.

Circuit diagram of
transistor controller


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.

 

In the circuit diagram above:

Q30, We can see that:

the base current + the collector curent = current

Q31, The current flowing into the leg,
controls the transistor and switches it on and off.


Q32, If the connection between the transistor's
emitter and the negative side of the battery were
removed the lamp would still work.

True
False

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