Transistor Switching and Control

The transistor was developed in 1948. Although it is often used to amplify current or voltage, we will be looking at how it can funtion as an electronic switch. In this section we will be using it in a simple control circuit to turn a light on and off. You can see it explained in further detail in the "FastTrack" section - where it is used in many more sensing control circuits.

The transistor was initially developed by a small team led by William Shockley. Their work on semiconductors was recognised to be very important and they received the Nobel prize for this in 1956.

The transistor was to replace the valve. Older products using valves (such as the radio on the right), were much larger, more fragile and less reliable than products we use today.

 

In the early 1950's, small pocket-sized transistor radios like these were developed.

The transistor has almost completey replaced the valve now, with the exception of some guitar amplifier manufacturers wishing to retain the earlier sound the valve produced.

 

An older valve and the
smaller transistor which has replaced it.

Semiconductor/transistor design is at the heart of the technology we use in computers today - where many millions of microscopic transistors are embedded into computer processors (as shown on the right).

The bipolar transistor uses two special types of material
called n-type and p-type semiconductors. These have
special properties which makes them part conductor and part
insulator. Being "semi-conductors" they can either allow current to flow or not to flow - which is very useful.

 

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. These are
very common. The connections to the layers, or legs of the
transistor, are known as the emitter, base and collector.

 

Q29, Open the Component and Symbols link above and find the transistor circuit symbol. Look at it carefully then return to this page where you need to drag the correct labels to the red highlighted connections on the transistor 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 to take a look at this happening again, then close the 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 make this happen as its contacts inside are made of a very good conducting material - sometimes they are gold plated.

 

Click on the switch
input
on the left to see this
represented on a
block diagram.


Block diagram
of the torch circuit

 

Control with Transistors

Using the following block 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


In the circuit below we will see how the “transistor switch” is made to turn “on” when a small current (shown green) flows into the base of the transistor, via a switch and current limiting resistor.

The output lamp switches on now as the larger current
(shown blue) is allowed to flow into the collector and through the transistor.

The two currents entering the transistor combine and flow out of the emitter and back to the negative side of the battery to complete the circuit.


Click on the switch
symbol to
see the
smaller & larger
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 base connection. This switches the transistor “off” again, stopping the larger output current flowing through the transistor. This causes the lamp to turn off.

Additional Points:

1, 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 reason a protection resistor is needed by an L.E.D.

2,In control systems, transistors can either be used to make sensitive electronic switches or they can be used as a "buffer" to provide higher power to an output device when operated, or switched, by a lower current input device.

 

From 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 transistor was "switched on" and the connection between the transistor's emitter and the negative side of the battery removed, the lamp in the circuit above would still light up. Is this true or false?

True
False


You have now completed the tutorials.
See and print your results certificate below.

(To improve your results, you may revisit any sections by using the links on the left.)

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