ElectronicsStudent

1
Electronics
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Always move the mouse before you click it.
2
Electronics
Introduction Ohms Law Power Calculations Resistor
s in Series and Parallel Capacitors Alternating
Current The Potential Divider Transistor Circuits
3
Introduction
Basic Concepts Simple Circuits
4
Basic Concepts
Electric current is due to the flow of
charge. In a solid conductor, the charge is
carried by electrons.
In a solid conductor, an electric current is due
to the flow of electrons.
5
Basic Concepts
Conductors include copper gold silver lead
All metals
And water (not distilled) which is why you should
not use mains appliances in the presence of water.
6
Basic Concepts
Insulators include Rubber Plastic Most solid non
metals Glass
Glass, unless it is very hot, is one of the best
insulators available.
7
Basic Concepts
Electric current (I) is measured in ampere (A) -
I is the symbol used to indicate current.
The amp is a rather large unit for most
electronic applications so we use the following
sub-multiples
1 mA 0.001A that is 1 / 1 000 th of an ampere
You already know that 1mm is 1/1000th of an metre
so there is nothing new here.
1 ?A 0.000 001A that is 1 / 1 000 000 th of an
ampere
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Basic Concepts
Voltage is measured in volt (V)
Common sub-multiples of the volt (less than a
volt) include
1 mV 0.001V that is 1 / 1 000 th of a volt
1 ?V 0.000 001V that is 1 / 1 000 000 th of a
volt
Common multiples of the volt (greater than a
volt) include
1 kV 1 000 V
1 MV 1 000 000 V
The kV and the MV are not common in electronics.
9
Basic Concepts
Resistance is measured in ohm (?)
Common sub-multiples of the ohm (less than an
ohm) include
1 m ? 0.001 ? that is 1 / 1 000 th of an ohm
1 ? ? 0.000 001 ? that is 1 / 1 000 000 th of
an ohm This is pronounced micro ohm
Common multiples of the ohm (greater than an ohm)
include
1 k ? 1 000 ?
1 M ? 1 000 000 ?
The m? and the ? ? are not common in electronics.
10
Basic Concepts
Capacitance is measured in farad (F)
Common sub-multiples of the farad (less than a
farad) include
1 m F 0.001 F that is 1 / 1 000 th of a farad
1 ? F 0.000 001 F that is 1 / 1 000 000 th of a
farad
1 n F 0.000 000 001 F that is 1 / 1 000 000 000
th of a farad this is written in full as a nano
farad
1 p F 0.000 000 000 001 F that is 1 / 1 000 000
000 000 th of a farad This is written in full as
pico farad
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Basic Concepts
Here is a summary of many of the available
multiples and sub-multiples
Symbol Prefix Multiplication factor T tera 1012
1 000 000 000 000 G giga 109 1 000 000
000 M mega 106 1 000 000 k kilo 103 1
000 h hecto 102 100 da deca 101 10 d deci 10
-1 0.1 c centi 10-2 0.01 m milli 10-3 0.001 ?
micro 10-6 0.000 001 n nano 10-9 0.000 000
001 p pico 10-12 0.000 000 000
001 f femto 10-15 0.000 000 000 000
001 a atto 10-18 0.000 000 000 000 000 001
The most frequently used are in bold.
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Basic Concepts
Resistors are marked with a series of coloured
rings to give us an idea of how big their
resistance is.
This gold band indicates that the
tolerance of the resistor is 5 (plus or minus
5 percent). This means that its resistance is
between 3 400 ? and 3 800 ?. We say it is
nominally 3 600 ?.
The third band is red. This means that there are
2 zeros.
The second band is blue. This means the second
digit is 6.
The first band is orange. This means the first
digit is 3.
So the resistor is nominally 3 600 ?.or 3k6
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Basic Concepts
The colours used for the first three bands and
their meanings are as follows
Colour Number Number of zeros Brown 0 none Blac
k 1 0 Red 2 00 Orange 3 000 Yellow 4 0
000 Green 5 00 000 Blue 6 000
000 Violet 7 0 000 000 Grey 8 00 000
000 White 9 000 000 000
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Basic Concepts
The colours used for the last band and their
meanings are as follows
Gold 5 Silver 10 No band 20
Resistors are manufactured in preferred values.
That means that you can only buy certain values.
The preferred values for resistors with a
tolerance of 20 are 10,15,22,33,47,68 and 100.
These are just the first two significant figures.
You can buy a 1500? but not a 2000 ?.
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Basic Concepts
The preferred values for 10 resistors are
10 12 15 18 22 27 33 39
47 56 58 82 100

16
Basic Concepts
The preferred values for 5 resistors are
10 11 12 13 15 16 18 20 22 24 27 30 33 36
39 43 47 51 56 62 68 75 82 91 100

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Basic Concepts
In summary
Voltage is measured in Current is measured
in Resistance is measured in Capacitance is
measured in
Volt (V) Ampere (A) ohm (?) farad
(F)
3MV 3 000 000 V 2kV 2 000 V 5mV 0.005 A 7?A
0.000 007 A

1nF 0.000 000 001 F 1pF 0.000 000 000 001 F
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Simple Circuits
Simple circuits have three main blocks of
components in common that perform the same type
of job. These are

• This is called a block diagram.
• The processor is the decision-making part of the
  system.
• The input is a sensor that transforms everyday
  phenomena such as temperature and heat to an
  electric signal that the processor can deal with.
• The output is a device that converts an electric
  signal from the processor into something that we
  want.

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Simple Circuits
Simple circuits have three main blocks of
components in common that perform the same type
of job. These are
Examples of input devices include Pressure
pads LDRs Thermistors Reed switches
Return to previous slide
20
Simple Circuits
Simple circuits have three main blocks of
components in common that perform the same type
of job. These are
Examples of output devices include Lamps LED
s Motors Solenoids
Return to previous slide
21
Simple Circuits
Simple circuits have three main blocks of
components in common that perform the same type
of job. These are
Examples of basic processors include Transistors
Operational amplifiers
Return to previous slide
22
Simple Circuits
Simple circuits have three main blocks of
components in common that perform the same type
of job. These are
What would the block diagram look like for a
system that brought on a light when it got dark?
Return to previous slide
23
Ohms Law
Ohms Law states that so long as the physical
conditions remain constant, the current through a
conductor is proportional to the voltage across
it. This gives us the formula
Voltage current x resistance V I R
We can rearrange this equation to give either R
V / I or I V / R
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Ohms Law
What does it mean?
Physical conditions remaining constant - This
really means as long as the temperature remains
constant. Usually it does.
The current through a conductor is proportional
to the voltage across it - this means that if
you double the voltage, you get twice the
current. Triple the voltage and you triple the
current etc.
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Ohms Law
Calculations using Ohms law fall into three
types
What is the resistance if ?
What is the current if ?
What is the voltage if ?
(Use R V / I)
(Use V I x R)
(Use I V / R)
E.G. What resistance could you use with a 10V
supply to limit the current to 15mA?
E.G. A 430 ? resistor protects an LED in a 5V
circuit. What is the current through the LED?
E.G. 12mA runs through a prorctive resistor of
resistance 820 ?. What is the voltage across the
resistor ?
R V / I 10 / 0.015
I V / R 5 / 430
V IR 0.012x820
667 ? so use 680 ?
12 mA
9.84 V
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Ohms Law
0.7 V Diode
The voltage across the diode is 0.7 V and the
cell produces 1.5 V. What is the current through
the resistor?
820 ?
1.5V
If you cant see how to do it straight away,
write the values given onto the diagram.
Voltage across the resistor 1.5V (provided by
the cell) - 0.7V (lost across the diode)
0.8V
Using I V / R 0.8 / 820 1mA
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Power Calculations
The detail from the bottom of an electrical
appliance shown here gives a very useful,
commonly used method of writing the power of the
appliance.
20 VA is exactly the same as 20 W (20 watts).
The more powerful an appliance is, the greater
the number will be. An electric fire might well
be 2 or 3 kW (2 000 or 3 000 W).
1W is sometimes called 1VA because you can
calculate the power by multiplying the volts by
the amps!
Power current x voltage or P I V
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Power Calculations
Remember that 20VA means a power of 20W and
that P I x V
This device runs from a 230V mains supply. What
can we learn from this information? Well, we know
that P IV we also know the voltage and the
power, so we can calculate the current I.
I P / V So I 20 / 230 87 mA
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Power Calculations - the formulae
Power current x voltage
P IV so I P / V and V P / I
But from Ohms Law, V I x R
So P I x IR P I2R
And from Ohms Law, I V / R
So P (V/R) x V P V2/R
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Resistors in Series and Parallel
Resistors are said to be connected in series when
the same current has to pass through each
resistor i.e. the current does not have to split.
These three resistors are connected in series.
And so are these 5 resistors
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Resistors in Series and Parallel
Resistors are said to be connected in parallel
when the current has to split to pass through
each resistor i.e. the current through each
resistor might not be the same.
These three resistors are connected in parallel.
And so are these two.
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Resistors in Series and Parallel
SERIES
R1 R2
R3
47 M? 47 M?
47 M?
These three resistors connected in series, could
be replaced by one resistor of resistance 141M?.
As a general formula we could write
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Resistors in Series and Parallel
PARALLEL
R1
The resistance of each of the resistors in this
parallel network is 47 M?. The effective
resistance of three resistors is 15.7 M? (use 16
M?).
R2
R3
You would expect the resistance to be less than
any of the individual resistances in the network
as there are three possible routes for the
electricity to take.
The formula used to add the resistances is
or
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Capacitors
Capacitors store charge. The greater the voltage
that you apply to them, the greater the charge
that they store. In fact the ratio of the charge
stored to the voltage applied is called the
capacitance.
Capacitance Charge / Voltage or C Q / V
Capacitance is measured in farad (F) but the
farad is a large unit of capacitance so you
usually see microfarad ?F (millionth of a farad
10-6 or 0. 000 001 F), nanofarad nF (10-9) or
picofarad pF (10-12).
The capacitor in the picture is a 470 ?F (0.000
47 F) polar (you must connect it the correct way
around in the circuit) capacitor rated at 40V
(the working voltage should not exceed 40V).
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Capacitors - Symbols
This is a non-polar capacitor - it does not
matter which way around you place it in the
circuit.
This is a polar capacitor. It is essential that
the capacitor is connected into the circuit the
correct way around.
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Capacitors
Capacitor Characteristics Time Constant
Calculations Capacitors in Series Capacitors in
Parallel
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Capacitor Characteristics
Closing the switch allows the capacitor to
charge. As this happens, the voltage across the
capacitor will rise in line with the fall of
current through it as it becomes fully charged.
Voltage
Time
Current
Time
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Capacitor Characteristics
The circuit has now been adapted so that closing
the switch allows the capacitor to discharge
through the resistor. Note now that the current
will fall as the voltage falls.
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Capacitor Characteristics
The circuit on the left allows us to investigate
the charging and discharging of a capacitor
simply.
Connecting the flying lead S to point X will
charge the capacitor from the cell through the
resistor R.
Connecting the flying lead S to Y will then
discharge the capacitor through the resistor.
It has been found that increasing either the
capacitance or the resistance will increase the
time taken for the capacitor to charge.
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Capacitor Characteristics
When the flying lead S is connected to X, the
capacitor will charge up through the resistor.
At first there will be little or no charge in the
capacitor so the current flows into the capacitor
(via the resistor), quite rapidly.
The current through the resistor develops a
voltage over the resistor. The voltage across the
capacitor will be proportional to the charge in
it. Since the charging has only just begun, it
will be small but growing.
The capacitor begins to charge It gets harder
for more charge to flow into the capacitor so the
current decreases. As the charge on the capacitor
is increasing, the voltage across it increases
too.
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Capacitor Characteristics
Eventually the capacitor will fill. This really
means that it approaches the condition such that
the voltage across it is equal to the supply
voltage.
There will no longer be any current flowing.
The time taken to achieve this increases with
increased capacitance and /or resistance.
The capacitor is said to be fully charged..
Increasing the supply voltage makes no difference
to the time taken for the voltage across the
capacitor to approach the voltage across the
supply.
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Time Constant Calculations
The time constant is the time taken for the
current or voltage to have fallen to 37 of its
original value or the voltage to have risen to
63 of its original value
We can calculate the time constant for a circuit
by multiplying the capacitance of the capacitor
by the resistance of the resistor
T C x R
The units of the time constant are seconds if the
resistance is in ohms and the capacitance in
farads.
43
Time Constant Calculations
E.G. One A 10M0 resistor is connected in series
with a 470 pF capacitor. How long will it take to
discharge the capacitor to 37V from an initial
voltage of 100V?
Note that the voltage is falling to 37 of its
initial value, so we are looking at one time
constant.
Using T C R
T 0.000 000 000 470 x 10 000 000 0.004 7 s
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Time Constant Calculations
E.G. Two A 10M0 resistor is connected in series
with a capacitor. If the time constant is 0.001s,
what is the capacitance of the capacitor?
Using T C R
0.001 C x 10 000 000 C 0.001 / 10 000 000
0.000 000 000 1 F
100 pF
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Capacitors in Series
These three capacitors are connected in series.
Their combined capacitance is given by
or
46
Capacitors in Series
4 What is the combined capacitance of a 10 ?F, 20
?F and 47 ?F capacitor connected in series?
The numbers here could get very small so let us
omit 5 zeros and give the answer in ?F as all the
capacitances are in ?F anyway.
C (10 x 20 x 47) / (10 20 47) 9400 /
77 122 ?F
47
Capacitors in Parallel
These three capacitors are connected in parallel
with each other.
Note that because they are in parallel, they must
have the same voltage across each other.
The combined capacitance of the network of
capacitances is given by
Ctotal C1 C2 C3
Return to menu
48
Capacitors in Parallel
Suppose that the capacitances are 10 ?F, 20 ?F
and 47 ?F.
Ctotal C1 C2 C3
So C 10 20 47 77 ?F
49
Alternating Current
Direct current (DC) is the current that comes
from a cell or battery.
It is unidirectional. That is to say that the net
drift of electrons is in one direction. This one
direction will always be from positive to
negative for electrons but negative to positive
for conventional flow.
It is easier to convert voltages from one value
to another if the direction of the current is
rapidly changing.
This is called an alternating current (AC).
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Alternating Current

• Alternating current has some strange properties
• it can appear to pass through a capacitor
• it produces the discharges that you see in a
  plasma ball
• it can be stepped up (to higher voltages and
  lower current)
• it can be stepped down (to lower voltages and
  higher current)

Mains voltage is always due to an alternating
current. It is used because it can be stepped up
or down easily.
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Alternating Current
Throughout Europe, mains voltage is supplied at a
frequency of 50 Hz.
You will remember that Hz is the abbreviation for
hertz - the unit of frequency.
This means that the electricity goes through one
complete cycle 50 times every second.

• This means that the voltage will
• start from zero and build up in one direction
  until it reaches a maximum value (about 325 V).

• Fall back to zero

• Change direction and start to build up to a
  maximum value (about - 325 V)

• Fall back to zero

• 50 times per second

Alternating voltages (and currents) can have
extremely high frequencies. The current that
produces radio waves can be many MHz (millions of
hertz).
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The Potential Divider
9V
Voltage is sometimes referred to as potential
difference. The potential divider simply divides
up a potential or voltage.
In its simplest form it is two resistors placed
across a power supply. The voltage across of each
resistor is less than the supply voltage. Adding
the voltage across each resistor will give the
supply voltage. It is probably easiest to
understand if you look at the diagram.
Here the power supply is 9V. Note that both the
resistances are the same.
0 V
The voltage from the supply will be split
(divided) equally as 4.5V. Of course 4.5V 4.5V
9V.
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The Potential Divider
In this potential divider circuit, the
resistances are not the same.
It develops 2/3rds of the voltage.
It is 2/3rds of the resistance.
The bigger resistance here means that there will
be a bigger voltage here.
2 000 ?
2/3 x 9V 6V
It develops 1/3rd of the voltage.
It is 1/3rd of the resistance
The smaller resistance here means that there will
be a smaller voltage here.
1 000 ?
1/3 x 9V 3V
If you would like to work through this again,
step back through the sequence using the left
arrow key.
54
The Potential Divider
You can calculate the voltage across each
resistor using a formula too.
V1 V x R1 / (R1 R2)
R1
V1
V1 9 x 2 000 / (1 000 2000) 9 x 2 / 3 6V
V2 V x R2 / (R1 R2)
V2
R2
V1 9 x 1 000 / (1 000 2000) 9 x 1 / 3 3V.
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The Potential Divider
The potential divider does not have to be made up
of two fixed resistors. One of them could be
variable, or even both.
As R1 increases so does V1 but V2 will fall.
As R1 decreases so does V1 but V2 will rise.
It is just as if there is only one cake to go
around (the voltage). If R1 increases then V1gets
more cake so there is less left for V2!
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The Potential Divider
Now the variable resistor has been moved to the
lower position in the network..
As R2 increases so does V2 but V1 will fall.
As R2 decreases so does V2 but V1 will rise.
It can be handy to change the position of the
variable resistor. Later you will see that it can
change the action of a transistor circuit so make
sure that you follow it.
57
The Potential Divider
With this potential divider, the tap in the
middle is a slider. It probably moves along a
track of carbon.
V
V1
R1
R1 and V1 will be the resistance and voltage
above the slider.
V2
R2 and V2 will be the resistance and voltage
below the slider.
R2
As R1 increases, R2 decreases. This will result
in V1 increasing and V2 decreasing.
0V
As R1 decreases, R2 increases. This will result
in V1 decreasing and V2 increasing.
58
The Potential Divider
Here is a component that could be used as a
potential divider.
59
The Potential Divider
Here is another
60
The Potential Divider
It is possible to use many different components
that vary their resistance in a potential divider
circuit. Here are a few that you might find and
the physical conditions that change their
resistance.
Light Dependent Resistor (LDR) - decreases
resistance with increased illumination.
Thermistor - decreases its resistance with
increased temperature (negative temperature
coefficient).
Microphone - changes resistance with sound.
Strain gauge - changes its resistance when
stressed.
Photodiode - decreases resistance with increased
illumination.
61
Transistor Circuits
The transistor is a three connection component
that can be used either as an amplifier or a
switch.
V
0V
Essentially the circuit is set up so as to try to
force electrons through the emitter and out of
the collector. This might be to light a bulb.
However, under normal circumstances, there is a
very high resistance between the emitter and the
collector so the bulb will not light.
But
If we make the base go positive, the collector /
emitter junction conducts and the bulb will light.
62
Transistor Circuits
The clever part now is to control the base bias
voltage that turns the transistor on.
The base bias voltage is the voltage between the
base and the emitter. If it is anything much less
that 0.7V, the transistor will be off.
The transistor switches on when it is 0.7V.
Rb
You should never allow the base bias voltage to
get too high as this will overheat the base and
burn out the transistor. For this reason you will
frequently find a resistor connected to the base.
63
Transistor Circuits
The clever part now is to control the base bias
voltage that turns the transistor on.
This can be achieved using a potential divider.
R1
Correct selection of the two resistors R1 and R2
will take the base to 0.7V and turn the
transistor on.
Rb
R2
Suppose R2 was much higher than R1. The voltage
across R2 would be high so the transistor would
switch on.
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Transistor Circuits
The clever part now is to control the base bias
voltage that turns the transistor on.
Now suppose R2 was an LDR.
R1
In the bright light, its resistance would be low
so the voltage across it would be low, the
transistor switched off and the lamp off.
Rb
R2
But suppose that it now goes dark!
65
Transistor Circuits
The clever part now is to control the base bias
voltage that turns the transistor on.
V
It has just gone dark!
R1
The resistance of the LDR rises.
Rb
The voltage across the LDR rises.
R2
The base bias voltage reaches 0.7V
0V
The transistor switches on.
The bulb lights.
66
Transistor Circuits
The clever part now is to control the base bias
voltage that turns the transistor on.
Suppose that we now swap the positions of the
resistor and the LDR.
R1
Rb
The bulb will now come on in daylight! It might
be useful as a warning light circuit in certain
circumstances.
R2
67
Transistor Circuits
The clever part now is to control the base bias
voltage that turns the transistor on.
Now let us consider
R1

• Ib the base current that flows into the transistor

Ic

• Ie the emitter current that flows out of the
  transistor

Rb
R2

• IC the collector current that flows into the
  transistor

Ie
Ie Ib Ic
68
The base current will be very small as it has
passed through R1 and Rb so it is almost true
that Ie Ic.
The ratio of Ic Ib is important. It shows that
the transistor is amplifying. It is often around
about 100.
69
That is to say that the collector current is a
always a constant amount bigger than the base
current. Feed a small current to the base and you
get a big current in the collector.
70
Transistor Circuits
The clever part now is to control the base bias
voltage that turns the transistor on.
Ie Ib Ic
R1
The ratio is called hfe.
Ic
hfe Ic / Ib
Rb
R2
Ie
71
Transistor Circuits
Click on a component to find out what it does.
V
R1
Relay
Rb
R2
C1
0V
CONTINUE
72
Transistor Circuits
Click on a component to find out what it does.
Capacitor This stores charge. It acts as a time
delay to any switching. If the transistor is on
and tries to go off, it will act as a reservoir
and keep the transistor on for a while longer.
V
R1
Relay
Rb
R2
C1
0V
CONTINUE
73
Transistor Circuits
Click on a component to find out what it does.
LDR Light Dependent Resistor Its resistance
decreases with increased illumination. In the
dark, the resistance goes up turning the
transistor on.
V
R1
Relay
Rb
R2
C1
0V
CONTINUE
74
Transistor Circuits
Click on a component to find out what it does.
Base Bias Resistor This fixed resistor protects
the base from too much current.
V
R1
Relay
Rb
R2
C1
0V
CONTINUE
75
Transistor Circuits
Click on a component to find out what it does.
Potential Divider The LDR and R2 are a potential
divider.
V
R1
Relay
Rb
R2
C1
0V
CONTINUE
76
Transistor Circuits
Click on a component to find out what it does.
Transistor A small voltage at the base will allow
current to flow through the emitter from the
collector.
V
R1
Relay
Rb
R2
C1
0V
CONTINUE
CONTINUE
77
Transistor Circuits
Click on a component to find out what it does.
Diode - this only allows current to flow in the
direction of the arrow head. Rapid changes in the
magnetic field of the relay can cause high
voltage that would damage the transistor.
V
R1
Relay
Rb
The diode diverts the currents formed by this
process.
R2
C1
0V
CONTINUE
78
Transistor Circuits
Click on a component to find out what it does.
Relay - current through the relay produces a
magnetic field that throws a switch in another
external circuit. The external circuit can be a
much higher powered circuit.
V
R1
Relay
Rb
R2
C1
0V
CONTINUE