Electrical and Electronic Systems
Submitted By
MD MARUFUR RAHMAN
A Level
Introduction To Electrical and Electronic Systems
Notes to accompany lecture 1
Electrical and Electronic Systems
Input
Process
Output
Most systems can be broken down into subsystems
Subsystems can often be broken down into further subsystems
Example:
1. Automatic door system ( e.g. entrance to a shop/train station)
2. Mobile phone
3. central heating system
4. television receiver
(Think how each system uses input, process and output differently)
1
BASICS
J1
We must start with some
basic ideas about
electricity, current flow,
voltage and resistances.
Key = Space
V1
12V
If we can get this over early
on we can get to the more
interesting stuff more
quickly. The diagrams and
text are incomplete without
the lecture to explain them.
All the diagrams are taken
from the circuit simulator
“Multisim” which you can
use in your studies (purchase
from the FESBE counter,
room J205.
P = VI
X2
21W 12V
CURRENT (labelled as I)
Conventional current I flows from battery positive (top of battery symbol here) to battery negative in a
loop. It is a flow of +ve charges. Measured in amperes (A)
Charge Q is measured in coulombs (C)
The size of the current is the number of charges flowing per second.
So, 1 Ampere = 1 Coulomb per second
VOLTAGE (labelled as V)
The battery does work on the charges and gives them enough energy to flow around the circuit. A
charge emerging from the +ve terminal is at a higher potential energy than a charge which is reentering the battery. Because of the difference in potential energy, we say that the battery has a
Potential Difference across its terminals. This potential difference is measured in volts and is also
called ‘voltage’. Therefore ‘potential difference’ and ‘voltage’ are the same thing. Confusingly, the
force which is applied to the charges inside the battery is called ‘electromotive force’ or emf and
sometimes ‘emf’ is used instead of ‘voltage’ or ‘potential difference’.
POWER (labelled as P)
Electrical energy can be used up at different rates. Energy is measured in joules, and therefore the
number of joules of energy used over a certain period of time is a measure of the electrical power.
Power P is measured in watts (W) . Electrical energy is usually used up in a circuit by conversion to
heat, light or movement. 1 watt = 1 joule/ second.
[More on this in “Principles of Electric Circuits” by Thomas Floyd]
RESISTANCE (labelled as R)
The size of a current flowing through a wire is dependent on the amount of resistance to current flow in
the circuit. A large resistance will reduce the current. Electrical resistance R is measured in ohms (Ω)
OHM’s LAW
XMM1
V1
12V
R1
6ohm
Ohm's Law
Relates I, V and R
together.
V = I.R
2
I= V
R
We can also say
V = IR
and
R=V
I
POWER (symbol P, unit Watts(W
A bit more about power.
If I=V/R and P = VI, we can see that P = V.V/R = V2/R . Also P = VI = (IR)I= I2R
Therefore we have 3 forms:P = VI
P = V2/R
P = I2R
SERIES RESISTANCES
Only one path for the current to flow.
Rtotal = R1 + R2 + R3
Current I is the same for all series components
R1
R2
R3
10ohm
20ohm
30ohm
V1
12V
Ohm’s Law tells us that when current flows through a resistance, a voltage difference is created across
the resistance. Therefore each resistance above has a different voltage across it. We can use voltmeters
to see the value. Note that the total current is the same as the current in flowing through each
resistance.
3
VOLTAGE DIVIDER CIRCUIT.
Two series resistances can be used in a special circuit called a voltage divider. It normally uses two
resistances. Note the modern symbols for a voltage source and the resistances.
The voltage divider
is usually shown like
this
Vin
Vout = Vin.R2
(R1 + R2)
diagram with old symbols (to be added)
diagram with new symbols
Variable Resistances (a component usually called a potentiometer or ‘pot’)
A strip of
carbon
works as an
electrical
resistance
R1
R1
R1+R2
R2
A slider can move
up and down the
carbon strip causing
the resistances
between the moving
connection ( wiper)
and the fixed
contacts to vary
R1+R2
R2
If connected like this, as a voltage divider, it’s called a potentiometer. We
use a voltmeter to read the value ( the current stays constant)
4
And we can use it to adjust the size of a dc or ac voltage
dc
R1
V1
12V
XMM1
65%
1kOhm
Key = a
XSC1
ac
V1
1V
0.71V_rms
1000Hz
0Deg
R1
G
95%
1kOhm
Key = a
A
B
T
We can also connect the wiper to one end and use it as a variable
resistance. In this case the current varies.
+
V1
12V
0.012 A
R1
100%
1kOhm
Key = a
5
RESISTANCES IN PARALLEL
With two or more parallel resistances the current has more than one circuit path to flow through, so the
total circuit resistance reduces. Two resistances in parallel are equal to one resistance whose value is
the product divided by the sum of the two separate resistances.
Resistors in Parallel
R1
R3
R2
Vs
12V
=
V2
12V
R3 = R1.R2/(R1 +R2)
What is the current in the circuits below?
R1
R3
47kohm
V1
12V
9.56kohm
R2
12kohm
-
=
V2
12V
-
+
0.000 A
+
0.000 A
?
-
0.000 A
+
V1
12V
0.000 A
R1
82kohm
R2
47kohm
R3
100kohm
+
V2
=
12V
6
AC WAVEFORMS ( SINEWAVES)
Let’s take a brief look at voltages which continuously change in a sinusoidal manner. The oscilloscope
will show us what it looks like!
Vpp
vertical distance
corresponds to voltage
Peak to Peak
Vpp
Horizontal distance
corresponds to time
Amplitude or Peak Voltage Vp is the measured voltage from the centre to one of the peaks
Vp = Vpp
2
Vrms
The effective value of this waveform for delivering power to a device is mathematically found by
finding the average of the square of the area under the curve and then taking the root of this value.
It’s called the ROOT MEAN SQUARE value (or rms value)
It works out quite simply as Vrms = (0.7071).Vp
(where 0.707 is the reciprocal of root2)
Frequency (f ) and Period (T)
The number of complete cycles per second is the frequency (f) measured in Hertz (Hz)
The time for one complete cycle is the Period (T) measured in seconds (s)
EXAMPLE
The UK mains supply is often quoted as 240 Vrms at 50 Hz.
Therefore the UK mains voltage reaches a peak voltage (Vp) of 339 V and has a period of 20 ms
MAINS SUPPLY, fuses and earths
The mains supply is a voltage sinewave on the live (L) terminal. The neutral is earthed ( zero voltage)
at the power station and is almost 0 V at the home. Mains is rated at 240 Vrms at 50 Hz which means a
peak voltage of + 339 V is reached during each cycle. The period T = 1/50 = 20 m s
To protect us from ‘live’ voltages touching the metal exterior of an appliance the earth (E) is used
along with a protection fuse circuit. Other more sensitive methods can also be used like the earth
leakage trip or the RCD Residual Current Detector (Device).
The fuse protection works by inserting a wire device in series with the Live wire feeding an appliance.
The device can carry a rated current but if this is exceeded then the wire gets hot and breaks. The
resulting break in the current loop means that the appliance no longer has an electric supply and
therefore stops.
7
How can the current to the appliance exceed its ‘normal ‘ value?
•
•
This can happen if there is an internal appliance fault
It can also happen if the mains current finds another (parallel) current path to the neutral or the
earth connection, causing the total resistance to reduce and the current to increase
8
Switches
SPST, SPDT, DPST, DPDT, SPSTSB, SPSTDB, NOSPSTSB
etc…………. ???What are these ? Find out in the lecture!
We’ll look at the following:XSC1
XFG1
G
A
B
T
J1
R1
1kohm
Key = Space
Flip the switch to
change the path
of the signal. This
uses a SPDT
XSC2
G
A
B
T
R2
1kohm
Using a DPDT
This is good for changing
the direction of current flow
through a device
S1
Key = Space
LED1
LED2
XMM1
V1
12V
R1
1kohm
+
! "
&
#
$
0.010 A
$
%
$
'
9