Prac 1: Using CRO to measure period and peak-peak value of
AC signal
Aims
1) To revise the use of a CRO to measure the period and hence determine the frequency of an AC outlet.
2) To revise the relationship between a peak-to-peak, peak and RMS voltage as measured by a CRO and
multimeter respectively.
Tasks
1)
Set up the following circuit involving a signal generator and a CRO.
signal
generator
On the signal generator, adjust the frequency and amplitude controls to generate a
sinusoidal signal that can be detected as a sine-wave on the CRO.
2)
Write down the frequency of the output signal as shown on the dial of the signal generator. For
example: f = 250 Hz.
3)
Now adjust the CRO, making sure that the time axis is calibrated so that you know in advance how
much time one centimetre corresponds to, i.e. 1cm = 10 ms for example.
4)
Measure the period of the sine-wave directly from the CRO. Convert this measurement into a
frequency: f 
1
.
T
5)
Compare the frequency determined from your measurement with the frequency as indicated on the
dial of the signal generator. To what extent do the two numbers agree?
6)
Now measure the peak-to-peak voltage of the signal using the CRO.
7)
Convert this peak-to-peak value into a peak value and then into an RMS value.
8)
Finally take a multimeter and set it to AC voltage and measure the RMS voltage value across the
signal generator.
9)
How do these two values (RMS value determined from the CRO and the RMS value as measured
from the multimeter) the compare?
Prac 2: The effect of a single diode in the active line of an AC
supply
Aims
To study the effect of placing a single diode in the active line of a simple circuit with an AC power supply.
diode
Equipment
AC power supply, diode, 1.0 kΩ resistor, CRO
CRO
active side
1.0 kΩ
6 V RMS
Tasks
1)
Construct the following circuit:
2)
Place the terminals of the CRO across the power supply.
Choose one of the yellow terminals to be the active side
of the power supply and maintain this convention
through out this and subsequent experiments. A simple
approach is to adopt the LEFT hand yellow terminal as
the active AC outlet.
neutral side
red
black
Vsupply
t
3)
Sketch the output voltage vsupply(t) over two cycles. On
your sketch, label the period and the peak values. You
should find that the period is 20 ms = 0.020 s since the
signal has a frequency of 50 Hz and that the peak value is about 8 to 9 volt.
4)
Now connect the CRO across the
resistor with the red lead of the CRO
closest to the active side of the power
supply.
Sketch the voltage across the resistor,
vresistor(t) carefully labelling the period and
the peak value.
5)
6)
VR
t
VDIODE
Place the CRO across the diode with the red terminal
closest to the active side of the power supply. Again
sketch the voltage across the diode vdiode(t) carefully.
t
7)
Describe what the diode does to the supply voltage by the time it has passed through to the
resistor. That is in what way does the diode filter the AC supply signal?
8)
For this type of circuit what is the voltage drop across the diode in forward bias?
9)
What is the voltage drop across the diode when reverse biased?
10)
Complete the sentence: A single diode in an active line converts an AC current into ________.
Prac 3: Capacitors
standard
Introduction
variable
polar

Capacitors consist of a sandwich of two metal plates separated by a thin insulator. They are usually
rolled up into a cylinder and put in a small can with two terminals, each connected to one of the two
plates.

Capacitors are used for storing electrical charge. This property is very useful in electronics circuits and
power supplies as charge with energy can be temporarily stored and released – it allows circuits to
have delays built in as one application or for a circuit to stay on temporarily after a master switch has
been opened.

The amount of charge Q that a capacitor can store is determined by its capacitance C and the
voltage V across the two plates.
Quantity of Stored Charge (Coulomb) = Capacitance (Farad) × Voltage (Volt)
Q = C × V or C = Q ÷ V
A Farad is a very large quantity. Capacitor values are normally much smaller and expressed in microfarads (
µF), nanofarads (nF) and picofarads (pF).
Find out what micro-, nano- and pico- mean:
micro
nano
pico
Calculate the quantity of charge stored [ in milli coulomb] in a capacitor of C = 100 µF, when V = 12 volts
is applied to it.
(Answer: 1.2 mC)
Prac Equipment
1000µF = 1 m F
6 V battery, switch, 1000 µF capacitor, 10 k resistor, stop
+
watch, multimeter, voltmeter and CRO
+
PART 1 THE CHARGING PROCESS
Connect the circuit up as shown. Ensure that the positive end
of the capacitor, the longer wire, will connect to the positive end
of the
6 V battery.
6V
10 k
The voltmeter monitors the voltage across the 10 kΩ resistor and thus by Ohm's Law, also the current
going through the circuit to charge up the capacitor.
V
If the voltage across the resistor changes, the current through the resistor also changes since V is
proportional to I for an ohmic resistor. Hence the current flowing onto the capacitor changes as well, as it
is in series with the resistor.
1) Now close the switch and observe the voltage across the resistor.
2) Describe how the voltage across the resistor changes during the charging process.
Now, since V is proportional to I for an ohmic resistor , describe how the current through the resistor
changes during the charging process.
The capacitor will now be charged since no more current flows in the circuit: a capacitor acts to store
charge and to eventually inhibits the flow of a DC current - in one way it is like damming up a river.
Open the switch to isolate the supply from the circuit.
3) Does the capacitor discharge into the 10 kΩ resistor? Why not?
4) Short circuit the capacitor to empty charge stored on it. This is done by connecting a lead to both
terminals of the capacitor. A current will run briefly in the lead to neutralise or discharge the capacitor.
5) Place the voltmeter across the capacitor, this time to measure the voltage across the capacitor over
time after the switch is closed.
6) Close the switch and describe how the voltage across the capacitor
1000µF = 1 mF
changes as it charges up. You will now have a fully charged capacitor for
which to do part 2 – the discharging process.
+
PART 2 THE DISCHARGING PROCESS
Now open the switch to isolate the battery from the circuit.
7) Describe what happens to the voltage across the capacitor. Why does it
not change? (Note: it will decrease slowly if you use a multimeter, but not
if you use an analogue voltmeter or a CRO – why is this?)
+
V
6V
We are going to discharge the capacitor into the 10 kΩ resistor and so you need only to disconnect the
cable from the negative terminal of the battery and connect it to the positive side of the capacitor. In this
way you will have a circuit that looks like this:
10 k
8) Connect up the circuit as above and observe and explain the reading on the voltmeter over time, that
is as a function of time.
PART 3 THE TIME CONSTANT

The time taken for the charging and discharging process to occur depends on the resistor and
capacitor values in the circuit.

The larger the resistor, the greater the time taken to charge up the capacitor.

The larger the capacitor, the greater the time taken to charge up the capacitor.

The time for the voltage to rise to 63% of its final value is called the Time Constant. The time
constant is given the symbol τ and pronounced “tau”.

9)
10)
11)
12)
You will need a stop watch for this part of the experiment as you will measure the time it take for
the voltage across the capacitor to rise from 0 to 63% of
10 k
6 = 3.8 V.
Connect the circuit up as follows using a 6.0 V battery
Adjust the timebase of the CRO to give a straight line on the
6V
1 mF
screen. Set the input controls so that you can observe a 6
Short circuit
Volt deflection. Make sure that the AC/DC switch is set to
lead
DC.
Connect the CRO across the battery to check how close to 6 Volts the battery actually is.
Close the switch and use the stopwatch to measure the time it takes for the voltage across the
capacitor to rise from 0 Volts to 6 Volts.
+
CRO
You will have found this time is difficult to measure as it is hard to tell when the capacitor is fully charged.
The Time Constant τ gets over these problems. The Time Constant, the time taken to charge to 63% of
the fully charged voltage. It can also be calculated from the relationship : τ = R × C
13) Open the switch and then discharge the capacitor with the short circuit lead and repeat the
experiment, but this time measuring the time to reach 63% of the charging voltage. For the
following values measure and calculate the time constants:
Capacitor
Resistor
1 mF
10 kΩ
1 mF
100 kΩ
100 µF
100 kΩ
Measured Time
Constant (s)
Calculated Time
Constant: RC (s)
1 mF
PART 4 Capacitors and AC Voltages – THE
CAPACITOR AS A DC FILTER
+
A.C.
Voltage
10 kΩ
CRO
DC.
setting
Connect the circuit as follows. Make sure the CRO is switched to the DC.
14) Switch on the AC voltage and note if there is any voltage across the resistor.
15) Is there any AC voltage across the resistor 1 minute after switching on? ________________
16) Describe how capacitors respond to AC and DC voltages.
A capacitor will allow AC voltages to pass through the capacitor without the capacitor charging up. A
capacitor can thus be regarded as a DC filter and an AC pass. Thus if a mixture of AC and DC presented
itself on the active side of a capacitor, the capacitor will charge up to stem the DC but transmit the AC to
the next part of the circuit.
Capacitor Problem
1)
When 10 V is applied across a capacitor, 4.5 mC is stored on each of the plates.
a) Determine the capacitance of the capacitor (in Farad)
b) If the voltage across the capacitor is now increased to 12 V determine the increase in the
amount of charge stored on the plates.
2) Consider the circuit below. At t = 0, the switch S is closed.
12 V
S
2.0 kΩ
4.7 mF
a) On a copy of the circuit indicate the movement of electrons in the circuit.
b) Calculate the time constant τ for the circuit.
V( t )
c) On the set of axes shown right,
sketch:
i) The supply voltage as a
function of time
ii) The voltage across the
capacitor as a function of time,
showing the time constant and the voltage across the capacitor at t = τ.
t
(ms)
iii) Sketch the voltage across the resistor as a function of time.
3) Capacitors are used to “DC decouple” a voltage signal v(t). Explain what the term “decoupling” means
and give an example.
4) Now consider the circuit below with an AC active rail:
a) Sketch the voltage across the capacitor and the voltage across the resistor.
12 V peak-to-peak, f = 10 Hz
V
S
t
4.7 mF
2.0 kΩ
Answers: 1) 450 F
be checked!
2. a) up through capacitor (charging) b) 9.4 s
Other answers will
Prac 4: Smoothing a rectified AC signal with a capacitor
Aims
1) To observe what happens when a capacitor is placed in parallel with a load resistor to the voltage across
the resistor Vresistor(t).
2) To observe what happens when the size of the capacitor is varied.
3) To learn more about the time constant τ = R×C for a circuit when there is a rectified sinusoidal input
voltage
Equipment
AC power supply, 180 Ω, 390 Ω and 1.0 kΩ, 10 µF, 100 µF and 1.0 mF capacitors
Tasks
1)
Construct the follow circuit, commencing with the 180 Ω resistor in parallel with the 10 µF
capacitor.
CRO
active side
6 V RMS
red
black
neutral side
The following tasks can be written up in the table on the following page.
2)
Connect a CRO across the resistor and adjust the screen until a clear picture is obtained. Set the
CRO to DC mode. Sketch a graph of the voltage across the resistor. Label the sketch “voltage
across the resistor with R =180 Ω and C = 10 µF”.
3)
4)
5)
6)
7)
Calculate the time constant τ = RC = 180 × 10 × 10-6 second = ____________ s.
Turn the supply off and change the capacitor to the 100 µF one and repeat steps 2 and 3, with the
power supply turned back on.
Turn the supply off and change the capacitor to the 1000 µF (ie 1.0 mF) one and repeat steps 2 and
3, with the power supply turned back on.
What do you notice about the change in shape of the graph of the voltage across the resistor as you
increase the capacitance of the capacitor?
For each of these three experiments calculate the fraction

T
where τ is the time constant and T is
the period of the supply, in this case T = 0.020 s or 20 ms.
8)
What can you say about the shape the graphs as the fraction increases?
9)
How might you design a smooth DC voltage if you only had an AC supply?
Table for results
C (µF)
R (Ω)
10
180
100
180
1000
180
10
390
100
390
τ =RC (s)
/T
Sketch of voltage across load resistor
C (µF)
R (Ω)
1000
390
10
1000
100
1000
1000
1000
τ =RC (s)
/T
Sketch of voltage across load resistor
Prac 5: Full rectification using a 4-way diode bridge rectifier
Aims
To investigate the effect of a 4-way diode bridge rectifier on an AC supply voltage.
Equipment
1 AC power supply, 4 diodes, 1.0 kΩ resistor, 1 CRO
active
1.0 kΩ
Tasks
1)
2)
3)
Construct the circuit below carefully. You are going to
make a 4-bridge rectifier from 4 diodes. They need to
be connected up the correct way. Make the rectifier first
then attach it to the power supply and load resistor.Do
not dismantle this circuit as you will use it in the next
Practical exercise - Prac 5.
6.0 V RMS
neutral, 0 V
Vsupply
Attach the CRO to the output of the supply voltage and
adjust the CRO to obtain a clear picture. Sketch a graph
of vsupply(t), which should be a simple sinusoidal
function.
Without adjusting the CRO attach the terminals
across the 1.0 kΩ resistor and sketch a graph of
the voltage across the resistor. [Remember to
designate one of the two terminals of the power
supply as the active and use the red cable on the
CRO as the positive terminal]
t
VR
t
4)
Describe the graph of the voltage across the resistor and how it is different to the voltage of the
power supply. [recall that the voltage drop across a diode in forward biased is approximately 0.70
V]
5)
During the half-cycle when the active
voltage is positive, ie greater than 0 V
draw in red the direction that
conventional current takes. During
the second half cycle when the active is
negative, ie less than zero, draw in
blue the direction that conventional
active
1.0 kΩ
6.0 V RMS
neutral, 0 V
current takes.
6)
Which way does the current pass through the 1.0 kΩ? Does this make sense with the CRO trace
that you observe when measuring the voltage across the resistor?
Prac 6: Full rectification and capacitor smoothing of an AC
supply
Aims
In this prac we are going to now fully rectify (fully sick!) an AC voltage and then smooth it with a
capacitor.
Equipment
1 AC power supply, 4 diodes, 1.0 kΩ resistor, 390 kΩ resistor,
1 CRO, 10 µF, 100 µF, 1.0 mF capacitors
capacitor
1.0 kΩ
Tasks
1)
2)
3)
Construct the circuit below carefully. You are going to
6.0 V RMS
make a 4-bridge rectifier from 4 diodes. They need to
be connected up the correct way. Hopefully you have
still got your set-up from the previous practical task.
Make the rectifier first then attach it to the power supply and load resistor.
Wire in the 10 µF capacitor but do not connect to the active line yet.
Attach the terminals of the CRO across the 1.0 kΩ resistor and adjust the CRO to obtain a clear
picture. Set the CRO to DC mode.
Sketch a graph of the voltage across the resistor in the table provided. [Remember the terminal of
the power supply as the active and use the red cable on the CRO as the positive terminal].
Turn off the voltage supply between each of the following tasks
4)
Repeat the procedure with various capacitors now connected. Calculate the time constant for the
circuit and sketch a graph of voltage across the resistor.
Resistor
Capacitor
Time Constant (s)
/TAC input
1.0k
NONE
N/A
N/A
1.0k
10F
1.0k
100F
1.0k
1.0mF
Sketch of Voltage across R
390
100F
Prac 7: Ripple voltage in a DC supply – a class demonstration
prac
Aims
To illustrate the variation in the amount of voltage-ripple when the resistance of a load is systematically
altered.
The supply voltage used is a commercial power supply with internal transforming, rectification and
smoothing.
A typical unit is shown below.
transformer
resetable fuse
4-way bridge rectifier
smoothing capacitors
Equipment
1 old power supply, ammeter, rheostat, CRO, 1 large capacitor.
Tasks
1)
Draw a circuit diagram of the experimental setup you would construct.
2)
Your teacher will vary the load resistance using a rheostat and in the table below you will record the
peak-to-peak ripple in the supply voltage using a CRO versus the load current using an ammeter.
The power supply will be set on 10 V DC.
Vripple
load current
(A)
Supply Vperk-to-peak ripple (mV)
3)
Comment on what happens to the size of the ripple in the supply voltage as the load resistance
decreases and hence the load current increases.
4)
State one way in which the peak-to-peak value of the ripple in the supply voltage could be reduced.
Return to the first load current setting and then turn the power supply off.
5)
The power pack will have an external capacitor placed across its terminals. State the size of the
capacitor used.
6)
Now turn the power supply back on and record the value of the peak-to-peak ripple voltage with
the extra capacitor in place.
7)
Comment on the changes that take place with the introduction of an external smoothing capacitor.
Prac 8: Zener diodes
Aims
This simple investigation serves to illustrate the use of zener diodes as a voltage regulator.
To understand the meaning of the phrase “avalanche point” and how it relates to the action of a zener
diode.
Introduction
Zener diodes are custom made diodes that have avalanche voltage at desired value. For this reason zener
diodes are always placed in reverse bias in a circuit and are placed in series with a resistor to make yet
again, a voltage divider.
The basic idea is that an unregulated voltage V(t) whose value is greater than the avalanche point of the
zener diode is placed across the series pair, acting as a standard voltage divider. This causes a constant
voltage drops across the zener diode since it is operating at its avalanche point.
Below is the characteristic curve for a zener diode. This type of voltage
regulation is used when the supply voltage has a ripple but the appliance
requires a constant DC level.
Typically appliances in these situations have a large resistance and do
not draw large currents to operate. Keeping in mind that the current to
operate the device must first pass through the resistor in the voltage
divider.
I(A)
avalanche voltage
Remember the avalanche voltage can be varied extensively due to
the manufacturing process when doping the semiconductors.
The 0.70 V cannot be altered however.
The avalanche point can be manufactured to specifications. The zener diodes you are using are
manufactured to have an avalanche voltage at -5.0 V.
Equipment
power supply, 390 Ω resistor, 5 V zener diode, ammeter, voltmeter
Tasks
1)
Design and construct a circuit with a 390 Ω resistor and zener diode as a potential divider. Place the
zener diode in the lower half of the divider with a DC power supply used to power the active rail.
2)
Draw a circuit diagram including the power supply, voltmeter and ammeter.
0.70 V
V(V)
3)
Set the supply voltage to 2, 4, 6, 8 and 10 V, each time recording the voltage across the terminals of
the power supply, the voltage across the zener diode with a voltmeter and the current through the
390 Ω resistor with an ammeter. The results are to be placed in a table like the one below in your
prac work book.
nominal voltage
of power supply
(V)
voltage across
power supply
(V)
voltage across
zener diode
(V)
current in 390 Ω
resistor
(mA)
calculated voltage
drop across 390 Ω
resistor (V)
2
4
6
8
10
4)
What do you notice about the voltage across the zener diode?
5)
Why is the 390 Ω resistor placed in the circuit?
6)
What is the largest current that this regulated power supply of 5.0V can supply when the nominal
supply is set at 6.0V?
Prac 9: Using the 7805 IC voltage regulator
Aims


To test and use a 7805 voltage regulator.
To learn about miniaturisation and the need for heat sinks in small high current systems.
Introduction
When a stable voltage and a large (I > 1.0 A) current is required, more complicated power supplies are
used. In the last 30 years with the advent of semi-conductors and the miniaturisation that has accompanied
this, small but robust stable power supplies have become common place. The 7805 regulated power supply
is an example of this.
As electrical systems have become smaller, the heat they generate is dispersed into a smaller volume. The
use of passive and active heat sinks has grown as a result of this miniaturisation.
Equipment
7805 regulator, power supply, 10Ω and 5Ω resistors, CRO.
Tasks
In each case, keep the power supplies only turned ON for a short while (< 10 seconds) otherwise
the 7805 units can get hot.
1)
Identify, using a sketch, the input, ground and output terminals of the 7805 regulator.
2)
Connect up the circuit below using the 7805. Ensure that the power is off.
7805
10 Ω
power
supply
3)
CRO
Turn the power supply to 4 V nominal and use the CRO (in DC mode) to observe the voltage
across the 10Ω resistor and the voltage across the power supply. Sketch the two traces.
Vsupply
VR
t
4)
t
Now turn the power supply to 6 V nominal and use the CRO to observe the voltage across the
10Ω resistor and the power supply. Again, compare the two traces.
Vsupply
VR
t
5)
t
Turn off the power supply and replace the 10 Ω resistor with a 5 Ω resistor. With the power
supply still set at 6 V nominal, observe the voltage trace across the 5 Ω resistor. Compare this trace
to the trace obtained with the 10 Ω resistor.
Vsupply
VR
t
6)
Is there any significant difference between the two traces?
t