Student : ……………………………

XXXSchool

PHYSICS

Unit 3

Electronics and photonics

2013

Index

1.Using the Multimeter and reviewing DC circuits 3

2. The LDR and Thermistor9

3. Learning to use the CRO13

4. Diodes15

5. LED18

6. Photodiodes21

7. Capacitors24

8. DC power supplies27

Selected pracs from CamberwellGrammar School

written by Dr Murray Anderson and Dan O'Keeffe

Resistor Colour Code

Resistors used in electronic circuits are so small that it is difficult to label them with their resistance value. Instead, a colour code is used, giving not only the resistance, but also the accuracy of the stated value of the resistor (its tolerance).

The first coloured band gives the value of the first digit, the second band the second digit. The third band indicates the number of zeros that are to be added to the first two numbers to give the value of the resistance in ohms. The fourth band shows whether the value of the actual resistance is within 5% or 10% of the stated value.

1st colour band
1st digit / 2nd colour band
2nd digit / 3rd colour band
number of zeros / 4th colour band
tolerance
black 0 / black0 / black no zeros
brown 1 / brown 1 / brown one zero
red 2 / red 2 / red two zeros / gold 5%
orange 3 / orange 3 / orange three zeros / silver 10%
yellow 4 / yellow 4 / yellow four zeros
green 5 / green 5 / green five zeros
blue 6 / blue 6 / blue six zeros
violet 7 / violet 7 / violet seven zeros
grey 8 / grey 8 / grey eight zeros
white 9 / white 9 / white nine zeros

Example: green blue orange

silver

1st colured band: green (5)

2nd coloured band: blue (6)

3rd coloured band: orange (three zeroes)

R = 56 000  = 56 k

Electronics Prac 1: Using the Multimeter and reviewing DC circuits

Aims

1) to review Year 11 knowledge by constructing a simple circuit

2) to use a multimeter to make resistance, voltage and current measurements

3) to compare experimental results with theory calculations

4) to get you to record your work for each prac in a systematic way so that you can refer to this work later

5) to write up your work as you go, not after the event

Equipment

1 × 6 volt dry cell8 leads

1 switch56 Ω, 68 Ω and 180 Ω resistors

1 multimeter

Tasks

1)Using the multimeter set on resistance measurement, measure the resistance of each of the 3 resistors. Write these values in the “measured” column and record their accuracy compared to stated value.

2)Calculate the total resistance of the circuit using your measured values of the resistors used.

3)Construct the circuit shown below.

Check your circuit with your teacher before proceeding.

Have the switch closed only when taking measurements.

4)VOLTAGE: Close the switch and measure the voltage across the battery, R1, R2 and R3 respectively using a multimeter set to voltage measurement, selecting an appropriate scale – typically the maximum 20 V range is best.Use the notation V56Ω to stand for the voltage across the 56Ω resistor and Vsupplyfor the voltage gain across the battery.Record your results in the table below (Measured)

5)CURRENT: Using the multimeter set to measure current through each of the 3 resistors. You will have to place the multimeter in series with each resistor separately.Use the notation I56Ω to stand for the current passing through the 56 Ω resistor.Record your results in the table below (Measured)

6)Now, you should have 4 voltage and 3 current measurements. This next task is to use DC circuit theory to compare theoretical measurements of voltage drops and currents through each of the 3 resistors.

  1. Use the total resistance and supply voltage to calculate the supply current from the battery.
  2. Work out the remaining quantities, I56Ω, V56Ω and I68Ω, V68Ω and I100Ω, V100Ω based on circuit theory from Year 11. Record your results in the table below (Calculated)
  3. Calculate the power absorbed in each of the 3 resistors.Use the notation P56Ω to stand for the power absorbed in the 56 Ω resistor for example.Record your results in the table below (Calculated)

Table of Results

Quantity / Measured / Calculated
“56 Ω” / Accuracy:
“68 Ω” / Accuracy:
“180 Ω” / Accuracy:
The total resistance of the circuit is ______
Vsupply
V56Ω
V68Ω
V180Ω
Isupply
I56Ω
I56Ω
I180Ω
P56Ω / N/A

Theory

Vsupply = VA + VB (A and B in series)

VA = VB (A and B in parallel)

Rtotal = RA + RB +…. (A and B in series)

(2 resistors A and B in parallel)

Isupply =

Voltage distribution in series: with IA = IB; voltage is distributed in direct proportion to resistance, the bigger the resistor the larger the voltage drop across it.

For example:

and and hence by substitution

V and hence V.

Current distribution in parallel: with VA = VB; current is distributed in inverse proportion to resistance, the bigger the resistance the smaller the current passing though it

For example

I5 = = mA and hence .

DC Circuit Problems

Problem A

Sally, Mustapha, Alexis and Wang-Chow have just finished wiring up an electrical circuit. They sincerely hope that it will work. It consists of a 12V battery and four globes A, B, C & D that they bought at a local hardware shop. Below is a diagram of the circuit that they have connected.

Globes A and D have a resistance of 4  each, while globes B & C each have a resistance of 24 Ω.

1)Determine the total resistance of the circuit

2)Calculate the current in light globe D

3)With the aid of calculations, list the globes in order of brightness

Problem B

A set of 60 party-lights run off a 240 V power supply. Each of the 3 rows of lights contains 20 globes. Below is a circuit diagram. The light globes are all identical and three globes X, M and N are labelled.

1)What is the voltage across each globe?

The power absorbed by each globe is 2.0W

2)What is the current in each of the light globes?

3)Calculate the resistance of the light globe marked X?

4)Calculate the power transferred from the power supply.

During the night, the light globe marked X breaks.

5)Describe what happens to the brightness of each of the globes marked M and N.

Problem C (Extension Only – This is HARD)

  • Alexis and Wang-Chow have become tired of their simple circuits and decide to head to the dodgem cars.
  • They board two different vehicles, each of which is equipped with a motor of resistance 4.0 Ω. The overhead line supplies 240 V DC and the ground is earthed at 0 V.
  • The upper supply line to the cars has a resistance of 0.025Ω/m
  • The lower supply line is earthed – effectively R = 0.
  • The circuit can be modelled as shown below:

1)Determine the ratio of Vm/Vw

2)Determine the ratio of Pm/Pw (the power dissipated by each motor)

Answers

Problem A

1) Rtotal = 31.4Ω

2) ID = 0.33A

3) C=0.07W, D=0.42W, A=0.58W, B=3.49W

Problem B

1) 12V

2) 0.167 A

3) 72 

4) 120 W

5) M is off due to break in circuit, N stays constant.

Problem C (HARD)

1) 1.075 2) 1.16

Electronics Prac 2: The LDR and Thermistor

Aims

1) to review the importance of a voltage divider circuit

2) to experiment with two non-ohmic conductors

Equipment

6 volt dry cell, switch, LDR, Thermistor, variable resistor, 6 cables, multimeter

Background

  • An LDR is a semi-conductor (often cadmium sulphide) whose resistance varies typically from 200 kΩ in darkness to a few kΩ in sunlight in response to the amount of light shone on the active surface.
  • A thermistor is also a variable resistor. At room temperature the resistance is typically in the kilo-ohm range and reduces to a few hundred ohms at temperatures of a few hundred degrees Celsius.
  • In each case both the devices conduct more readily with the input of energy, either light or heat: essentially what happens is that more free electrons are made in the semi-conductor in the presence of light or heat and hence the material conducts better and thus the resistance falls.

Tasks

1)Using a multimeter, measure the resistance of the LDR in darkness and then in the available ambient light in the class-room.

2)Draw a sketch of the device and draw the correct electrical symbol for the device.

Resistance of LDR in darkness / Resistance of LDR in ambient light / Sketch and symbol

3)Using a multimeter, measure the resistance of the thermistor after it has been in hot water for 1 minute.

4)Place the thermistor in icy water and repeat task 2.

5)Draw a sketch of the device and draw the correct electrical symbol for the device.

Resistance of thermistor in hot water / Resistance of thermistor in icy water / Sketch and symbol

3)Construct the following circuit:

Check with your teacher, to see if the circuit is correct.

6)Close the switch S.Measure Vsupply; it should be close to 6 V, but may be a little below it.

7)Adjust the upper resistor so that the output voltage for this voltage divider has the following properties.

  1. The output voltage is less than 1 volt in ambient light and
  2. The output voltage is more than 1.5 volt in darkness

When this is achieved, open the switch to isolate the divider from the active line and do not adjust the dial on the variable resistor.

8)Measure the resistance of the upper resistor with a multimeter.

9)To finish off the prac, you are going to check the voltage divider formula to verify both it and your results.

Here, the voltage divider formula is ; Vsupply = 6.0 V.

Quantity / Calculated/Measured Value
Rupper
RLDR (dark)
Vout (dark)
RLDR (light)
Vout (light)

10)Use your textbook to find out the following words and write definitions in the table below.

Device / Definition
Transducer
Optical – electrical transducer
Electrical – optical transducer

LDR and Thermistor problems

Problem A

Solve for the unknown in each of the following problems. In each case the switch is closed. Use the potential divider formula

Problem B: A thermistor-based voltage divider question

A simple electric thermometer is based on a voltage divider circuit below.The thermistor has a characteristic curve shown below.

1)State the temperature when the resistance of the thermistor is 50 kΩ. ______

The designers of the unit require the voltage input to the display unit to be 4.0 V when the temperature is 20 C.

2)Calculate the value of the resistance R that will achieve this.

3)Using your value for R in Question2, determine the temperature when the voltage across the display unit is 3.0 V.

4)What is the output voltage to the display unit when the temperature is 50 C?

Problem C: Voltage divider problem solving task.

A light dependent resistor or LDR is placed in series with a resistor of resistance R in a potential divider circuit. This is shown in Figure 1.

The table below gives the resistance of the light dependent resistor in light and dark conditions.

light / 0.40 k
dark / 30 k

It is required that the output voltage Vout of the potential divider is

a)greater than 1.5 V in the dark and

b)less than 1.0 V in light conditions.

1) Use calculations and explain your reasoning to choose an appropriate value for R that satisfies both conditions a) and b).

2)Show that your value for R chosen works.

Answers

Problem A 1) 5.16V 2) 35k 3) 116k Problem B 1) 35o 2) 80k 3) 16o 4) 6.9 VProblem C To be marked by teacher!

Electronics Prac 3 – Learning how to use a CRO

Aim

To revise the confident use of a CRO as a voltage measuring device but one that gives a visual display.

Equipment

CRO1 × 6 volt dry cell

1 × power supply1 × multimeter

Background

  • A CRO is just a visual voltmeter which can be used to measure constant and time-varying voltages. If the voltages are periodic, they can be used to measure the period and hence the frequency of the signal. The CRO will give you a graph of voltage as a function of time, v(t). The controls on the CRO translate and stretch the time and voltage axes to obtain a clear picture.
  • As a convention in this lab, make the left hand yellow AC outlet the ACTIVE with the right hand AC outlet the neutral as a default. This will mean that there is only one correct way to connect the CRO to the circuit where the red wire of the co-axial cable is the ACTIVE input for the CRO.

Tasks

1)Your first task is to obtain a horizontal trace on the screen.

a) Switch on the CRO, turn the brightness/ intensity on full and adjust the focus to mid range. Adjust the intensity to produce a small sharp dot.

b)Set the sweep range/ time base to 0.1 kHz or 10 ms/ cm. Soon a horizontal trace will appear.

c)Adjust the Horizontal and Vertical Shift Controls to centre the line and adjust the Focus and Brightness to obtain a clear, sharp line.

2)MEASURING DC VOLTAGES

You are now going to use a dry-cell (6 V) for the CRO to measure a DC voltage.

a)Set the AC/ DC switch on the CRO to DC and the vertical range to 5 V/division (or 2 V/cm on old CRO)

b)Connect the leads of the CRO to the dry cell and describe what happens.

c)Now reverse the leads and describe what happens this time.

3)MEASURING AC VOLTAGES

This time we will use the AC outlets of the power supplies.

a)Set the AC/ DC switch on the CRO to AC. Connect the leads
of the CRO to the AC terminals of the power pack at 10V

b)Adjust the range and time-base controls to stabilise the trace.

c)Draw and describe the pattern. Include a scale on your graph.

Note: 1 millisecond = 1 ms = 1/1000th of a second or 1.0 × 10-3 s.

______

4)From your trace measure the following quantities:

a)The peak voltage______V

b)The peak to peak voltage______V

c)The period of the signal______ms

5)From the results above calculate the following quantities:

a)The RMS voltage______V

b)The frequency of the signal______Hz

6)Now switch the top right dial to the XY position. This turns off the trace so that only the y-axis variation is shown without the t-axis being swept. This is excellent for measuring peak to peak voltage variations. What this does is allows the y-axis variation to be seen without any x-axis; For you Mathematical Methods nuts, it allows the range of the function to be observed without the domain being selected.Draw and describe what you see and label the axes (units)

7)Use a CRO to measure the peak-to-peak voltage and a multimeter to measure the RMS voltage across the power supply. Fill out the table below.Note: Use the AC voltage setting on the multimeter.

Power Pack Setting
(Volts) / Multimeter Reading (Volts) / Calculated
Peak-to-Peak Voltage (Multimeter x 22) / CRO
Peak-to-Peak Voltage (V)
2
4
6
8

8)Comment on the results of the last two columns.

Electronics Prac 4 – Diodes

Introduction

  • Diodes are one of many electronic devices that are made from semiconductors. Semiconductors are Group 4 elements such as Silicon and Germanium that have been doped with small amounts of Group 3 and 5 elements. These group 4 elements in the periodic table form crystals in their pure state.
  • Semiconductors doped with group 3 elements are called p-type semiconductors. Though the material is elecrically neutral, they have insufficient electrons for the number of atoms and hence the material has a virtual positive charge.
  • Semiconductors doped with group 5 elements are called n-type semiconductors. These materials have virtual negative charges.
  • When p-type and n-type are joined a p-n junction is formed and this junction is the heart of the diode and gives it its unique properties. At the join the virtual positive charges neutralise the virtual negative charges to create a thin wafer of material that does not conduct, this wafer is called the depletion zone. Adding a positive voltage (this is called forward biasing) overcomes this and after the applied voltage exceeds about 0.70 V the junction conducts. Adding a voltage in the opposite direction (this is called reverse biasing) only adds to making the wafer thicker and consequentally the diode insulates even better
  • Diodes have the property that they allow current to flow through them in one direction only. This is reflected in the fact their electrical resistance depends on which way they are connected to the circuit.

Diodes are used mostly in converting AC voltages to DC voltages and hence will re-appear later in the course.

Tasks

The aim of this prac is to investigate the electrical properties of a typical diode

1)Testing a Diode

Connect the diode and a light globe in series across a supply voltage of 6 V using a dry-cell battery. Determine which way the diode needs to be placed in the circuit for the light to go on.

Diodes conduct electricity one way(provided the forward bias is great enough) but not the other way.

Using the positive terminal of the battery and the markings on the diode, draw in the space below the circuit diagram for when the light works, showing the polarity of the battery and the orientation of the diode.

2)Voltage-Current Characteristics of a Diode

Sketch the graph of the diode characteristics in Fig 4.12 on page 87 of the text book.

3)Using a Diode in a Circuit

Connect up in series a diode and a 1 kΩ resistor to a DC power supply. Use a multimeter to measure the voltage across both the diode and the 1 kΩ resistor separately for each of the DC voltage settings on the power supply.

DC Supply
Dial setting (V) / DC Supply voltage measured (V) / Voltage across
the Diode (V) / Voltage across
the Resistor (V)
2
4
6
8
10
12

A silicon based semiconductor will have a maximum voltage of about 0.70 V across it whereas a germanium based semiconductor will have a maximum voltage of only 0.10 V across it when forward biased. What type of diode are you working with? ______

Diode Problems