1. First Year Lab Introductory Electronics
A small atomic physics experiment here (015 Blackett)
We are Physicists. Why do electronics?
You will probably also end up using computers!
You may end up using optics too.
Start at 9:05

2. First Year Lab Introductory Electronics
Aims - to introduce…
The equipment
Good lab book keeping
An awareness of measurement and errors
A bit of physics/electronics!
Remember…
To use the demonstrators
To colour code your circuits
Be adventurous and inquisitive with your experimentation

3. Equipment Benchtop Power Supply – Gives DC power
Digital Multimeter – Measures AC/DC voltage levels, resistance
Function Generator – makes sine, square, triangle oscillating waveforms.
Oscilloscope
Breadboard
Wire clippers
Resistors/Capacitors/Wire/Banana-banana wires
Headphones
BNC-banana cables (co-axial, two wires in one cable, a sheath which is usually grounded and a core).
10 mins
Conductors
Insulators
BNC cable
Cross-section

4. Benchtop power supply Meter Buttons: Knobs Connectors
Displays output voltage or current
Buttons:
On/off
Range - 30V/15V
Meter - amps/volts
Knobs
Coarse and fine voltage adjustment
Connectors +V -V
Ground !??

5. TTi Power Supply On

6. Digital multimeter Connectors Accuracy depends on: Buttons
Range (see manual)
How recently it was calibrated
Connectors
Common
Volts/ohms
Current
High (<20A)
Low (<2A)
Buttons
On/off
Measurement type
Current
Voltage
Resistance
Measurement range
Assume 0.5% + 1 digits
2.738 reading has error
0.5% =
1 in last digit = 0.001
2.738 ± 0.015

7. Use your digital multimeter to meaure the voltage on your benchtop power supply
Set power supply to give 10V output
Set multimeter to “DC V” & “20V” range
Connect using banana leads
Do the digital and analog meters agree?
How accurate is each meter?

8. Measuring resistance Resistor colour code
Choose one of the resistors in your component box
Attach banana leads to the common and V/ terminals of your DMM and switch to  mode
Attach your resistor between the other end of the leads using 2 croc clips
Is your resistor within the stated tolerance?
a b c d
47k±10%
1st digit
2nd digit
Power of 10
Tolerance (accuracy)
0. Black
Brown
Red
Orange
Yellow
10% Silver
Green
Blue
Violet
Grey
White
5% Gold

9. The protoboard
Rows and columns of holes on the breadboard are electrically connected
Use your multimeter in resistance mode to check exactly how
Make simple probes:
Banana lead + croc clip
Short length of single strand wire
Need photo of breadboards:
Hand round decapitated breadboard to show connections

10. The protoboard
Rows and columns of holes on the breadboard are electrically connected
Use your multimeter in resistance mode to check exactly how
Make simple probes:
Banana lead + croc clip
Short length of single strand wire
Need photo of breadboards:
Hand round decapitated breadboard to show connections

11. The protoboard
Rows and columns of holes on the breadboard are electrically connected
Use your multimeter in resistance mode to check exactly how
Make simple probes:
Banana lead + croc clip
Short length of single strand wire
Need photo of breadboards:
Hand round decapitated breadboard to show connections

12. The protoboard
Rows and columns of holes on the breadboard are electrically connected
Use your multimeter in resistance mode to check exactly how
Make simple probes:
Banana lead + croc clip
Short length of single strand wire
Need photo of breadboards:
Hand round decapitated breadboard to show connections

13. Checking Ohm’s Law Power supply R + V A
When measuring current what do you assume
about the resistance of the ammeter?
Now measure the voltage across R - what do you assume about the resistance of the voltmeter ?

14. Checking Ohm’s law - what you should have in your lab book
A circuit diagram
Switch meter from “DC V” to “DC A” to measure current I and voltage V for your different resistors
Record values in a table - include estimates of errors
Calculate resistance from measured I and V
Compare to multimeter measured value of Rmeas
I (/mA)
V (/V)
R=V/I (/)
Rmeas (/)
27.1±0.2
14.78±0.08
545±5
548±4 …

15. Voltage Divider
Set up the circuit shown here with two resistors in series
Use the ammeter to measure the current in the circuit – how does it compare with the value you found for the previous circuit?
Would this value change if you placed the ammeter at different points in the circuit? Why?
Can you deduce the rule for resistors in series?
Now measure the voltage across R2 and show that the voltage is given by:
Power supply + R1 I R2 V
node
Ammeter shunt resistance at 20mA range is 10
Voltmeter series resistance on 20V range is 10M

16. Current Divider
Now set up the circuit shown here with resistors R1 and R2 in parallel
Use the ammeter to measure currents I, I1 and I2 – can you deduce the rule for currents at a node?
Can you deduce the rule for resistors in parallel?
Show that the current through R1 is given by:
Power supply + R1 I R2 I1 I2
Ammeter shunt resistance at 20mA range is 10
Voltmeter series resistance on 20V range is 10M

17. Function or signal generator
On/off switch!
Trigger
DC offset
Outputs
Vout
Com/0V
(Ground)
Frequency range (buttons) and value (dial)
Signal shape
Signal amplitude

18. Function generator + headphones
Set the generator to give a 1kHz, 4V peak-to-peak sine wave.
Connect your 3.5mm jack socket to the function generator terminals and plug in the headphones
What does it sound like?
Over what range of frequencies can you hear signals?
Middle C is 262 Hz, what do 131, 524 and 1048 Hz sound like?
An octave in musical terms is a doubling in frequency
How does the volume change when you change the voltage range
Music is logarithmic!
Set the generator to give square and triangle waves
Square and triangle waves contain higher harmonics (multiples of the fundamental frequency)

19. Measuring voltage as a function of time The oscilloscope: like OMG!
Think of groups (horizontal, vertical)
Horizontal = time
Vertical = voltage (2 identical channels)
Channel 1 (vert)
Channel 2 (vert)
Time (horizontal)

20. Oscilloscope Basics e- beam in evacuated tube.
dc voltages applied to X and Y plates deflect e-.
Apply sawtooth voltage in time to X-plates (timebase)
Apply voltage you want to monitor to Y-plates
Electron
gun
X plates
Y plates
Phosphor screen t Vx

21. Exploring (some of) the Controls
Turn on `scope, Set CAL knobs fully clockwise
Set function generator to 4V p-p, 1kHz sinusoidal.
Set ‘trigger’ control to ~ (line)
Check ‘coupling’ is DC, not ground
Input into channel 1 of 'scope (use banana-BNC cable)
Y-sensitivity knob – multi position rotary
Sets ‘volts per division’ vertically, 1div=1cm. Set to 1V/div
Time base knob – multi position rotary
Sets period of saw-tooth, ‘seconds per div’ horizontally. Set to 0.2ms/div
If you see a mess DON’T PANIC
Change ‘trigger’ control to AC 2
V/V
t/ms t Vx
Screenshot

22. Trigger to the rescue!
Reference voltage
source internal to ‘scope,
set by knob on front panel
– ‘Trigger Level’
Comparator – gives out pulse
when inputs are equal
Input voltage
Go signal
to timebase
Input voltage compared with an internally set level – the trigger level
After a single sweep of the screen the e- gun waits
When the input equals the trigger level the next tooth of the sawtooth is executed t Vx
wait

23. Trigger explained Sinusoidally oscillating voltage 4V p-p V/V
For a trigger level at 1.6V, say
As soon as signal goes above 1.6 V the sawtooth triggers
At end of sawtooth, `scope waits for next trigger event
Play with the trigger level and see what effect it has on the leading edge of the waveform
You may need to press the AT/Norm button
Check to see what the +/- or ‘slope’ button does 2
V/V
t/ms
1.6 25
Trigger point
Edge of screen for
chosen timebase
Wait time
10 ms/div
0.5 V/div
Screenshot

24. Trigger Source Can trigger off the signal applied to the channel
Or can trigger off a separate signal – external trigger
e.g. a sig. gen. may simultaneously give out a TTL (square) pulse train and a sinusoid. Use the TTL pulse as an external trigger
Or can trigger from the mains frequency (‘line’ trigger). Useful for seeing if a ‘noise signal’ is correlated with mains frequency.
Plug a BNC-banana connector into the ‘scope
Trigger the ‘scope from line
Hold the positive banana connector between your fingers
Wave your free hand near a mains plug socket
Sketch what you see in your lab book. Explanation?
45 mins

25. Other Notes Cal – ‘Calibrated’ Input Coupling
Change from the calibrated position to make arbitrary sized wave ‘fit’ between grid lines to aid measurement
Input Coupling
Ground – shorts scope input to ground – kills signal, allows you to find 0V and set using Vert Position
DC – the ‘normal’ mode, what you see is what you got
AC – removes any DC component of a signal, useful for seeing a small oscillating voltage on a big DC background

26. Output/Input resistances
Remove headphones - attach oscilloscope to function generator
Equivalent circuit for function generator shown on left: ideal source V0 in series with output resistance R0
Equivalent circuit for oscilloscope shown on right with input impedance R0
Potential divider VL=Vo RL/(Ro+RL)
Calculate VL given:
Oscilloscope input resistance, RL=1M
Stated output resistance on generator, Ro (600)
Check values over range of frequencies (use a sequence to cover 1 kHz to 1 MHz) – do they change?
What do you conclude about the frequency dependence of resistors? Ro Vo ~ VL RL I
Signal generator
Oscilloscope

27. Capacitors
A capacitor consists of two metallic plates separated by an insulating slab
(called a dielectric) + -
With the plates short circuited as shown any
free charge will distribute equally throughout
the circuit.
The inclusion of a battery pushes electrons onto one of the plates (leaving a deficiency of electrons on the other). Such a separation of charge creates an electric field that opposes any further charge transfer and the voltage across the capacitor equals that of the battery. + -

28. Capacitors Why? How can they possibly be useful?+ -
How can they possibly be useful?
They store electrical energy which can be released when required
In ac circuits they allow ac currents to flow
They act like frequency dependent resistors impeding current flow (they have impedance)

29. Impedance of capacitors
Set up the circuit shown on your breadboard using a 0.1F capacitor across the oscilloscope input
Starting with a 4V p-p 1kHz sine-wave output measure VC over the range 1kHz – 1MHz
How does VC compare with V0?
What can you say about the frequency dependence of a capacitor?
Function
generator VC V0 ~
600 RL C
Oscilloscope
Change the function generator to give a square wave output at 1 kHz, 10 kHz and 1 MHz
Sketch your results
Can you explain what you see?

30. Impedance - resistance and reactance
Impedance describes how an electronic device impedes the flow of current in response to an applied voltage
For a resistor the impedance is simply its resistance=R
But a capacitor can also impede the flow of current - its impedance is given by 1/(2fC)
Actually it has an effect on the phase of signals too which you will meet later in terms of complex numbers and complex impedances!
A capacitor impedes lower frequencies more than higher ones

31. Input impedance of headphones
Remove the capacitor and adjust the function generator to give a 4 V p-p 1 kHz signal
Replace the capacitor with the headphones
What has happened to the signal voltage!!?
Connect the headphone jack to the multi-meter and measure the headphone resistance …if you don’t measure about 16 then you aren’t measuring the right thing! (check the jack plug)
Use voltage divider rule to estimate the voltage you should have expected
Function
generator VH V0 ~
600 RL
Scope

32. The impedance of the headphones
Starting with a 4V p-p 1kHz sine-wave output measure VH over the range 1kHz – 1MHz
You should find that the voltage increases
Consult your practical notes on inductors and explain what you see.
Function
generator VH V0 ~
600 RL