Transcript Intro_Elec 2010

First Year Lab
Introductory Electronics
•
•
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We are Physicists. Why do electronics?
You will probably also end up using computers!
You may end up using optics too.
A small atomic
physics
experiment here
(015 Blackett)
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
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).
BNC cable
Cross-section
Conductors
Insulators
Benchtop power supply
• Meter
– Displays output voltage or
current
• Buttons:
– On/off
– Range - 30V/15V
– Meter - amps/volts
• Knobs
– Coarse and fine voltage
adjustment
• Connectors
– +V
– -V
– Ground !??
TTi Power Supply
On
Digital multimeter
• Buttons
• on:
Connectors
• Accuracy depends
– Common
– Range (see manual)
– On/off
Volts/ohms
– How recently it was– calibrated
– Measurement type
– Current
• Current
• Voltage
• Resistance
• Assume 0.5% + 1 digits
• High (<20A)
– 2.738 reading has error• Low (<2A)
0.014
– Measurement range– 0.5% =
– 1 in last digit = 0.001
– 2.738 ± 0.015
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?
Measuring resistance
Resistor colour code
a b c
a.
b.
c.
d.
d
47k±10%• Attach banana leads to the common
and V/ terminals of your DMM and
switch to  mode
1st digit
2nd digit
Power of 10
Tolerance (accuracy)
0. Black
1. Brown
2. Red
3. Orange
4. Yellow
10% Silver
• Choose one of the resistors in your
component box
5. Green
6. Blue
7. Violet
8. Grey
9. White
5% Gold
• Attach your resistor between the other
end of the leads using 2 croc clips
• Is your resistor within the stated
tolerance?
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
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
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
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
Checking Ohm’s Law
Power
supply
R
+
V
A
•When
measuring
do R
you
assume
•Now measure
thecurrent
voltagewhat
across
- what
do you
about the
resistance
of the ammeter?
assume
about
the resistance
of the voltmeter ?
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
I (/mA)
V (/V)
R=V/I (/)
Rmeas (/)
27.1±0.2
14.78±0.08
545±5
548±4
…
…
…
…
…
…
…
…
• Calculate resistance from measured I and V
• Compare to multimeter measured value of Rmeas
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:
I
Power
supply +
R1
R2 V
node
R2
VR 2 
VIN
R1  R2
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:
I
Power
supply +
I2
I1
R1
R2
I1 
I
R1  R2
R2
Function or signal generator
On/off
switch!
Frequency range
(buttons) and
value (dial)
Trigger
DC
offset
Outputs
Vout
Com/0V
(Ground)
Signal
shape
Signal
amplitude
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)
Measuring voltage as a function of time
The oscilloscope: like OMG!
• Think of groups (horizontal, vertical)
• Horizontal = time
• Vertical = voltage (2 identical channels)
Time
(horizontal)
Channel
1 (vert)
Channel
2 (vert)
Oscilloscope Basics
• e- beam in evacuated tube.
• dc voltages applied to X and Y
plates deflect e-.
Electron
gun
X plates
Y plates
• Apply sawtooth voltage in time to Xplates (timebase)
• Apply voltage you want to monitor to
Y-plates
Phosphor screen
Vx
t
Exploring (some of) the Controls
V/V
• Turn on `scope, Set CAL knobs fully
clockwise
2
• 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)
Vx
• 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
t/ms
t
Screenshot
Trigger to the rescue!
• 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
Reference voltage
source internal to ‘scope,
set by knob on front panel
– ‘Trigger Level’
Go signal
to timebase
Input voltage
Comparator – gives out pulse
when inputs are equal
Vx
wait
t
Trigger explained
• Sinusoidally oscillating voltage
4V p-p
• 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
Edge of screen for
chosen timebase Wait time
V/V
2
1.6
t/ms
25
Trigger point
Trigger point
0.5 V/div
– You may need to press the
AT/Norm button
• Check to see what the +/- or
‘slope’ button does
10 ms/div
Screenshot
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?
Other Notes
• Cal – ‘Calibrated’
– 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
Output/Input resistances
– Oscilloscope input resistance, RL=1M
– Stated output resistance on generator,
Ro (600)
Ro
~
• 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:
Vo
VL
RL
I
• Check values over range of frequencies Signal generator Oscilloscope
(use a 1-3-10 sequence to cover 1 kHz
to 1 MHz) – do they change?
• What do you conclude about the
frequency dependence of resistors?
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.
-
+
+
-
+
-
+
-
+
-
+
-
Capacitors
Why?
+
-
+
-
+
-
+
-
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)
Impedance of capacitors
• Set up the circuit shown on
600
your breadboard using a 0.1F
capacitor across the
C
oscilloscope input
VC
RL
V0 ~
• Starting with a 4V p-p 1kHz
sine-wave output measure VC
over the range 1kHz – 1MHz
Function
Oscilloscope
• How does VC compare with V0?
generator
• What can you say about the
frequency dependence of a
•Change the function generator to give a
capacitor?
square wave output at 1 kHz, 10 kHz
and 1 MHz
•Sketch your results
•Can you explain what you see?
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
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
600
V0
~
Function
generator
VH
RL
Scope
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.
600
V0
~
VH
Function
generator
http://electronics.howstuffworks.com/speaker5.htm
RL