Transcript First Year Lab Introductory Electronics

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 uncertainties
• 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
• Protoboard
• Wire clippers
• Resistors/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
TTi Power Supply
• Meter
– Displays output voltage &
current
• Buttons:
– On/off
• Knobs
– Coarse and fine voltage
adjustment
– Current limit
• Connectors
– +V
– -V
– Ground !??
On
Digital multimeter
002.734
• Buttons
• Accuracy depends on:
– Range (specified in manual)
– On/off
• Connectors
– Measurement type – How recently it was calibrated
– Common
• Assume 0.5% + 1 digits
• Current
• Voltage
– Volts/ohms
– 2.738 reading has error
• Resistance
– Current
– 0.5% =
0.014
– Measurement range
• High (<10A)
– 1 in last digit = 0.001
• Low (<1A)
– 2.738 ± 0.015
Use your digital multimeter to meaure the
voltage on your benchtop power supply
• Set power supply to give 5V output
• Set multimeter to “DC V”
• 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
• Find the 1kΩ resistor 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 protoboard 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 protoboard 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 protoboard 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 protoboard 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
+
A
•When measuring current what do you assume
about the resistance of the ammeter?
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 - include estimates of the error in your measurement
I (/mA)
V (/V)
R=V/I (/)
Rmeas (/)
4.75±0.02
4.78±0.02
1006±6
1001±4
…
…
…
…
…
…
…
…
• Calculate resistance from measured I and V
• Compare to multimeter measured value of Rmeas
Tip: Formulae for combining uncertainties are summarised in
the inside rear cover of the lab manual.
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:
• 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 AC
• 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
t/ms
t
» Press AT/NM button
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/NM button
• Check to see what the ‘slope’
button does
10 ms/div
Screenshot
Trigger mode settings
• Can trigger off the signal applied to
the channel.
– Auto Trigger (AT) always ensures the
trigger level never exceeds the amplitude
of the waveform. Always results in a trace.
– Normal Trigger (NM) allows an arbitary
trigger level to be set. If greater than the
amplitude of the waveform the screen will
remain blank.
• 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
More trigger mode settings
• AC trigger mode supports Auto Trigger
and suitable for signals >20Hz.
• DC trigger suitable for signals <20Hz,
only Normal Mode trigger is supported.
• LF trigger is for low-frequency triggering
(<1.5kHz) and used if the signal is noisy.
• TV trigger is used for synchronising to
video signals.
• Line trigger (~) triggers from the mains
frequency. Useful for seeing if a ‘noise
signal’ is correlated with mains
frequency. To activate the line trigger depress both the AT/NM and ALT
buttons. Hold the red banana plug and wave your other
hand close to mains cables. The oscilloscope should start
triggering off the signal that is picked up from your body!
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
Ro
~
• All instruments possess an effective
resistance, known as impedance when
dealing with AC signals.
• The Function generator and
Oscilloscope contain complex
electronics, but we can approximate the
interior electronics with an effective
resistance.
• 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
• Voltage will divide according to the
potential divider VL=Vo RL/(Ro+RL)
Vo
VL
RL
I
Signal generator Oscilloscope
Input impedance of headphones
Adjust the function generator to give a
4 V p-p 1 kHz signal
• Insert headphones into the circuit
• What has happened to the signal
voltage!!?
• Connect the headphone jack to the
multi-meter and measure the
headphone resistance …if you do
not measure about 32 then you
are not measuring the right thing!
(check the jack plug)
• Can you explain why the voltage
measured on the oscilloscope
drops when the headphones are
connected?
600
V0
~
Function
generator
VH
RL
Scope