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First Year Lab Introductory Electronics • • • 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 0.1F 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/(2fC) – 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