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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.

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Presentation on theme: "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."— Presentation transcript:

1 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)

2 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

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 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). Conductors Insulators BNC cable Cross-section

4 TTi Power Supply On Meter –Displays output voltage & current Buttons: –On/off Knobs –Coarse and fine voltage adjustment –Current limit Connectors –+V –-V –Ground !??

5 Digital multimeter Buttons –On/off –Measurement type Current Voltage Resistance –Measurement range Connectors –Common –Volts/ohms –Current High (<10A) Low (<1A) Accuracy depends on: –Range (specified in manual) –How recently it was calibrated Assume 0.5% + 1 digits –2.738 reading has error –0.5% = –1 in last digit = –2.738 ±

6 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?

7 Measuring resistance Find the 1kΩ resistor 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? Resistor colour code a b c d a.1st digit b.2nd digit c.Power of 10 d.Tolerance (accuracy) 0.Black 1.Brown 2.Red 3.Orange 4.Yellow 10% Silver 5.Green 6.Blue 7.Violet 8.Grey 9.White 5% Gold 47k  ±10%

8 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

9 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

10 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

11 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

12 Checking Ohm’s Law Power supply R + A When measuring current what do you assume about the resistance of the ammeter?

13 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 Calculate resistance from measured I and V Compare to multimeter measured value of R meas I (/mA)V (/V) R=V/I (/  )R meas (/  ) 4.75± ± ±61001±4 ………… ………… Tip: Formulae for combining uncertainties are summarised in the inside rear cover of the lab manual.

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

15 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)

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

17 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 Y plates X plates Phosphor screen Electron gun t VxVx

18 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 AC 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 »Press AT/NM button 2 V/V t/ms t VxVx Screenshot

19 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 t VxVx wait 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

20 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 –You may need to press the AT/NM button Check to see what the ‘slope’ button does 10 ms/div 0.5 V/div Screenshot 2 V/V t/ms Trigger point Edge of screen for chosen timebase Trigger point Wait time

21 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

22 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!

23 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

24 Output/Input resistances 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 V 0 in series with output resistance R 0 Equivalent circuit for oscilloscope shown on right with input impedance R 0 Voltage will divide according to the potential divider V L =V o R L /(R o +R L ) VoVo ~ RoRo RLRL VLVL Signal generator Oscilloscope I

25 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? Function generator VHVH V0V0 ~ 600  RLRL Scope


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