Lab Assignment 3: Resonance and Diodes

Transcription

1 Physics 105, Analog Electronics Page 1 Lab Assignment 3: esonance and Diodes eadg: Meyer Chapter 4 (Semiconductors and Diodes) First lab day for the week: Parts 1, 2 Second lab day: Parts 3, 4 PELAB Part 1 Figure 1a C L Figure 1b C L GND r (resistance of ductor) GND The circuit shown Figure 1a is similar to Fig 3.15 Meyer. As derived class, the transfer function G (v out /v ) for Figure 1a is: ( ) (i) Figure 1b is the same circuit, with the addition of the herent series resistance of the ductor shown explicitly as r ; real ductors are made of wire which has a fite resistivity; hence there can be a small resistance, r, the ductor. The transfer function, accountg for the resistance, r, of the ductor, is r G 1 2 ( L) r 2 1 j C L ( r 2 / ) L 1 (ii) You can easily verify that (ii) reduces to (i) when r = 0 A. For L = 10mH, C=0.01uF and = 100k, use Eq (i) to generate a predicted Bode plot of ga, G(dB) vs. log f (Hz) for the case where r = 0. Identify: a. the resonant frequency, f 0, b. 3dB pots, defed as -3dB below the maximum ga, G max. c. Q of the circuit f 0 / f 3dB B. Generate a phase plot vs. log f, identifyg the phase shift at the resonant frequency, f 0, and for frequencies far above and far below the resonant frequency. C. Now, use Eq (ii) to predict what will happen to this pair of plots when you assume r = 25, and under the followg conditions (i.e., fill out the table below). Thk carefully about the low and high frequency limits.

2 Physics 105, Analog Electronics Page 2 r L C f 0 Hz) G f 0 f f f 0 (ideal) 100 k 10 mh 0.01uF k 10 mh 0.01uF mh 0.01uF k 1 mh 0.1 uf emember that = angular frequency rad/sec, and f = frequency Hz. Part 2.1: The circuit Fig 3 (page 3 of this handout) is driven by a 10(peak) se wave. Sketch the predicted waveforms of both diode and, usg i) a Si diode, d =0.6, and ii) a zener diode, z = 5.1. Also predict the peak current the circuit. Part 2.2: Part 3: Part 4: A 10 volt (peak) se wave is to be used to light a red LED with a forward voltage drop of 1.9. A resistance placed series with the LED will be used to control the current. Approximately What resistor values are necessary to drive the LED with 30 ma (peak)? 60 ma? In the circuit of Figure 4, predict the maximum values of x and y (referenced to ground). Predict the maximum current to with 10%. Assume a 0.6 drop the forward direction for the photodiode. For the power supply Figure 5 of this lab, predict: the discharge time constant, max. DC output, AC ripple amplitude, and percent ripple for both a 15 μf and 470 μf capacitor. HOMEWOK 1. Derive Eq ii. That s all, folks.

3 Physics 105, Analog Electronics Page 3 LAB Figure 2 (repeat of Fig 1b) PAT 1. LC esonance (50 pts) Lab Note: Measure, C and L on the LC meter the lab; also the series resistance, r, of the ductor. C L 1.0 Update your Prelab table with your actual values of, L and C. r (resistance of ductor) 1.1 Measure the attenuation (db) and phase shift vs. log(f) for the LC circuit Fig 2. Use these values: = 100k, C =.01 uf, L = 10mH. Take sufficient data to plot on the predicted Bode plot and phase plot from Prelab. GND Submit: a. A pair of plots with both predicted and measured i) ga, G, and ii) phase shifts b. Determe the Q of the circuit, Q ( f 0 / f 3dB ). c. Compare with the predicted results from your Prelab table (for this set of component values). d. Expla, usg Eq (ii), why a fite resistance the non-ideal ductor, r, causes G max to be < 1 (or < 0dB, if you prefer) 1.2 Change to 100 ohm. a. Note the changes G max, f 0, f 3dB, and Q (don t need to plot this). Compare with Prelab prediction. b. Expla what happened to Q, usg the derivation of Q shown class. c. As 1d, expla why G max is < 1 ( or < 0dB, if you prefer) 1.3 eturn to = 100k, change L to 1 mh, C to 0.1 uf. a. Note the changes G max, f 0, f 3dB, and Q (don t need to plot this). Compare with Prelab prediction. b. Expla what happened to Q, usg the derivation of Q shown class. c. As 1d, expla why G max is < 1 ( or < 0dB, if you prefer) Moral of the story: eal ductors have non-ideal properties that can make a BIG difference. It turns out real ductors also have a capacitance associated with them. But now you see how you can account for these non-idealities.

4 Physics 105, Analog Electronics Page 4 Part 2: Diode esponse to AC (45 pts) D D FIGUE 3A FIGUE 3B Part 2.1: Half-wave ectifier Drive the circuits of Figure 3A and 3B with a 10 (peak), 1 khz se wave. Use a BNC tee at the function generator to split the signal so that one coaxial cable provides put to the circuit while the other cable allows you to view that signal on the first channel of the scope. Use a 1N914 diode and = 1k (what is the function of?). Look at both the voltage r across the resistor (Fig 3A) and the voltage d across the diode (Figure 3B) usg the oscilloscope. Acquire these waveforms to Excel on the computer. Note that one side of an oscilloscope is ALWAYS connected to ground, so you can t measure a directly unless one side of the component is already at ground, as with the resistor Fig 3A ( r.= ). For d swap the location of the two components, resistor and diode, and then read d directly. a. Submit: Show one period of each of d (t) and r (t) on the same plot. Note important voltages detail: the peak voltages and the behavior around the pots where the diode turns on and off. b. Confirm that Kirchhoff s voltage law is always satisfied by addg the two waveforms. You may need to align the waveforms, as the scope might trigger differently for each measurement. c. From your r waveform determe the peak current the circuit. Compare with Prelab calculation. Now replace the diode with a 1N4733 zener diode, with a z = 5.1 (see the spec sheets the lab), and acquire a waveform of the diode voltage. d. Submit your waveform. On this plot, show the zener voltage z and confirm that it meets the manufacturer s specs. Also confirm that the forward voltage drop, d = 0.6 Part 2.2: LEDs epeat the circuit of Fig 3B usg a clear eally Bright ed LED.. Care and feedg of LEDs: The cathode end of the device is often identified some way on LEDs, just as it is marked by a band on regular signal diodes. This lead is usually dicated by a flat edge on the plastic capsule closest to the cathode. And often, the leads are different lengths. If you can t figure out which end is which, use the diode function on your DMM to determe this. As with other diodes, you should never connect them directly across a voltage source. They should always be used series with a current-limitg resistor, or more generally, powered by a current source. a. Acquire one cycle of the waveform, and dicate d, the forward voltage drop. b. What is the peak current seen by the LED (measured from your plot)? c. To get the LED to turn on even brighter, calculate the value of (stead of 1k) to get approximately 60 ma peak through the LED usg the 10 peak se wave put. The LED housg is a lens so you need to look down the axis of the housg. d. Use this value of the circuit and repeat steps a and b.

5 Physics 105, Analog Electronics Page 5 The wavelength (and hence, energy) of photons from an LED are directly related to the energy lost by charge carriers as they overcome the diode voltage drop. Sce red photons are the lowest energy visible photons, red LED s require the lowest forward voltage drop. If the LED housg itself is colored, you may not see this effect, but clear LEDs will exhibit this. e. Estimate the true wavelength of your red LED by comparg your d to the diode drops of the set of calibrated colored LEDs (appended to this lab writeup; these were taken from LED s mounted on a small patch panel sittg at the curve tracer the lab). Infrared (I) LED s used remote controls require even lower voltage drops, which is why some remotes can be powered by a 1.5 battery. PAT 3: Photodiodes (35 pts) A photodiode changes its conduction characteristics response to comg photons. When used without external bias, it is the photovoltaic mode and is known as a solar cell. Usg external bias is called the photoconductive mode, and such a photodiode is used as a light (photon) detector. Your goal this part of the lab is to observe the effect of light on the i- curve of the photodiode operated the photoconductive mode. To learn more about scopesmanship, you ll measure the i- curve yet another way: connect the photodiode ( cyldrical case with lens and BNC connection) this circuit, and use the scope the XY mode. The scope will display the i- curve directly. Check the polarity of the BNC connector usg the diode function on the DMM. Use a small desk lamp as the light source. Figure 4 a. Which channel on the scope measures d, and which measures i d? b. How do you convert the scope voltage readg to (be quantitative)? The function of the 10 resistor is to "measure" the current the circuit by readg the voltage across it. This is a standard way to measure, or sense, a current electronic circuits. You may also thk of a resistor as a simple current-to-voltage converter. c. Why is the 10 resistor so small? d. Drive the photodiode with a 10 peak se wave from a function generator at 1 Hz make sure you see it trace the exponential i- curve. Then crease the frequency to ~50Hz until you have a contuous curve. The computer acquisition doesn t work XY mode, so hand sketch i- curve waveforms both with and without light shg on the photodiode, while varyg the light level. It s possible to give the photodiode too much light. This is called saturation and is dicated by the i- curve becomg a straight le. Make sure your photodiode is not saturated. e. Submit one I- curve showg the photodiode both with and without light on one hand sketch. What is d both cases? f. From the scope readg, calculate the maximum value of i d. Compare with Prelab prediction. g. What quantity is proportional to the tensity of the comg light? Your i- curves should reveal this.

6 +_ +_ Department of Physics, Stanford University Lab 3.12b Physics 105, Analog Electronics Page 6 PAT 4: Power Supplies (50 pts) Lab details: Use a 6.3 AC filament transformer. This is an rms value the peak voltage is 6.3 Measure the peak voltage of this with the scope (do this while the transformer secondary is disconnected from the circuit). Don t worry if it s a volt or two higher than you expect. use the AC and DC put couplg on the scope--this will make the ripple measurement much easier. 4.1 Full wave rectifier: Build the circuit of Fig. 5, usg the DF02. Measure output on the scope: a. Submit output waveform. b. Measure peak voltage. c. There will be flat regions near zero volts. Measure their duration and expla. 110 AC 6.3 AC Figure 5 2.2k 4.2 Add electrolytic capacitor for pulsatg DC: use both 15uF (anythg is ok), and 470uF. +_ d. Submit two output waveforms usg both the 110 AC 6.3 AC 15uF capacitor and 470uF capacitor (or greater). 2.2k e. (Prelab) calculate expected ripple amplitude and max DC level with both a 15uF and 470uF capacitor f. measure the actual ripple amplitude, max DC level and % ripple with both capacitors. Compare with calculated (Prelab). Figure Now, you will build a regulated power supply, which means a voltage output dependent of load. One of the most common uses of a zener diode is voltage regulation. To make a regulated supply, use a reverse-biased 5.1 zener diode as shown below. Use a 15uF capacitor. Instead of 4 diodes, use a bridge rectifier. The part number for this is the DF02. This is an IC that contas the 4 diodes arranged a bridge, side one convenient package with 4 termals: 2 for the AC (~), 2 for the pulsatg DC out (+, -). Use the "trench" your breadboard.. A datasheet for the DF02 is posted on Coursework. 110 AC 6.3 AC A 470 B z = DC 2.2k g. Submit waveforms at pots A and B the circuit. h. From waveform plots, measure % ripple at both pots A and B. Compare with Prelab calculations (you did the Prelab without accountg for the zener) any improvement? i. From this, describe the operation of the zener diode regulatg voltage. If it's still not clear to you, try removg the capacitor from the circuit, and see what happens j. What s the function of the 470 resistor?