Laboratory Instrumentation I

Villanova University

ECE 2053 Fundamentals of Electrical Engineering I Lab

Spring 2012

Exp 6 – Silicon Diodes and LEDs Learning Objectives At the end of this session, you should be able to: 1. Set up a circuit to measure the i-v characteristics of a silicon diode and an LED. 2. Examine the characteristics of a diode half wave rectifier and full wave rectifier circuit.

Introduction Silicon diodes, in discrete form, are devices with two terminals that are similar in size to a ¼ W resistor. They are made of semiconductor material. One distinguishing characteristic of the diode is that current flows through the device only in one direction. The diode is like a one-way valve. The i-v characteristic, unlike that of a resistor, is highly nonlinear. As shown on the i-v graph to the right, the maximum voltage

is around 1.2 volts. If you put 5 volts across the diode, then POOF.

A Little Quiz A silicon pn-junction diode can be described as follows. Current flows through the diode in the direction of the arrowhead, but not in the reverse direction – see circuit below. When current flows through the diode in the direction of the arrowhead we say it is forward biased, and when no current passes through the diode we say it is reverse biased. In this circuit the current labeled ID flows from the + terminal to the  terminal. Under this condition, the voltage, VD, is zero volts if we assume an ideal diode. (a) Calculate the current, ID, through the diode in this circuit and give the answer in milliamperes (mA). Answer __________

Realistically, the voltage across a forward biased diode is not zero. In fact it is approximately 0.7 volts. This is called the forward voltage and is designated with the symbol VF. (b) Calculate the new value of ID through the diode using the forward voltage VF in your calculation. Again, give the answer in milliamperes (mA).

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(c) Compare this calculation with the previous one. What is the error? Answer __________

Answer __________

Exercise 1: I-V Characteristics of a Silicon Diode In Experiment 2, you measured the i-v characteristics of a #47 lamp and a carbon resistor. In this exercise, you will measure the i-v characteristics of a silicon diode. The DVM shown in the circuit in Figure 1 will be used to measure the diode voltage vD and, indirectly, the diode current iD. With the red lead and black connected as shown, the DVM reads the voltage vD . However, when the red lead of the DVM is moved to node C, the DVM measures the voltage, vcs, across the resistor Rcs. This resistor voltage is proportional to the diode current, iD, so the diode current can be computed from Ohm’s law by iD = – vcs / Rcs If resistor Rcs is chosen as 1 kΩ (as shown), then the DVM reading can be directly interpreted as the diode current, in mA.

blk lead

red lead

DVM

D Node A

Node B

+ Vd

-

Vs

Rcs 1k Node C

Figure 1 Circuit to measure the diode i-v characteristics. 1. Review Appendix 1 on silicon diodes. 2. Obtain a silicon diode, and a 1 kΩ resistor whose measured value is very close to 1000 Ω; record its value. 3. Wire the circuit shown in Figure 1 onto your PB by connecting these components across the + and the COM terminals of the +25-volt dc supply as shown in Figure 1. Connect the DVM last.

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4. Perform the following sequence of steps to determine the data for the diode current and the diode voltage. a) Clip the red lead of the DVM to node A as shown in Fig. 1. b) Adjust the dc supply so that the DVM reads 0.75 volt. Record it. c) Move the red lead to node C – don’t move the black lead. Record the voltage reading. This is the voltage across Rcs; it should be a negative value. d) The magnitude of this measurement is the diode current, in mA. Record the current, in mA. Start a new set of readings. e) Move the red lead of the DVM to node A – don’t move the black lead. f) Adjust the dc supply so that the DVM now reads 0.7 volt. Record it. g) Move the red lead to node C – don’t move the black lead. Record the voltage reading. h) The magnitude of this measurement is the diode current in mA. Record the current. 5. Repeat the above sequence of steps for these diode voltages: 0.65, 0.55, 0.45, 0.35, 0.3 volts. Put the data into a table like the table in Appendix 3. 6. Reverse the orientation of the diode and measure the diode current and voltage as above for a diode voltage of 1.0 volts. Note: the reverse diode current is extremely small. 7. Make an accurate plot of your data in your lab notebook with vD on the x-axis and iD on the y-axis. The length of the vD axis should be around 4 inches; the scale goes from − 1 to + 2 volts. The length of the iD axis should be around 4 inches; the scale starts at 0.

Exercise 2: I-V Characteristics of an LED An LED is a silicon diode that emits light; hence, LED means light emitting diode. The i-v characteristics are similar to a silicon diode. 1. Review Appendix 2 for information on the LED. 2. Obtain a red or green LED. Replace the silicon diode in Figure 1 with the LED – make sure the P-side (anode) of the LED is at node A. 3. Measure and record current and voltage using the procedure in Exercise 1; however, start the process with an LED voltage of 2.5 volts. If the LED does not light, reverse the LED. Repeat the sequence of steps for these LED voltages: 2.0, 1.5, 1.0, 0.5 volts. Note the voltage when the LED goes out. 4. Place the data into a table like the table in Appendix 3 5. Make an accurate plot of your data in your lab notebook.

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Exercise 3: DC Diode Circuit 1. Build the series circuit shown in Figure 2 on your PB. The 2.7 kΩ resistor is selected to limit the current through the diode. Measure the resistor. The voltage source will be initially set to 1.5 volts. Make sure the physical orientation of the diode is in the forward biased direction. A

B

Silicon Diode D + Vd

Vs

-

+ VR

1.5 V

R 2.7Kohm

0

Figure 2 2. Measure and record all voltages. 3. Calculate the diode current using the resistor voltage and Ohm’s law. 4. Change the voltage source to 2.5 V. Repeat steps 2 and 3. 5. Change the voltage source to 15 V. Repeat steps 2 and 3.

Exercise 4: DC LED Circuit 1. Replace the silicon diode with the LED as shown in Figure 3. Make sure the physical orientation of the LED is in the forward biased direction. Initially set the voltage source to 1.5 volts. A

B

LED + Vd

-

Vs

R 2.7Kohm

1.5 V 0

Figure 3 2. Measure and record all voltages. Note if the LED is lit. 3. Calculate the diode current using the resistor voltage and Ohm’s law. 4. Change the voltage source, first to 2.5 V, and then to 15 V. Repeat steps 2 and 3.

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Exercise 5: Half-Wave Rectifier Diode Circuit with Sinusoidal Input 1. Replace the dc source with the function generator (FG), and replace the LED with the silicon diode as shown in Figure 4. A

+ Vd 8 V pk-pk sine wave 0 V offset

B

Silicon Diode D -

+ VR

Vs

-

R 2.7Kohm Load

0

Figure 4 Half wave rectifier 2. Set the FG according to the procedure in Appendix 4. Set the FG for a 1 kHz sine wave. 3. Turn on the oscilloscope. Connect the oscilloscope’s CH 1 probe to node A and CH 2 probe to node B (the output). Connect the ground clips to node 0 (ground). 4. Set the FG amplitude to 8 volts peak-to-peak. With both waveforms displayed, you should observe that the output waveform has its bottom wave clipped off. In other words, the circuit cuts off half of the wave – the circuit is called a half wave rectifier circuit. 5. Sketch both waveforms, noting scales on the horizontal and vertical axes. 6. To observe the waveform across the diode, interchange the resistor and diode as shown in Figure. 5. Connect CH1 probe to node A and CH2 probe to node B. The ground clips go to ground as before. A

8 V pk-pk sine wave

Vs

R

B

2.7Kohm

+ Vd

0 V offset

Silicon Diode D

0

Figure 5 7. With both waveforms displayed, note how the output waveform, vd, has changed. Sketch both waveforms, noting scales on the horizontal and vertical axes.

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Exercise 6: Full-Wave Rectifier Using LEDs A circuit that performs the absolute value operation on its input signal vs is called a full-wave rectifier diode circuit. A nifty visual circuit can be built with 2 red LEDs and 2 green LEDs in a bridge configuration as shown in Figure 6. The LED orientation is very important. A

//

D1

Vs

D3

//

R B

4 Vp-p, 0.8 Hz

C 1.2k

0Vdc //

D4

D2

//

D

Figure 6 Full wave rectifier 1. Build the circuit on the PB and lay out the circuit as seen in the schematic. Let R equal 1.2 kΩ. 2. Set the FG for a sine wave at 800 mHz (0.8 Hz) and an 8 V p-p value. 3. Connect the CH1 probe and ground clip across the FG (on the PB). Select auto scale on the scope and readjust vertical sensitivity to 1 V/div and lower the sweep speed to 500 ms/div. Observe the input signal. 4. Remove the CH1 probe and ground clip. 5. If all goes well you should see a nifty LED light display. 6. To observe the waveform across a diode, connect the CH1 probe to the anode of diode 1, and the ground clip to the cathode of diode 1. The observed signal has a positive peak of about + 2 volts, and a negative peak of about − 6 volts. 7. Remove CH1 probe and ground clip. 8. Switch the input to a triangle wave and then try a square wave.

Parts List Silicon diode: 1N4148 LED: Red, green or yellow Resistors: 1 kΩ, 1.2 kΩ, 2.7 kΩ all 5 % tolerance, ¼ W

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Appendix 1: Silicon Diode Information Figure A-1 shows an outline drawing in part (a) and the schematic symbol in part (b) for a small-signal silicon diode. Notice the band located on the n-side end of the diode – this end is the cathode. The other end is the anode. Note the band at one end Cathode n-side

Anode p-side

(a)

Anode p-side

Cathode n-side (b)

Figure A-1 Physical outline and schematic symbol for diodes

Appendix 2: Light Emitting Diode (LED) Information Figure A-2 (a) is a side view of an LED – note that one lead is longer than the other. However, the length of the leads is not the way to identify the anode and cathode. Figure A-2 (b) shows the bottom view of the LED, where the wires are pointing at you! You should see a flat side on the body of the LED. The wire nearest the flat side is the cathode or n-side. Case

Note the flat side

Anode p-side

Cathode n-side

Anode p-side

Cathode n-side

Leads

(a) Side view

(b) Bottom view

(c) Schematic symbol

Figure A-2 Physical outline and schematic symbol for an LED An LED produces light if a minimum current of around 3 to 4 mA passes through it. Then the voltage across the LED is between 1.5 and 1.7 volts.

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Appendix 3: Sample Data Table Voltage at A (V) 0.75 0.7 0.6 ….

Table A-1. Voltage at C Diode Current, iD (V) (mA) − 4.67 sample − 4.67 sample … …0 … … …. ….

Appendix 4: Function Generator Settings Whenever you use the function generator, reset the FG default display as follows. Enter the following commands on the front panel of the FG. Then the actual peak-topeak value will appear in the readout. Keys to Press Shift Menu On/Off → → → ↓ ↓ → Enter

What is in the Display A: MOD MENU D: SYS MENU 1: OUT TERM 50 OHM HIGH Z ENTERED

Now the default FG display is the same as what is read on the scope. Note: If you turn off the FG, you lose the above settings.

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