Lab2 for EE 100A

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EE1o0A Electronic Circuits IDepartment of Electrical EngineeringUniversity of California Riverside

Laboratory 2EE 100 A

LABORATORY # 2

Diode Circuits

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Lab 2 Diode Circuits

EE100A Electronic Circuits IUniversity of California - Riverside

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Objectives

Lab 2contains three parts; objectives are to get familiar with:

1. Diode based half-wave rectifiers and their characterization;2. Full-wave rectifiers;

3. Basic amplitude and positive half-cycle limiters with a level shift;

Equipment

PC or compatible DMM (digital multimeter) Oscilloscope Function/Waveform Generator; Power supply (+6V) Jumper wires to connect components on solderless breadboards Solderless breadboard (you need to bring your own)

Parts

4 each diode 1N914 (switching diode) 3 each resistors (all 1/4W): 1k

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WARNING

Polarized Capacitors must be handled with great caution:

1. Polarized capacitors must be properly connected in a circuit:+ terminal of a cap to + terminal in the circuit, - to -.Otherwise the cap can be permanently damaged;

2. Never touch both legs of a polarized capacitor with yourfingers after usage since it may contain a substantial amountof charge that can electrocute you or at best burn the skin;

3. Neverdischarge a polarized capacitor by short-circuiting thelegs. Such a discharge will create a great amount of currentthat can overheat the cap and cause an explosion (nokidding). Since polarized capacitors contain liquid chemical

acid matter such an event may cause permanent damage toyour eyes. It is a good habit to wear safety glasseswhilehandling polarized capacitors;

4. Neverdischarge a capacitor while still in circuit;

5. In order to safely discharge a capacitor after handling,use a high wattage low value resistor (say, 100 Ohm, 1W),connect the resistor to the cap legs and wait for a couple ofseconds to fully discharge the cap (10-20 seconds may beenough but it may vary, easy to compute though by RC circuitanalysis);

6. Never store (long-term storage) used polarized capacitorswithout properly discharging them.

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SPECIFICATION

PART 1. Diode Based Half-Wave Rectifiers

1.1 Theoret ical B ackgroun d

The diode circuit applications can be subdivided into categories shown in FigureL1-1.

Figure L1-1.Diode circuit applications

However in this laboratory only the first two will be practically analyzed. The

action of the half-wave recitfier circuit of Figure L1-2 a)is shown in Figure L1-2 b)-d).Observe that if the simplest diode model is applied to the analysis of the half-wave circuit, it will demonstrate that the output voltage will follow the inputvoltage exactly for positive values of the input voltage and will clip off thenegative values.

Figure L1-2.Half-wave rectifier characterization assuming the idealdiode model behavior:a)the half-wave rectifier circuit; b)Vin(t); c)Vout(Vin); d)Vout(t)

Mathematically speaking the relationship for the voltage transfer function can beexpressed as

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(1.1), 0 V

( ) {0, 0 V

in in

out in

in

V if V V V

if V

(1.1) can be efficiently verified by applying the Load Line analysis as in Figure L1-3where the ideal diode acts as a load for the resistor.

Figure L1-3.Load Line analysis of the half-wave rectifier using the idealdiode model. Observethat Voutfollows Vinexactly when Vin> 0 and is 0 when Vin< 0

A more realistic behavior of the half-wave rectifier is obtained by applying the0.7diode model. Analysis leads to the results shown in Figures L1-4and L1-5.

Figure L1-4.Load Line analysis of the half-wave rectifier using the 0.7Vdiode model. Observethat Voutfollows (Vin-0.7V) exactly when Vin> 0.7V and is 0 when Vin< 0.7V

solution when Vin > 0which corresponds to Vout = Vin

i

v

Vin

iR= 1/R vR

solution when Vin < 0which corresponds to Vout = 0

The i-vcharacteristic of theideal diode, mirrored w.r.t. the i-axis and shifted by Vin

solution when Vin > 0.7Vwhich corresponds to Vout = Vin 0.7V

i

v

Vin0.7V

iR= 1/R vR

solution when Vin < 0.7Vwhich corresponds to Vout = 0

The i-vcharacteristic of the0.7V diode model, mirroredw.r.t. the i-axis and shifted by

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For a sinusoidal input signal, it will be transformed as shown grahically in FigureL1-5.

Figure L1-5.Voltage signal transfer function (a), and the response to the sinusoidal input signal

(b)of the half-wave rectifier using the 0.7Vdiode model where VD= 0.7V. Note that the diode isON (conducting) only when Vin> VD.

The voltage transfer function in this case can be expressed as

(1.2)0.7V, 0.7 V

( ) {0, 0.7 V

in in

out in

in

V if V V V

if V

1.2 Schematic and Procedures

D1

D1N914

R1

1k

VS1

FREQ = 100Hz

VAMPL = 5V

VOFF = 0V

VIN

0

0

VIN

VOUT

Figure L1-6.Half-wave circuit to be used in the experiment

Assemble the circuit schematically shown in Figure L1-6. Observe that in thisexperiment the function generator is used as a power supply. In general, functiongenerators MUST NEVER BE USED as power supplies. Their purpose is togenerate signal (voltage) waveforms, not power. If there is a need to use them aspower supplies, the circuit must consume no more than 100 mW in the worstcase scenario when something goes terribly wrong, and no more than about 10 -20 mW in routine experiments. Verify and report that in this experiment, in theworst case scenario, the function generator will supply no more than 15 mW ofpower. Otherwise the function generator may be permanently destroyed.

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1.3 Repo rt

Figure L1-7.Oscilloscope observations of the half-wave circuit performance for a sawtooth inputsignal of amplitude Vin,peak= 2V at f = 500 Hz. Shown are Vout (yellow) and Vin (green) asfunctions of time. Observe the VP = 671 mV voltage difference between the peak voltages (equalto the voltage drop of the diode when ON, or conducting).

1. Using the function generator produce a sinusoidal input signal and usetwo channels of the oscilloscope to observe BOTH the input voltage VinAND the output voltage Vout. Utilizing the oscilloscopes facilities, make asnapshot of the monitor screen and save it as an image on a USB drive forfurther analysis, and include it in your lab report;

2. Repeat the above procedure for a sawtooth input signal (not ramp);

3. Measure the difference between peak voltages Vin,peak and Vout,peak;

4. Measure the voltage Vin at which a non-zero output appears across theoutput terminals.

5. Measure the time during which the diode is conducting (or in jargonterms, is ON). What is the fraction of the period (in percents) that theoutput is different from 0V?

6. Discuss observations and include theoretical analysis which supports it;

0 1 2 N

Vin= VS, V

Vout= VR, V

VD= Vin- Vout, V

7. Disconnect the function generator. Connect a regular (+6V) power supplyto the input Vin. Using two channels of a multimeter (front and rear), varythe power supply voltage (Vin) in the range from 0 to 5V and record theoutput voltage Vout. When done, flip the positive and negative power

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terminals on the breadboards power bus (whatever was a positiveterminal, connect it to the negative terminal of the power supply, and viceversa, whatever was a negative terminal, connect it to the positiveterminal). Repeat measurements in the range from 0 to 2V using the same+6V power supply (observe that these are negative values compared to the

previous measurement). NOTE: it is required to record the value of thepower supply voltage using the multimeter!Do not rely on the valuesdisplayed by the power supply, they are not accurate.Double checkthat

bothchannels of the multimeter inputs are voltage inputs(not currentinputs as we had during our previous lab on the rear terminal panel of themultimeter);

8. Plot two relationships Voutvs Vin, and (VD = Vin Vout) vs Vin. By drawingan approximate line through the data points on the positive side of theplot, determine the voltage Vin at which the non-zero voltage appears atthe output. Discuss the results. Where the 0.7 diode model is moresuitable at higher values of Vin or lower? Compare multimeter

measurement results with the results obtained using the oscilloscope(explain the non-collinearity of the input/output lines in the sawtoothinput signal measurements).

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PART 2. Diode Based Full-Wave Rectifiers (H-Bridge Circuit)


The full-wave rectifier circuit shown in Figure L2-1fixes the problem of loosing thenegative part of the input signal (either a lost information or a lost power). It alsofinds applications in the DC electric drives where this circuit is called the H-Bridge circuit.

Figure L2-1.a)Full-wave rectifier circuit, and b)its input/output characteristic

By analyzing this circuit*it can be shown that during positive half-cycles onlydiodes D1 and D2 are conducting, while during negative half-cycles only D3 andD4 are conducting. Make an important observation that during the whole time

the current is flowing in the same direction, and the resistors actual voltagepolarity (the output of the circuit, Vout) stays the same.


D4

D1N914

D1

D1N914

D2

D1N914

D3

D1N914

R1

1k

0

VS1

FREQ = 100Hz

VAMPL = 5V

VOFF = 0V

0

VIN

VIN

+Vout

-Vout

Figure L2-2.Full-wave rectifier circuit

*See textbook for a complete discussion.

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Assemble the circuit schematically shown in Figure L2-2. Again, in this experimentthe function generator is used as a power supply. Use an input signal Vinoffrequencyf= 100 Hz and voltage amplitude Vp= 5V.

2.3 Repo rt

1. Using the function generator produce a sawtooth input signal andindividuallysave a snapshot of both the input voltage Vinand the outputvoltage Vout. Include them in your lab report. What is the period of eachsignal? Discuss it.

2. Using the oscilloscope measure the peak voltages Vin,peak and Vout,peak; andcompute the difference between them. Discuss the results.

3. Measure the voltage Vin at which a non-zero output appears across theoutput terminals. Discuss the results.

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PART 3. Limiters


Some circuits require limiting the amplitude of an incoming signal to a specific

range, probably linearly rescaling it at the same time. Circuits performing thisfunction are called limiters.

Figure L3-1.a)voltage transfer characteristic of (hard) limiters; b)sample input/output

Selected diode based limiters and their function are sketched in Figures L3-2, L3-3.

Figure L3-2.a)simple (positive cycle) limiter; b)limiter with a level shift

Do not be misled !!!In Figure L3-2 b)VB1is a reference voltage, not an actual power supply. A circuit

containing such a power supply would destroy the power supply. This notation is a theoretical convenience

for analysis only. A quick calculation/observation will show that such a power supply would need to

consume power in this case instead of generating it. The last experiment in this lab will demonstrate an

appropriate circuit for such purposes.

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Figure L3-3.a)negative cycle limiter; b)amplitude limiter

Voltage transfer characteristics can be efficiently analyzed by applying the LoadLine analysis as in Figure L3-4.

Figure L3-4.Individual i-vcharacteristics of a diode connected with respect to opposite polarities

and current directions, and a series of load lines for same load resistor, two diodes combined inparallel and a variable set of input voltages. Voutfollows Vinonly in the range [-0.7V,0.7V] andtake extreme values (+0.7V or -0.7V) otherwise, for any R value. Current defines the choice of R.

i

v

0.7V

i

v- 0.7V

v- 0.7V

0.7V

Vin1Vin2Vin3Vin4Vin5

Vin6Vin7

i

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VS1

FREQ = 500Hz

VAMPL = 5V

VOFF = 0V

VIN

0

D2

D1N914

D1

D1N914

R1

1k

VIN

VOUT

0

Figure L3-5.Basic amplitude limiter

VS1

FREQ = 500Hz

VAMPL = 5V

VOFF = 0V

VIN

0

D1

D1N914

R1

1k

VIN

0

VOUT

R2

1k

R3

1kVS2

4V

VREF

0

VREF

Figure L3-6.Half-cycle limiter with a reference level shift

VS1

FREQ = 500Hz

VAMPL = 5V

VOFF = 0V

VIN

0

D1

D1N914

R1

1k

VIN

0

VOUT

R2

1k

R3

1kVS2

4V

VREF

0

VREF

C1

100uF

Figure L3-7.Half-cycle limiter with a reference level shift and a bypass cap

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3.3 Repo rt

1. Assemble the basic amplitude limiter shown in Figure L3-5;

2. Using the function generator produce a sawtooth input signal and use twochannels of the oscilloscope to observe BOTH the input voltage Vin ANDthe output voltage Vout. Utilizing the oscilloscopes facilities, make asnapshot of the monitor screen and save it as an image on a USB drive forfurther analysis, and include it in your lab report;

3. Measure the peak voltage at the clipped off part of the output signal andcompare it with the input voltage at that point. Discuss the discrepancywith the theory;

4. Assemble the positive half-cycle limiter shown in Figure L3-6; with areference voltage obtained by using the voltage divider R2, R3 from aseparate power supply source VREF.

5. Repeat procedures 2 and 3 above;6. Discuss and explain observed results by computing the Thevenins

equivalent circuit of the active subcircuit created by VREF, R1, R2;

7. Add the bypass capacitor as shown in Figure L3-7 (observe the electrolyticcaps polarity, remembering that an arrow on the cap points toward thenegativeterminal, and/or at the same time the positive terminal has arubber insulationwhile the negative terminal is inserted into analuminum plate);

8. Repeat procedures 2 and 3 above;

9. Why did the capacitor improve the output characteristic of the limiter?

10.Use proper procedures to safely discharge the capacitors before storingthem!!!

Presentation and Report

Must be presented according to the general EE 100lab guidelines.

Prelab

1. Review lectures and textbook on the subject of Lab experiment.

2. Obtain the theoretical behavior of circuits in the Lab Experiments.

3. Study a procedure on how to deal with polarized, electrolytic capacitors.

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Lab2 for EE 100A