To measure characteristics of a rectifier and a Zener diodes and to understand
the difference between an "ideal diode" and a real device. To familiarize
students with basic rectifiers and other diode circuits.
PRELAB
What are the differences between characteristics of a model “ideal diode” and a real device?
What is the difference between a Zener
diode and a "standard" rectifier diode?
Draw a “load line” plot for a circuit with a d.c. source, forward biased diode and a resistor. The plot should have the diode voltage on the horizontal axis and current on the vertical axis. Mark both scales with numerical values in volts and mA, respectively. Draw two straight load lines representing two resistors with specific values (between 1 and 10 kohms). Draw a reasonable “diode characteristic” curve on the same graph; you will measure it later in the lab. Mark two operating points corresponding to the two resistors.
LABORATORY
Equipment needed from the stockroom: Proto-board, analog universal
meter, leads.
1. MEASUREMENTS OF I-V
CHARACTERISTICS OF A RECTIFIER DIODE.
a) Measure the I-V characteristic of a
rectifier diode or the dependence of current through the diode on voltage across the diode. For measurements with forward diode bias you need a voltmeter
and an ammeter; use the analog and the digital meters. An incandescent lamp
in series with the diode makes a handy "high power" resistor. Do not burn the bulb (stop increasing current when it becomes bright)! Cover a wide range of current values, from a fraction of a mA to about 100mA. Plot the current as a function of the bias voltage. Make a preliminary graph in the laboratory.
b) Apply a reverse bias to the diode of a few volts to see how large the reverse current is. Even your digital ammeter may not be able to measure a very small reverse current. Can you determine it by measuring voltage across a high value resistor (MΩ) connected in series with the diode? Think which of the two voltmeters to use in this measurement (consider their internal resistances!).
2. ZENER DIODE
Measure the I-V characteristic of a Zener diode with reverse bias. Use an appropriate series resistor (the light bulb may do it) to permit a few tens of mA of current after achieving a breakdown voltage. Make a preliminary graph in the laboratory.
Plot reverse bias characteristic of the Zener diode and determine the Zener
breakdown voltage.
3. A DIODE IN A CIRCUIT;
THE LOAD LINE
a) Assemble the circuit shown below using the diode which characteristic you just measured. Choose the resistor in a few kohm range. Measure the voltage across it while increasing slowly the power supply voltage to obtain current of a few mA. Measure also the voltage across the circuit (Vs) and across the diode.
Fig. 8.1 A diode in a circuit with a resistor
a) Repeat a) for a lower resistance (100 ohm) and in a few tens to 100 mA range. Do not exceed the power rating of the resistor (1/4 W). Make a quick calculation of the dissipated power (for example, 100 mA is too much for a 100 ohms small resistor!)
Make the load line plots for the two circuits tested in a) and b), using the diode characteristic you measured in 1.
Advice: Make sure that the current is in the range you covered in the measurement of the diode characteristic in section 1. If not, you may need to add a few points to the characteristic now. You will need it to prepare the load line plot.
4. DIODE CLAMP CIRCUITS
Assemble circuits shown in Fig. 8.2 and test the output voltage with an oscilloscope while supplying waveforms with different amplitudes to the input. Try a sinewave and a triangle or square wave with different dc bias. The second circuit, known as a diode limiter, is often used to protect inputs of sensitive devices, such as ammeters or high gain amplifiers. Choose R for good clipping performance. Sketch input and output waveforms. Explain how these circuits work (Experiments in 4. should help).
Tabulate results of 1 a). Plot the I-V characteristic of the rectifier diode for forward biasing. At what forward voltage does the rectifier diode effectively conduct current? Do your measurements agree with a known theoretical equation relating diode current and voltage? Show it on a graph.
Hint:A semi-log graph may be the most informative here, plot it in addition to a standard linear graph.
Tabulate the results of the rectifier and Zener diode measurements. Plot the I-V characteristic of the rectifier diode for forward biasing using (a) linear graph and (b) semi-log graph. Plot the Zener diode characteristic for reverse biasing using a linear graph. At what forward voltage does the rectifier
diode effectively conduct current? What is the value of the Zener breakdown
voltage?
Include the load line plots for the two circuits tested in 3 a) and 3 b) using the diode characteristic you measured in 1.
Answer all other questions printed in bold letters in this manual.
