EE 110 Introduction to Engineering & Laboratory Experience Saeid Rahimi, Ph.D. Lab 6 Diodes: Half-Wave and Full-Wave Rectifiers Converting AC to DC

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1 EE 110 Introduction to Engineering & Laboratory Experience Saeid Rahimi, Ph.D. Lab 6 Diodes: Half-Wave and Full-Wave Rectifiers Converting C to DC The process of converting a sinusoidal C voltage to a DC voltage consists of two steps: (i) The first step is to remove or invert the negative half-cycles of the input voltage. Diodes are used for this purpose. (ii) The second step is to reduce the fluctuations of positive half-cycles. Capacitors are used for this purpose. Such a system is generally referred to as a rectifying bridge. Note that inexpensive rectifying bridges are available commercially. For example, the figure below shows an IC with two pins for C input and two pins for DC output. This device is, of course, far simpler to use than to construct a bridge from scratch. The main purpose of this lab is to make students familiar with the concepts and some applications of diodes and capacitors. Diodes are one of the most basic electronic devices that have many applications. They are used as rectifiers (rectifying diodes), voltage regulators (Zener diodes), light sources (LEDs and Laser Diodes), photodetectors, and optical power generators (solar cells). The most basic application of diodes is for C to DC signal conversion. y definition, an C signal fluctuates with time. Imagine a sinusoidal C signal that half of the time provides positive current (voltage), and the other half negative current (voltage) as shown on the left side of the figure below. We employ a rectifying device (diode) to filter out the negative current and pass through the positive current as shown on the right hand side. Of course, in order to observe the signal we need to monitor the voltage across a resistor which is called a load resistor indicated by R L in the figure. The diode allows only the forward current I F to go through and it blocks the reverse current which leads to the elimination of the negative parts of the waveform. The rectifying properties 1

2 of a diode can be understood through its IV diagram shown below. When a positive voltage is applied to the diode it allows current to flow through it. The current increases with the "forward biased" voltage. It turns out that the current increases exponentially with the applied voltage. When a negative voltage is applied, the diode allows very little current to pass through. The diode appears to block the current when a "reverse bias" voltage is applied. Note that a diode will turn on at threshold voltage which depends on the diode material. For example, for silicon diodes the threshold voltage is about 0.7 volts. The diode symbol shown below consists of an arrowhead, which indicates the direction of current allowed by the diode, and a vertical line which indicates that the current in the opposite direction will be blocked. The left side of the diode is called the anode and the right side is the cathode. The diode will break down and conduct if the negative voltage is increased beyond a maximum allowed value. The "breakdown" voltage and other characteristics of diodes are described in their manufacturer's specification sheets. Measurement 1: simple way to visually observe the rectification properties of a diode assemble the following simple circuit. pply a 5 V pp sine wave with a frequency of 10 Hz. The 300 Ω resistor is used as a current-limiting resistor to protect the diode from burning due to too much current. R1 XFG1 300Ω LED1 2

3 Redraw the circuit in your lab book and explain why the diode blinks. Gradually increase the frequency and observe that at some threshold frequency the diode will appear as if it is not blinking anymore. Record that particular frequency and explain why! Measurement 2: Now that you understand the on and off function of a Light Emitting Diode (LED), let us experiment with a rectifying diode. You should have a rectifying diode in your toolbox. If not, you will be provided with a general purpose diode 1N4001 to examine its forward and reverse IV characteristics in a qualitative way. The actual shape of a rectifying diode is shown below. Note the band on the right hand side. That band indicates the location of the cathode pin of the diode. Current flows from the anode toward the cathode and not the other way round. The 1N4001 diode has a voltage rating of 50 V and current rating of 1. Look up the data sheet of this diode on the internet and verify its specifications. If in doubt about the anode and cathode ends of the diode, use the continuity function of your multimeter for identification (see below). The 1Nxxxx numbering system is an merican standard (now adopted globally) used to mark semiconductor devices. The "1N" means that it is a single junction semiconductor device (i.e. a diode). "4001" is a number given to the smallest diode in the 400x series (4001, 4002, and so on) - the number indicates the voltage, current and power ratings of the diode. transistor (which has 2 junctions) would be numbered 2Nxxxx. Use the continuity function of your multimeter to examine the forward bias direction of the diode. Connect the red lead of the meter to one side of the diode and the black lead to the other. If you hear a beeping sound then the forward bias direction of the diode is from the red side to the black side. Next, observe the forward and reverse resistance of the diode by selecting the resistance function of your meter and connecting the leads to the diode in the forward and reverse direction. Record your readings for each case. The diode resistance in the reverse direction would be very large. Half-Wave Rectifiers half-wave rectifier removes half of the cycles of the input sine wave. This concept is illustrated in the first figure at the top of the document. ssemble the circuit shown below and set the frequency of the function generator to 1 khz and its amplitude to 4 V pp. You can 3

4 use either the laboratory instruments or your DS. Note that points and indicate the circuit input and points C and D indicate the output points of the circuit. We wish to observe both the signal connected to the input of the circuit (from the function generator) and the output signal of the circuit. Connect channel 1 of the scope to the circuit input and channel 2 of the scope to the output. First, let us focus on the input signal, which in fact is the output of the function generator. Observe two full cycles of the signal by manipulating the vertical and horizontal knobs of the scope. Using the cursors measure the period and amplitude of the signal. Calculate the frequency of the waveform by inverting the period and verify that it is 1 khz. Change the frequency of the signal on the wave generator and observe the corresponding change on the scope. In electronic circuit diagrams the actual points of contact are indicated with a solid dot. In the absence of the dot, the two wires are not physically connected to each other. Observe the four solid dots in the circuit. Next, observe the output signal which is the voltage across the 1 k resistor, also called the load resistance. XSC1 Ext Trig D1 XFG1 1N4001G C R1 1kΩ D Measurement 3: Observe the shape of the output waveform and explain the differences between the input and output voltages. Carefully sketch V in and V out in your lab book. lso Record the Peak voltage of the output waveform. What do you expect the waveform to look like if you reverse the diode? Reverse the diode and record your observation. Using a Capacitor for Reduction of Fluctuation s we saw above, the net effect of a half-wave rectifier is to remove the negative segments of the input sine wave. The output is a positive voltage that varies between aero and the signal amplitude. However, this output is far from a desired constant DC signal. dding a capacitor in parallel to the load resistor will cushion the fluctuation. The capacitor acts similar to a shock absorber in an oscillating mechanical system. The reduction in fluctuation of the output signal is due to charging and discharging of the capacitor. lthough the end result is not still a perfect DC signal, it would be acceptable if we can reduce the amount of ripple in the output. The ripple is reduced by using a larger capacitor. Figure below shows three waveforms: (i) the full sine wave input signal, (ii) the half-wave rectified signal, and the DC signal with ripple after adding the capacitor. 4

5 Measurement 4: In this part we will place a capacitor in parallel to the load resistor. The RC time-constant plays a critical role in modifying the fluctuations of the signal. The larger the product of R and C, the closer the output signal will be to a DC signal. You can increase the RC time-constant by increasing the load resistance R, the capacitance C, or both. Let us first keep the 1 kω resistor and insert a small capacitor in parallel. Select a value of about 20 nf. Monitor the output signal and record the waveform in your lab book. Next, use a larger capacitor (about 100 nf) and observe and record the output. Finally, select a larger capacitor (470 nf or so) and observe the output signal (DC with ripple) on the scope. Use the capacitors in the lab if you don t find the appropriate capacitors in your toolbox. Calculation: Calculate the RC time-constant (in units of seconds) for each of the above three cases and record in your lab book. Compare the time constants with the period of the signal. Note the ineffectiveness of the rectifier when the RC time-constant is too small. Question: In order to select a large RC time-constant, would you select a large R or a large C? What factors do you consider for your choices? With the largest capacitor in the circuit, record the peak voltage and observe the difference between this waveform and that of the case without a capacitor. Make a note of the peak voltage and the ripple voltage and carefully draw the waveform in your lab book. e sure to indicate the scale of the voltage and time axes on your graphs. XSC1 Ext Trig D1 XFG1 1N4001G C R1 1kΩ C1 470nF D Measurement 5: In this part, while using a small capacitor, we attempt to increase the RC time-constant by increasing the resistance. Use the 20 nf capacitor and replace the 1 kω resistor with a larger10 kω resistor. Measure and record the new ripple voltage. Carefully draw the waveform in your lab book. 5

6 Measurement 6: Now let us select a large capacitor and a large resistor. Use a 10 kω resistor in parallel to the 470 nf capacitor and observe the difference with the previous case. Measure the ripple voltage (the difference between the top and the bottom of the output signal) and carefully sketch the waveform in your lab book. Create a table and show three sets of values for R, C and ripple voltage. Full-Wave Rectifier (optional): You will need four rectifying diodes for this part and you may borrow diodes from the lab and carry out this part of the experiment at home. You can build a simple full-wave rectifier using four diodes as shown in the diagram below. The difference between the output of this rectifier compared to the half-wave rectifier is that here the negative segments of the input signal are not eliminated. Rather, they are inverted, which means all half cycles of the wave are positive. Of course, we still do not have a DC output signal. Similar to the half-wave rectifier case, we need to add a capacitor in parallel to the load resistor. Placing a capacitor in parallel with the load resistor will reduce the ripple voltage, thus creating nearly a DC signal. The height of the ripples can be reduced by using larger capacitors. Construct this full-wave rectifier using four diodes and a 10 kω load resistor without a capacitor. Observe the full wave consisting of positive half cycles (the dotted red lines). Note that we can only choose one ground connection on this circuit. Therefore, the input and output signals may not be both displayed on one oscilloscope. Due to grounding complications, it is recommended that only the output signal (the voltage across the load resistor) be displayed on the scope. sk your instructor for help if you encounter difficulty. Next, insert a 0.4 µf capacitor in parallel to the load resistor as shown in the diagram and observe the DC line with ripples. You may have to use an electrolytic capacitor since the ceramic capacitors do not have large capacitances. e very careful about connecting electrolytic capacitors in your circuit. Unlike ceramic capacitors, the electrolytic capacitors have positive and negative polarity and must be connected correctly in the circuit. You can insert a larger capacitor in parallel to the load resistor to obtain a virtually flat DC line without any significant ripples. 6