Diode Applications Half-Wave Rectifying

Transcription

1 Lab 5 Diode Applications Half-Wave ectifying Objectives: Study the half-wave rectifying and smoothing with a capacitor for a simple diode circuit. Study the use of a Zener diode in a circuit with an AC power supply. Equipment: Function Generator Diode 1N4007 Zener Diode ZPD 9.1 esistor (10 kω) Capacitor (1 µf) - Theory A) Large Signal Operation (ectification) n the forward bias region (positive voltage v diode ), the current i turns out to be exponentially related to v diode. For typically used values or the current, the resulting steepness of the diode -V characteristic curve means that a large range of current variation can be obtained by varying the voltage in a narrow range, as seen in previous Lab. For this reason, for commonly used current values the forward-biased diode voltage v D is sometimes considered to be approximately constant. A typical value often used for this voltage is 0.7 V. We will assume this value in the discussion, but it should be kept in mind that other values may be more reasonable, depending on the diode type and current range used. Figure 1: Half Wave ectifying Circuit

2 Consider now the circuit of Figure 1. The output voltage cannot be negative since this would require a negative value for, which cannot pass through the diode. The voltage can be positive, though, and this will occur when is positive (i.e., when the diode conducts). n this case, v diode will be approximately equal to 0.7 V (see above). From Kirchoff s voltage law, we have v diode + Vout = Vin, and thus the output will be Vout = Vin v diode. These observations should help you interpret what you will be measuring. Figure 2: the input and output signals of the Half -wave ectifier circuit Assume now that Vin in Figure 1 is a sinusoidal voltage. Form the above observations, it can be seen that the output voltage waveform Vout is as shown in Figure 2. The negative parts of each input cycle are cut off, since Vout cannot be negative. The positive parts of each input cycle appear at the output, but are lowered by the voltage of the forward-biased diode (assumed to be about 0.7 the figure). The circuit in Figure 1. is called a rectifier. The output voltage Vout in Figure 2 has only one polarity, in contrast to the input voltage. However, it is not a DC voltage, as it is not constant with time. We can obtain a voltage that is almost DC by adding a capacitor to the circuit, as shown in Figure 3. This results in the behavior shown in Figure 4, as will now be explained. When the diode conducts, Vout = Vin v diode, and the capacitor charges up to this value. Let the peak value of the input voltage be V P. Near the peaks of the input voltage, the capacitor voltage is approximately Vout = V P 0.7 V. When Vin decreases below its peak value, the value of v diode = Vin Vout decreases since Vout is held almost constant by the capacitor. Thus the diode becomes cut off, and its current reduces practically to zero; it now behaves virtually as an open circuit. The capacitor is then effectively connected only to the resistor and begins discharging through it. f the C time constant is large, the discharge will be slow, as shown in Figure 4. At some later point, the input rises again and reaches a value about 0.7 V higher than the value of Vout; then the diode starts conducting again, and Vout = Vin V D Vin -0.7V. The output thus starts following the rising input (while

3 staying below it by about 0.7 V). The capacitor charges up again, and the whole cycle is repeated. t can be seen that, for the circuit of Figure 3, the output voltage is almost DC; it is constant within a small variation, called the ripple. The ripple can be made very small by choosing an appropriate C time constant. C Figure 3: Half-wave ectifier with Smoothing Capacitor Figure 4: The input and output signals of the Half -wave ectifier and Smoothing circuit B) Zener Diode Clipping Circuits Diode clipping and clamping circuits are circuits that are used to shape or modify an input AC waveform (or any sinusoid) producing a differently shape output waveform depending on the circuit arrangement. Diode clipper circuits are also called limiters because they limit or clip-off the positive (or negative) part of an input AC signal. As zener clipper circuits limit or cut-off part of the waveform across them, they are mainly used for circuit protection or in waveform shaping circuits. For example, if we wanted to clip an output waveform at +7.5V, we would use a 7.5V zener diode. f the output waveform tries to exceed the 7.5V limit, the zener diode will clip-off the excess voltage from the input producing a waveform with a flat top still keeping the output constant at +7.5V. Note that in the forward bias condition a zener diode is still a diode and when

4 the AC waveform output goes negative below -0.7V, the zener diode turns ON like any normal silicon diode would and clips the output at -0.7V as shown below. Figure 5: Single and Double Zener Diode Clipping Circuits - Experience A) ectifier 1. Connect the function generator to the circuit, as shown in Figure 1. Use a sinusoidal voltage with a frequency of 1 khz and an amplitude of 4V. Observe both the generator and the resistor voltage waveforms, using the two channels of the scope. Be very careful with the grounds. (The ground of the scope should be attached to the grounded terminal of the function generator; otherwise, the generator may be damaged.) Use DC input coupling on the scope for both channels. You need to think about how you would trigger to obtain a stable display. Adjust the time/div control so that you can observe several cycles on the screen. Keep a record of the waveforms obtained. The circuit you have just studied is a rectifier. Questions for Lab eport: 1) epresent and for the circuit of figure 1.

5 B) ectifier with smoothing capacitor Keep the same circuit and add a 1 µf capacitor in parallel, as shown in Figure 3. The circuit you have just made is an AC-to-DC converter a simple power supply. t is the basis of most power supplies, which convert a 60 Hz or 50 Hz AC voltage to a DC voltage. n this experiment, we have used instead a frequency of 1 khz for convenience in measurements. Questions for Lab eport: 2) epresent for the circuit of figure 3 and determine: a. The average value of the waveform Vout? b. The ripple (variation) of the waveform Vout in volts? c. The ripple as a percentage of the average value found in step (a.)? C) Zener Diode Clipping Circuits 1. Set up the circuit as shown in Figure 6. V diode Figure 6: Zener Diode Clipping Circuit 2. Using the oscilloscope, Set the function generator at a sinusoidal signal having a frequency f = 1 khz and an amplitude 10 V. Questions for Lab eport: 3) eproduce the diode and the resistance voltages V diode (t) and V (t). 4) Deduct the value of V D (the forward working voltage) and V Z (the zener voltage). 5) Show these values (V D and V Z ) on the figure.