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UK EE 462G - EE 462: Laboratory # 4 DC Power Supply Circuits Using Diodes

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Fig. 3 Definition of percent rippleIV. Pre-LabV. Lab procedureEE 462: Laboratory # 4DC Power Supply Circuits Using DiodesbyDr. A.V. Radunmodified by Dr. K.D. Donohue (10/08/04)Department of Electrical and Computer EngineeringUniversity of KentuckyLexington, KY 40506Laboratory # 4Pre-lab due at lab sessions October 12, 13, 14.Lab due at lab sessions October 19, 20, and 21.I. Instructional Objectives- Design and construct circuits that transform sinusoidal (AC) voltages into constant (DC) voltages.- Design and construct a voltage regulator based on the characteristics of the Zener diode.- Study the performance of simple rectifier and regulator circuits.See Horenstein 4.3 and 4.4II. RectificationElectric power transmits best over long distances at high voltages. Since P = I V, a larger voltageV implies a smaller current I for the same transmitted power. Smaller currents allow for the use of smaller wires with less loss. The high voltages used for power transmission, however, must be reduced to be compatible with the needs of most consumer and industrial electric equipment. This is done with transformers that only operate with AC (DC does not pass through a transformer). Since most electronic circuits require DC (constant) voltages, normal AC power voltage must be transformed into a DC (constant) voltage.The terminology "DC" is somewhat ambiguous. DC can mean the voltage or current always has the same polarity but changes with time (pulsating DC), or it can mean a constant value. In this lab assignment DC will refer to a constant voltage or current. If the voltage or current always have the same polarity but changes with time, it will be referred to as having both a DC and an AC component. The process of changing an AC signal that has both plus and minus values, to a signal with only plus values is called rectification, and the circuits that perform this operation arecalled rectifier circuits. The rectifier circuit operates in a way similar to the clipping circuits usedin a previous lab. Figure 1 a) shows a half wave rectifier and Fig. 1 b) shows a full wave rectifier.RLVsVout(a)+-RLVsVout(b)+-Fig. 1 a) Half wave rectifier. b) Full wave rectifier.Although the output of a rectifier is always positive, (or always negative) it is generally not constant, often going from zero to a peak value. Thus, the output of the rectifier must be filtered to obtain a DC (constant) output. Since DC has a frequency of 0 Hz, a low-pass filter can be applied to attenuate the higher frequency signal components. The simplest low pass filter is a capacitor. Figure 2 shows examples of passing rectified signals through a low-pass filter. This low-pass filtering is sometimes called smoothing. No filter can totally eliminate unwanted frequencies so the actual output of the filter will always have some AC content, often called ripple (ripple voltage or ripple current). A rectifier and its filter is a simple example of a DC power supply. RL Vs Vout (a) + - RL Vs Vout (b) + - Fig. 2 a) Half wave rectifier with capacitor filter. b) Full wave rectifier with capacitor filter.One performance measure of a DC power supply is the percent output ripple computed from the ratio of the (peak-to-peak) output voltage to the average (DC) output voltage. Output ripple can be expressed as r in the equation below: opopVVr(1)where popV is the peak-to-peak output voltage and oV is the mean of the output voltage, which is equivalent to the DC component. Multiply r by 100 for percent output ripple. A typicaloutput signal is illustrated in Fig. 3. The best performance occurs when the percent ripple is zero(a battery produces ideal DC). This lab examines and compares the two rectifier schemes in Fig. 2.Peak to peak ripple Average DC oV t popV oV Fig. 3 Definition of percent rippleIII. RegulationA DC power supply provides constant DC voltage to some load, which will be modeled as a resistor. This constant value should be independent of the load and fluctuations of input voltage. When a load is applied to the output of a simple power supply (as in Figs. 1 and 2), the output voltage decreases due to increased resistive voltage drops, diode voltage drops, and increased voltage ripple. A voltage regulator circuit is used to prevent this change in output voltage. A Zener diode can be used to make a voltage regulator circuit (see Fig 4) by taking advantages of the Zener diode’s reverse breakdown characteristic. Recall that once a Zener diode breaks down, its voltage remains essentially constant independent of its reverse current. The regulator's resistor, Rreg, limits the current through the Zener to reduce the power dissipated in the Zener. This is done, however, at the price of limiting the maximum load current that can be supplied with a regulated output voltage. Vin Vout + - + - Rreg Fig. 4 Zener voltage regulatorAn important characteristic of a voltage regulator is its percent regulation defined as the difference between the average no-load voltage (implies zero load current and thus infinite load resistance) and the average full-load voltage (the load draws its maximum current and has its minimum resistance) divided by the average full-load voltage. Regulation can be expressed in the equation below:oFLoFLoNLVVV Regulation(2)where oNLVis the average output no-load voltage and oFLV is the average output full-load voltage. Percent regulation is obtained by multiplying Regulation in Eq. 2 by 100. The best regulation performance is achieved with a 0 % regulation.A DC power supply is made up of a combination of a rectifier, a filter, and a voltage regulator circuits. A simple power supply using this combination is shown in Fig. 5. Vin Vout + - + - Rreg RL Vs + - Rectifier Filter Regulator Load AC source DC Power Supply Fig. 5 Basic power supply consisting of a rectifier, filter and regulator.IV. Pre-Lab1. For the half-wave and full-wave rectifiers in Fig. 1, determine the output voltage and the current through a 5.1k- load with a sinusoidal, 8V rms, 60Hz input voltage. Use suitable approximations.2. Estimate the oscilloscope horizontal setting (in ms / div) and the oscilloscope vertical setting (in volts / div) to observe the output voltage in Pre-lab Problem 1. Often the major divisions on an oscilloscope are 1 cm wide so volts / cm is sometime used instead of volts / div.3. For the full-wave rectifier case, sketch a schematic showing how you will place your probes to measure the output


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UK EE 462G - EE 462: Laboratory # 4 DC Power Supply Circuits Using Diodes

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