Dc Power Supplies

EE 312 & EE 352

Experiment No. 8

DC POWER SUPPLIES

November 10, 1999

James J. Whalen

Summary

Students are to write a two-page summary. The summary replaces the sections entitled Abstract and Conclusions indicated on p. 70 of the lab manual. The two-page limit is intended as a guide. There is not a penalty for exceeding two pages. However, it is quite probable that there will be a score reduction for exceeding two pages when the Staff believes that too much extraneous information is included. The rule is simple: make every word count.

1. Introduction

A dc power supply using an isolation transformer, a half-wave rectifier, a RC filter, and a Zener Diode was designed by the EE 312 & EE 352 Staff. The design is described in the EE 312 & EE 352 Lab Manual and Lecture Slides. Additional information is given in the course textbook by Wolf & Smith. See references given below. The dc power supply was assembled and tested for variations in ac voltage over a 90% to 110%.

This report is organized in the following manner. Following the Introduction is Section 2: Procedure. The procedures were given in detail in the EE 312 & EE 352 Lab Manual and are repeated in the report for completeness. Section 3: Design of Power Supply & Selection of Component Value was written by the EE 312 & EE 352 Staff. Section 4: Calculation of DC & Ripple Voltages and Comparison to Measured Values and Section 5: Effects of Varying Input Voltage, Discussion and Explanations are the major sections of the report written by (your name). Section 6: PSPICE Simulations presents computer simulations in support of the design. Students write this section also. The last section prepared by students is a two-page Summary. To summarize: Students write Sections 2, 4, 5, & 6 and the Summary.

REFERENCES

1. K. Etemadi, Laboratory Manual for EE 312 Basic Electronic Instruments Lab. & EE 352 Introductory Electronic Circuits Lab. Buffalo (NY): 1999, pp. 61-70.

2. J. Whalen, Lecture 8: Slides on DC Power Supplies. Buffalo (NY): 1999. (Slides available at htttp://www-ee.eng.buffalo.edu/~whalen/ee352

3. S. Wolf & R. F. M. Smith, Student Reference Manual for Electronic Instrumentation Laboratories. Englewood Cliffs (NJ): Prentice-Hall, 1990, 284-291 & 344-351.

2: Procedure

The procedures were given in detail in the EE 312 & EE 352 Lab Manual on pp. 66-68 and are repeated in this section of the report for completeness. Each student is required to enter the procedures in this section.

Section 3: Design of Power Supply & Selection of Component Value

Shown in Figure 1 of the EE 312 & EE 352 Lab Manual on p. 62 is the power supply circuit. The circuit consists of an input section that includes an isolation transformer, a half-wave rectifier and a capacitor CI. Not shown is a ground connection to one terminal of the output side of the isolation transformer. The rms ac voltage at the output of the isolation transformer is VT. The voltage across the capacitor CI is measured at Node 1. The next section is the RC low-pass filter consisting of RF & CF. . The voltage across the capacitor CF is measured at Node 2.The third section is the Zener regulator section consisting of a current limiting resistor RS and a Zener Diode. The load resistor is labeled RL. The voltage across the load resistor RL is measured at Node 3.

Shown in Figure 2 of the EE 312 & EE 352 Lab Manual on p. 62 is the voltage waveform across the capacitor CI at Node 1. The voltage waveform consists of two parts: (1) sections of the positive part of sinusoids with a peak value 2 X VT; (2) sections showing droop that is approximated as a straight line. (Note: subtracting the dc voltage drop of ~ 0.7 V across the rectifier diode would lead to a more exact design.) The equation for the droop is

where R is the appropriate resistor that controls the discharge of capacitor CI. What value should be used for R will be discussed later. It is important to make the time constant RCI sufficiently large so that the percentage droop is not too large. A 20% to 30% droop is usually acceptable for a half-wave rectifier. Let t be the time that the capacitor CI discharges before being re-charged on the next positive part of the sinusoid. The value for t shown in Figure 2 is t = 1/f where f = 60 Hz. Actually the value for t is less than 1/f. For a specific droop the value for t can be calculated. For a 20% droop the value calculated for t was 0.9 X 1/f. The percentage droop is defined as 100 X {V1/ (2 X VT)} where V1 is the maximum drop in voltage v1(t). For a 20% to 30% droop, Equation (3-1) can be approximated as

The droop V1 is given by substituting t for t in Eq. (3-2) and subtracting the result from the peak voltage 2 X VT. The result is given by

The dc voltage at Node 1 is denoted by V1DC and is equal to the average value of the voltage across the capacitor CI and that is given approximately by

If the design value for the droop is 20%, then the dc voltage at Node 1 is given by

Equation (3-5) neglects the dc voltage drop across the rectifier diode. To account for that voltage 0.7 V should be subtracted from the peak value 2 X VT and that result multiplied by 0.9.

For a 20% droop Equation (3-3) indicates that

The values available for the capacitor CI are 200 F, 100 F, and 50 F. It was decided to use the 100 F capacitor for the RC filter section and to use the other two capacitors in parallel for CI. For CI = 200 F + 50 F = 250 F a value for the total resistance R can be calculated using Eq. (3-6) with t = 1/f where f = 60 Hz as follows

The resistor R is not quite equal to sum of the resistors RF + RS + RL on account of the Zener Diode in parallel with RL. If the dc resistance RZ of the Zener diode is defined to be RZ = VZ/IZ where VZ & IZ are the Zener diode dc voltage and current, then the resistor R is given by

The dc load current was specified to be 50 mA. For a 12 V Zener diode the appropriate value for RL was determined to be

The Zener diode dc current was selected to be IZ = 21 mA. For a 12 V Zener diode the appropriate value for RZ was determined to be

Using the values calculated with Equations (3-9) & (3-10), the value calculated for the RZRL was RZRL = 571240 = 170 . Inserting the value for RZRL = 170  into Eq. (3-8) yields a value for RF + RS.

Note: In the lecture slides the value obtained was RF + RS = 138 .

The last step is to decide upon the division of RF + RS in to separate parts. The ac voltage ratio V2ac/V1ac for the low-pass filter at a frequency f is given by

The RC low-pass filter was assigned a voltage reduction factor of 1/10 at a frequency of 60 Hz. Equation (3-12) was set equal to 1/10 and solved for RF. The result is given by

The value for RS is given by

The values available included 100 , 67 , and 47 . A decision was made to use RF = 100  and RS = 47. A value RS = 67  might have been a better choice. The design choices are listed in Table 3-1.

TABLE 3-1 DESIGN & ACTUAL VALUES FOR CIRCUIT COMPONENTS

Figures to be inserted by students

Figure 3-1 Power supply circuit.

Figure 3-2 Voltage waveform in input section.

4: Calculation of DC & Ripple Voltages and Comparison to Measured Values

Students write this section. Section 4 starts at the top of a page.

5: Effects of Varying Input Voltage, Discussion and Explanations

Students write this section. . Section 5 starts at the top of a page.

6: PSPICE Simulations

Students present PSpice or Electronics Workbench computer simulations in support of the design in this section. . Section 6 starts at the top of a page.

Summary

The Summary is the last section prepared by students, but it is placed immediately after the title page.