Introduction:



DC Power Supply

Teberih Kiflay

November 22, 2000

Class #: TrAM P#26946910

Partner: James Ross

EE 312

Introduction

The objective was to design, construct, and test a dc power supply as an application of the transformers, diodes, and filters investigated in previous experiments. The dc power supply was the half-wave rectifier type and consisted of four sections: transformer, rectifier, filter, and regulator. A transformer was used to transform the 120 VAC line voltage to 18 VAC. The transformer also provided isolation so that one node of the dc power supply could be connected to ground. A rectifier section that consisted of one diode and a capacitor CI followed the transformer section. A low-pass filter section that consisted of a resistor RF and capacitor CF followed the rectifier section. A regulator section that consisted of a resistor RS and a Zener diode followed the filter section. The load resistor was labeled RL.

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 drop 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 drop 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 drop 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% drop, 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% drop 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

|Component |CI |CF |RL |RF |RS |

|Units |(F |(F |( |( |( |

|Design Value |NA |NA |240 |80 |83 |

|Actual Value |250 |100 |240 |100 |47 |

Figure 3-1 Power Supply Circuit.

Figure 3-2 Voltage Waveform in Input Section.

These are the Waveform that we tested at these three points. The values that were taken for these three are the average voltage and the voltage peak to peak. The values for the output waveforms for the values 1, 2, and 3 are found in Table 1.

The values that the components had are shown in the figure below.

From these values we calculated and found the voltage peak to peak and the average voltage. From the voltages we were able to calculate the values of IL, IZ + IL, and IZ.

[pic]

TABLE 1

This is the measured value for the DC power supply with full wave rectifier.

C = RL CONNECTED & NC = RL NOT CONNECTED.

[pic]

Table 2

PSPICE was also used to simulate the half-wave rectifier dc power supply circuit and to determine values for all the voltages and currents measured.

The worst case that was found was

110% AC VZ1BD = 10.8V RL NC IZ = < max IZ

90% AC VZ1BD = 13.2V RL C IZ > 0

[pic]

SUMMARY

The objective was to design, construct, and test a dc power supply as an application of the transformers, diodes, and filters investigated in previous experiments. The dc power supply was the half-wave rectifier type and consisted of four sections: transformer, rectifier, filter, and regulator. A transformer was used to transform the 120 VAC line voltage to 18 VAC. The transformer also provided isolation so that one node of the dc power supply could be connected to ground. A rectifier section that consisted of one diode and a capacitor CI followed the transformer section. A low-pass filter section that consisted of a resistor RF and capacitor CF followed the rectifier section. A regulator section that consisted of a resistor RS and a Zener diode followed the filter section. The load resistor was labeled RL.

The design was given in the EE 312 & 352 Lecture Slides available on the EE 352 Website (htttp://www-ee.eng.buffalo.edu/~etemadi/ee352). The available capacitors had values 50 (F, 100 (F, & 200 (F. The 50 (F & 200 (F capacitors were used for CI. The 100 (F capacitor was used for CF. The design values for the resistors were RF = 100 (, RS = 100 (, & RL = 5111 (. The values 75 ( & 63 ( are not standard values. The standard values used were RF = 100 ( & RS = 100 (. A 12W 1000-( variable resistor was used to obtain RL = 511 (.

The measurement equipment used was the Fluke 8000A & 8010A Digital Multimeters (DMM) and a HP 56400B CRO. The Fluke 8000A DMM was used to measure the rms ac voltage at the transformer output. The Fluke 8010A DMM was used to measure the dc voltage across the load. Channel 1 of the CRO was used to measure the average voltage V1(av) and peak-to-peak voltage V1(pp) across the capacitor CI at Node 1 or to measure the average voltage V2(av) and peak-to-peak voltage V2(pp) across the capacitor CF at Node 2. Channel 2 of the CRO was used to measure the peak-to-peak ac voltage across the load V3(pp). Since the values for V3(pp) were as low as 1.3 mV and there was considerable noise present, averaging = 64 was used. The CRO trigger mode used was “Line”.

A Variac was used to adjust the transformer output voltage. A rms value = 18 VAC was the 87% value. The following results were obtained for 18 VAC: V1(av) = 22.38 V & V1(pp) = 2.563 V; V2(av) = 16.82 V & V2(pp) = 450 mV; VLOAD = V3(av) = 11.43 V & V3(pp) = 1.3 mV. The current through the load and the Zener diode was determined to be 55 mA. The current through the load was 22 mA. The current through the Zener diode was 33 mA. A transformer output rms voltage = 19.8 VAC was the 96% value. The following results were obtained for 19.8 VAC: VLOAD = V3(av) = 11.4 V & V3(pp) = 1.9 mV. The current through the load and the Zener diode was not measured. It was estimated to be 44 mA. If the load resistor were disconnected, the current through the Zener diode would exceed its 66 mA maximum current rating. A transformer output rms voltage = 16.2 VAC was the 80% value. The following results were obtained for 16.2 VAC: VLOAD = V3(av) = 11.24 V & V3(pp) = 2.1 mV. The current through the load and the Zener diode was not measured. It was estimated to be 45 mA. The current through the load was 22 mA. The current through the Zener diode was 23 mA. The ac ripple voltage increased to 2.1 mV. It was concluded that the dc power supply should be re-designed so that the load could be disconnected without destroying the Zener diode at 96% VAC and to obtain a larger Zener diode current at 80% VAC to reduce the ac ripple voltage.

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.

-----------------------

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download