DC Power Supply

5/28/2018 DC Power Supply

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Creating a DC Power Supply

2/22/2014

Abstract:

This report discusses the design and testing of a DC power supply, using a transformer, diode

rectifier, filter and voltage regulator. DC power supplies takes standard wall outlet power andoutputs a DC voltage. The diode rectifier uses a diode to invert negative parts of the input. The

voltage regulator uses a Zener diode in regulation to maintain a constant voltage. The DC power

supply must provide 12 V at a maximum of 20 mA, with a ripple of less than 30 mV. We tested

the circuit with no current, 20 mA, and more than 20 mA. The DC power supply successfully

provided at least 12 V for all loads above 600 and met the design specifications.


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1 IntroductionThe purpose of this design project is to produce a DC power supply from an AC source. DC

supplies are useful for powering devices which do not respond well to the changing voltage of an

AC signal. DC supplies are used to charge batteries and power computers. A simple DC supply

design consists of a transformer, rectifier, filter and regulator. Transformers lower the amplitude

of the AC signal to a more reasonable voltage, which is safer to use with diodes. Rectifiers often

use diodes to ensure that voltage never drops below zero. The filter attenuates high frequency

signals to produce a flatter input. The voltage regulator consists of a Zener diode in parallel with

the load. The Zener only takes a certain amount of voltage at its reverse breakdown current,

which is duplicated across the load, providing a nearly DC signal.

2 Methods

Figure 1: DC power supply with nominal values

The DC power supply specifications are a 12 V nominal supply, with maximum output current of

20 mA and a maximum peak to peak ripple of less than 30 mV [1]. The DC power supply we


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built consisted of four different modules: transformer, diode rectifier, filter, and voltage

regulator. To determine the range of loads our circuit can handle, we use (1).

(1)

The minimum load our circuit can function with is 600 . Any load larger than 600 can be

used. However, anything smaller will draw a current larger than 20 mA.

2.1 Voltage Regulator

Figure 2: schematic of voltage regulator with load

The voltage regulator uses a Zener diode in regulation to provide a constant voltage to the

load. The Zener provides the same voltage as long as the current is within its breakdown region.

To choose a Zener for the voltage regulator, we needed a diode providing 12 V during

reverse breakdown. We decided to choose the 1N4742A Zener. To find the maximum safe

current allowed through the Zener, we used (2) and the listed value of 1 watt for maximum

power.


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(2)

The maximum safe current for the Zener is 83 mA, and the minimum is 0.25 mA. For our

design, we chose to make the minimum current 1 mA as a safety factor.

To find the value of resistor R, we found the output voltage of the rectifier, which is the

node before R from Figure 1.

(3)

Using the battery model, we replaced the Zener with a 12 V battery and a 9 resistor,

using specifications [2].

Figure 3: Voltage regulator with Zener battery model and open circuit load

Using the results from (2) (3), and Ohms law, we solved for the minimum resistance of R

using the maximum Zener current and an open circuit load. The minimum value for R is 93 .

To find the maximum value of R, we assume the ripple in VRis 1 V, which makes VRmin

19.51 V. Using our minimum load of 600 , we used Ohms Law to find (4). When the

resistance of the Zener and the load are added, their equivalent resistance is approximately 9 .

(4)

We set IDto be the minimum current allowed through the Zener (1 mA) and ILoadto be the

maximum current through the load (20 mA). We solved for the maximum value of R to be 357

, assuming the ripple is 1 V.


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We used a 330 resistor for R because the larger the resistor, the smaller the ripple on

VLoad. Using superposition, we killed all DC voltages to consider only the AC ripple. To find the

ripple on VLoad, we used voltage division rule (5).

(5)

Using the 330 resistor, the ripple (26.5 mV) is below our design specification limit (30

mV). To find the maximum ripple VRcan have, we set VACloadto be 30 mV in (5) and solve for

VRripple, which is 1.13 V.

To ensure our components can handle the circuit, we calculated the power through the

resistor R.

(6)

According to (6), the resistor R dissipates 0.22 W at the highest stress, which is below the 0.25

W limit on some resistors. However, we used a 0.5 W resistor for increased safety.

2.2 Diode Rectifier and Filter

Figure 4: schematic of diode rectifier and filter


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The diode rectifier takes the 15 V RMS sine wave from the transformer and makes the

negative regions positive. When the diode is on, the capacitor charges and provides voltage when

the diode is off. Diode rectifiers and filters together produce a signal at the maximum amplitude

with ripple caused by capacitor discharge.

To find the necessary capacitance of C, we used (6) using the battery model of the Zener

diode, and assuming D is switched on.

Figure 5: Filter and voltage regulator with Zener battery model and open circuit load

(7)

We used (6) to solve for the value of C where the ripple is 1.13 V, which we determined is the

maximum ripple in VRwhere we meet design specifications. Because the ripple increases as C

decreases, we found the minimum value of C is 345 F. We used a 1.5 mF electrolytic capacitor

to further reduce ripple in our DC supply.


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2.3 TransformerThe transformer module lowers the voltage of the mains power from the wall socket from 120 V

RMS down to 15 V RMS. Reducing the voltage helps protect diodes from current overload. For

the transformer in our circuit, we used the Elenco Digital/Analog Trainer Model XK-150 with its

built in transformer.

2.4 SimulationAfter designing our circuit, we built a model on Circuit Lab for simulation [2].

Figure 6: Circuit Lab simulation schematic


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Figure 7: Plot of transformer voltage (VS), rectifier voltage (VR) and load voltage (VL) over

time. We used time domain simulation with an interval of 0.05 s and a step of 0.01 ms.

Figure 7 shows that the simulation of our design meets specifications. The load voltage is stable

at 12 V.


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Figure 8: Time domain simulation of load voltage over time with different load resistances.

The interval and step times are the same as Figure 7.

Figure 8 shows the performance of the DC power supply with loads below 600 . With

resistance below 600 , the load voltage does not reach a stable 12 V, and the ripple is much

larger.

2.5 TestingWe built our DC power supply with the components described in the above sections on the

Elenco Digital/Analog Trainer Model XK-150. Using an open circuit load we measured the

voltages at the nodes highlighted in Figure 6 using the Agilent Infiniium DSO-2002 oscilloscope.

We used Line triggering, because the input has the same frequency as the main supply. We

compared the nodal voltages to the ones we got from our Circuit Lab simulations.


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To verify the performance of our design, we tested the circuit three different load

currents: 0 A, 20 mA and overload. For 0 A, we used an open circuit load. For 20 mA and

overload, we used a 1 k potentiometer in series with two 470 0.25 W resistors in parallel.

To test 20 mA, we used an Agilent U3401A Digital Multimeter to measure current through the

load. We adjusted the potentiometer until the DMM showed 20 mA of load current. For

overload, we set the potentiometer to its minimum resistance. We followed the same procedure

to test the DC power supply at each load. We measured the DC voltage using the DMM at nodes

VR and VL. Using the oscilloscope, we measured the ripple at the same nodes. We used AC

coupling, averaging over 256 cycles, and BW limit settings on the scope to get a clear view of

the ripple and its magnitude.

We also tested our DC supply by powering a uA741 operational amplifier. We set up a

unity-gain buffer amplifier with a 1 kHz sine wave input from an Instek GFG 8210 Function

Generator. We adjusted the DC offset and amplitude to fit the largest possible wave without

clipping.

3 ResultsComponents Nominal Values Actual Values % Error (%)

R 330 327.01 .906

Rpot min 0 0.76 -

Rpot max 1 k 1.0262 k 2.62

2 470 in parallel 235 234.08 .391

Capacitor 1.5 mF 1.2015 mF 19.9

Table 1: Components nominal and actual values with percent error

These are the values of the components we used in our DC power supply with the exception of

the transformer and diodes.


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Theoretical Calculated Percent Error (%)

Ripple on VLoad 27 mV 26.4 mV 2.222

Minimum C 354.5 F 357.7 F 0.903

Power Through R 0.219 W 0.221 W 0.913

Table 2: Theoretical and calculated values of design specifications using actual values of

components with percent error

These values were the ones we found in class using the various assumptions in the methods

section. The calculated values use the actual values of components tested in the lab.

Figure 10: Ripple at VRwith open circuit load

This is a plot of the signal in the DC supply after it has gone through the transformer. The plots

of the signal after the rectifier is shown by Figure A-1 and at the load would be shown by Figure

A-2.


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Peak to Peak Voltages

After Transformer 47.0 V

After Rectifier 281.4 mV

At Load 5.5 mV

Table 3: Peak to peak voltages at nodes of interest with an open circuit load

DC voltage

VR0 A 21.657 V

VL0 A 12.685 V

VR20 mA 19.32 V

VL20 mA 12.287 V

VRoverload 18.08 V

VLoverload 6.875 V

Table 4: DC voltages over VRand VLat 0A, 20mA and overload over the load

This table shows the values of the DC voltages over VRand VLwhile testing the power supply at

three different load currents. The saved plots of the ripples over VRand VL are in Appendix A.

At overload the DC current is below 12 V 30 mV by about 5 V and this is because the

load resistance is far below that of our design specifications. The current pulled by the load is

38.2079 mA which we measured with the Agilent U3401A Digital Multimeter.

After testing the DC supply under these three conditions we used it to power a uA741

operational amplifier. We ran a 1-kHz sine wave through the amplifier and modified its

amplitude until we had the largest waveform we could without clipping.


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Figure 11: 1-kHz sine wave through amplifier where clipping begins

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Conclusion

The DC power supply we designed met the specifications in [1]. Our design successfully

powered an operational amplifier. Our tests show that the DC supply was able to provide at least

12 V and up to 20 mA of current to loads above 600 .


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Appendix A

Figure A-1: Ripple over VR, 0 A load current

Figure A-2: Ripple over VL, 0 A load current


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Figure A-3: Ripple over VR, 20 mA load current

Figure A-4: Ripple over VL, 20 mA load current


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Figure A-5: Ripple over VR, overload

Figure A-6: Ripple over VL, Overload

DC Power Supply

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