Download Experiment 3: Power Supply Design Project Design Team A

Alex Hench
September 23, 2014
EE 310
Section 6
Experiment 3:
Power Supply Design Project
Design Team A
Introduction:
In this lab we were tasked to design and build a complete AC to DC low-voltage power
supply. This power supply circuit is meant to take a typical 120 Vrms input, operating at 60 Hz,
and convert this to a filtered and regulated DC output. This circuit consisted of a power
transformer, rectifier, capacitor, and voltage regulator supplying the output to a load. Our team
was tasked to designing the power supply with a filtered voltage of 12 V and a regulated output
voltage of 5 ± 0.5 V, using a full-wave rectifier with two 1N4004 rectifier diodes. We also used a
12:1 step-down transformer, a 470 µF electrolytic capacitor, and a 1N4733 zener diode rated at
5.1 V.
Circuit Design:
The full-wave rectifier inverts the negative portion of the sinusoidal so that the output is
generated during both halve of the sine wave, seen in figure 1. Our rectifier circuit was designed
with two diodes on the positive and negative terminals of the transformer respectively. This
allowed for the first diode to be conducting during the positive inputs of the sine wave while the
second diode was off, and vice versa for the negative inputs. The full-wave rectifier decreases
the peak to peak ripple voltage compared to a half-wave rectifier. This allows for less
capacitance required in the filter circuit and less power is dissipated by the regulator.
In this part we built and tested two voltage regulators, the first one using a zener diode in
breakdown region and the second circuit using a commercial IC voltage regulator for comparison
of accuracy. The voltage regulator is necessary to keep the DC output constant even when the
input signal or load current changes. In the second part of this experiment, we designed the
regulator to give us an output voltage of 5 V and minimize the changes in this voltage. We used a
zener diode rated at 5.1 V in breakdown region to give us that 5 V output. This diode was built in
parallel with our filter capacitor and load. The filtered, unregulated voltage of 12 V can be
represented as our input battery for the regulator.
Supporting Analysis:
The first step in designing this circuit was stepping down the 120 V input using a power
transformer. The configuration for our transformer utilized terminals A and C as our input to the
rectifier, and terminal B connected to ground. Since this transformer is not ideal, there are some
internal resistance in the windings. So we first calculated this resistance from our measurements
of the voltages from terminals A-C and a test current. Adding the two diodes, one connected to
terminal A and the other to C, was the first step in the reducing the ripple voltage and creating a
DC output. This rectified signal can be seen in figure 1 with an rms voltage of 9.65 V. With our
nominal DC voltage across the capacitor of 12 V, we were able to determine the appropriate load
resistance and used a 150Ω load. After taking an oscilloscope plot of our filtered voltage (figure
2) we were able to take measurements for our maximum, minimum, and peak to peak voltages.
Using this information, we could predict the maximum diode current.
For the voltage regulator, we needed to determine several parameters based on the
wanted output voltage 5 V. The maximum zener current could be determined from the power
dissipated from the regulator divided by our 5 V output. For a safety margin, we used less than 1
W of the power dissipation, 0.5 W. Knowing that the load current would increase when a load
resistance is connected to the output, the minimum zener current would be 30% of the maximum.
Having our regulator voltage input and output and our maximum zener current, we could
determine the resistance Ri, connected to our input. Finally, our load resistance was determined
by dividing the output voltage by the difference in our maximum and minimum zener currents.
Data:
As mentioned above, the transformer was configured with terminals A and C connected
to the diodes and terminal B connected to ground. We took several voltage measurements from
the transformer. The voltage measured from A-B was 10.5 V, B-C was 10.4 V, and as expected
A-C was 20.7 V. These measurements were taken under no-load conditions. From here, we
determined the internal resistance of the transformer to be 6.82 Ω. Figure 1 shows the rectifier
output voltage and shows the rippled voltage signal that was created from the diodes. In figure 4,
we compared the unregulated filtered voltage to the output from the transformer. You can see
that the transformer is a typical AC sine wave, while the filtered voltage is beginning to look like
a constant DC voltage with small ripples occurring. The diode current waveform shown in figure
5 demonstrates the fast spikes in current that happens when the capacitor discharges. The peak of
the waveform is the maximum diode current. The output voltages with and without the load
generated a constant DC voltage seen in figure 6. Using a multi-meter to measure the output, our
no-load voltage was 4.88 V while our load voltage was 5.2 V. The IC regulator gave us better
results and the load voltage is shown in figure 7. Our load voltage using the DMM was 4.93 V
and the no-load voltage was 4.92 V. This gave us a significantly low percent regulation, roughly
0.2%. The regulation when using a zener diode was 6.15%. This shows the improved voltage
regulation of the IC regulator compared to the zener diode.
Discussion:
Considering the power dissipations we added a safety margin when choosing our
components by multiplying our calculated values by 2 to ensure they can operate under these
conditions. It was important to understand that the load resistance can take 1 W when
considering the power dissipated across the load. Our calculated values varied from the actual we
used in our circuit due to what was available to us. For example, we calculated the resistance Ri,
to be 76.5Ω but the closest value we could use was a 75Ω resistor. This discrepancy could affect
our measured values as well as understanding the devices we used are non-ideal. The output
voltage when using a zener diode measured by the oscilloscope was 5.12 V, which was in the
range of our desired voltage, but could be closer. However, the IC regulator gave us a much
better result, measured with oscilloscope the output voltage was exactly 5.0 V. This shows that
the commercial IC regulator is a much better way of regulating our voltage.
Summary and Conclusions:
Overall, this lab demonstrated the importance of a properly designed power supply. A
poorly design circuit can result in unregulated output that can change with any changes in the
input, resulting in unwanted AC ripples. Throughout the design process, careful considerations
were taken so that we did not short any components and the devices we used would result in the
appropriate output. This lab demonstrated the uses for rectifiers, filters, and regulators, and gave
us experience in the actual design of these circuits.