Jacob Summerill
EE 310
Section 6
Experiment 3 Formal Report
Introduction
The task for this lab was to design and build a power supply circuit that was capable of taking a
standard 120V 60Hz AC input and outputting -15V DC with a 0.5 V tolerance. In order to
simplify the design process, the circuit was broken down into the following sections: Power
Transformer, Rectifier, Filter Capacitor, and Voltage Regulator. The power supply was to be
designed and constructed during a 2 week period with the transformer, rectifier, and filter
capacitor being built during the first week, and the voltage regular added in the second week.
In addition to the voltage output requirements, it was also required to have the nominal
voltage into the regulator be -24V DC, have a maximum ripple voltage of 3.6v (15%) at the
regulator input, and be rated to provide 1w to the load. Lastly, as well as having the circuit
meet all the requirements, it had to be built to be reliable. It was suggested to use components
that were rated to operate at 1.5-2 times the calculated values.
Circuit Design
When designing this power supply we were confined to using the components available in the
EE stockroom. This caused some of the sections of the circuit to only have one applicable
component, while other sections had a list of possible components to choose from. We were
limited to the following components for each section:
Power Transformer- 12:1:1 Center Tapped Transformer. The circuit diagram of this transformer
can be seen in figure 1 below.
Figure 1
Rectifier- Full wave using any of the diodes located in table 1 below
Table 1
Diode Part Number Rated Peak-Inverse-Voltage Average Current Rating Peak Current Rating
1N4004
400 V
1.0 A
30 A
1N4934
100 V
1.0 A
30 A
1N4935
200 V
1.0 A
30 A
Filter Capacitor- Electrolytic with any of the following capacitance:
47µF, 100µF, 220µF, or 470µF rated at 50V DC working voltage
1000µF rated at 16v DC working voltage
Voltage Regulator- Two different types of regulators were to be built and compared. The first was a
Zener Diode regulator using any of the diodes from table 2. The second was an integrated-circuit (IC)
regulator using any of the IC’s from table 3.
Table 2
Diode Part Number Zener Voltage
1N4728
3.3 V
1N4732
4.7 V
1N4735
6.2 V
1N4736
6.8 V
1N4738
8.2 V
1N4739
9.1 V
1N4742
13 V
Table 3
IC Regulator Part Number Input Voltage Range Typical Output Voltage
LM7805
7.5 V – 20 V
5V
LM7815
17.5 V – 30 V
15 V
LM7905
-20 V – -7 V
-5 V
LM7915
-30 V – -17.5 V
-15 V
In addition to these components we also had access to a variety of ¼ watt resistors and a few 2 watt
resistors.
Power Transformer
Since there was only one transformer available to us it made for an easy decision when designing this
part of the power supply. However, the transformer does have a small internal winding resistance which
must be taken into account. This resistance can be modeled by figure 2 below.
Figure 2
With a 12:1:1 transformer connected to 120 Vrms source the output will be VAB = 10 Vrms, VBC = 10 Vrms,
and VAC = 20 Vrms. These and all other calculations can be found in “Supporting Analysis” under
the corresponding heading.
Rectifier
The combination of having a center tapped transformer and the information we’ve learned so far in EE
310 gave us two options for building a full wave rectifier; center-tap rectifier or a bridge configuration
rectifier. Since the input to the voltage regulator was required to be -24 V, the rectifier had to output at
least this much magnitude since nothing else in the circuit will change the voltage. The power
transformer we chose to use will provide 20 Vrms max when not using the center tap. 20 Vrms is about
28.3 Vp which will be enough to satisfy the 24 V DC requirement. Since the transformer has to be used
without the center tap than a bridge configuration rectifier must be used. Figure 3 on the following page
shows what this rectifier circuit looks like with the input from the transformer modeled as an AC source.
Figure 3
The maximum voltage seen at Vout is the peak AC voltage minus the voltage across any diodes that are
on. At any given time there are going to be two diodes on and they can be represented simply by a
constant voltage drop of VD = 1 V. Since the peak AC voltage is 28.3 V, this gives a maximum Vout of 26.3
V. The directions also say that RL should be absorbing 1 watt which will require a resistance of 576 Ω
assuming that Vout is going to be our desired 24 V once we add the filter capacitor. Also, since we
ultimately want a negative voltage setting the positive terminal as ground will give us the negative
output we desire. Lastly, the diode will see a PIV of 28.3 V so any of the diodes available should work for
this application. Choosing to use the 1N4004 would be the best decision as it has the highest PIV rating
so it will see the least stress.
Filter Capacitor
Now that we have a rectified DC signal we need to reduce the ripple voltage to meet the specifications
of the power supply. This can be done by adding a rather large capacitor in parallel with RL. In order to
save on cost and size we will be using the smallest electrolytic capacitor that will meet the
requirements. Since C and Vr are inversely related using the largest allowable Vr (3.6v) will give us our
smallest possible C value of 105.7 µF. Normally I would treat this as the absolute minimum and use the
220 µF capacitor to be safe. However, when using electrolytic capacitors there is often a rather large
tolerance that is commonly +80%/-20%. With this in mind using a 100 µF capacitor should be acceptable
but it may need to be swapped with a 220 µF if the specifications aren’t met when taking
measurements. Also, a 100 µF capacitor will give an average Vout of -24.5 V which is much closer to the
desired Vc than it would be if using the 220 µF.
Supporting Analysis
This section contains all of the calculations for the values that were referenced in the circuit design.
Power Transformer
12:1:1 Transformer means Vin/12 = VAB/1 = VBC/1 -> VAB = VBC = 120Vrms/12 = 10 Vrms
VAC = VAB + VBC = 10 Vrms + 10 Vrms = 20 Vrms
Rectifier
Vpeak = Vrms * √ = 20 * √ = 28.28 Vpeak
Vout max = Vpeak – 2 * VD = 28.3 V – 2 * 1 V = 26.3 V
RL = (VRL)2/PRL = (Vc)2/PRL = (-24 V)2/ 1 W = 576 Ω
PIV = Vpeak = 28.28 V
Filter Capacitor
C = Voutmax/(2*f*R’L*Vr) = 26.3 V / (2 * 60 Hz * 576 Ω * 3.6) = 105.7 * 10-6 F = 105.7 µF
Data
Power Transformer
When measuring the open circuit voltages of the power transformer we found that VOC of
AB was 9.9 Vrms, VOC of BC was 9.9 Vrms, and VOC of AC was 20.4 Vrms. By connecting a 432 Ω
test resistor across AC and measuring a Vtest of 20.15 we were able to calculate the internal
winding resistance to be 2.68 Ω.
Rectifier
Once the internal resistance of the transformer was known it was time to build and connect
the rectifier portion of the circuit. Once the circuit was built we connected it to the
oscilloscope to ensure that the rectifier was working correctly. The figure below shows the
capture from the oscilloscope of these measurements.
Figure 4
As you can see the circuit was able to correctly rectify the AC input to our required -27 V
DC component. Of course since there is no filter capacitor installed yet the voltage has an
extreme ripple. The figure on the next page shows the improved output after connecting
the filter capacitor. This is our Vc that will be fed into the voltage regulator.
Figure 5
From the measurements in the figure you can see that the ripple voltage at VC is max – min
= 4.2 V. When comparing this voltage to the input from the transformer in the figure on the
next page you can easily see how the rectifier and filter capacitor were successfully
converting the AC source into DC.
Figure 6
Now that we had our Vc it was time to connect the voltage regulator to filter the remaining
ripple voltage from our power supply. The next figure shows the waveform of the voltage
coming from the zener diode shunt regulator.
Figure 7
As you can see the ripple voltage has practically been eliminated by using this regulator. We
were asked to compare the waveforms from the regulator with and without a load attached
but the graphs were identical when attached to a 665Ω load because the % regulation was so
low. When measured with the DMM, the open circuit voltage was -14.92 V while the voltage
across the load was -14.73. This gave us a % regulation of 1.27%. After gathering all the data
from the shunt regulator it was removed from the circuit and replaced with an LM7915 IC
regulator. With this regulator we compared the ripple across the filter capacitor to the ripple
from 3 different load resistances: 470Ω, 360Ω, and 270Ω. The following 3 figures show the
results from these different tests.
Figure 8
Figure 9
Figure 10
As you can see, the LM7915 did an exceptionally well at regulating the voltage from the filter
capacitor. Even with such a small load of 270 Ω the regulator only produced 14.8 mV of ripple.
When measured with the DMM we saw an open circuit voltage of -14.94 and a voltage across a
454 Ω load of -15.19. This gave us a % regulation of 1.67% which was slightly higher than the
shunt regulator.
Discussion
From the information in the previous section a few deductions can be made about this
experiment. First we can look at the design parameters and see how our circuit performed.
First, the instructions said to design the circuit for -15 V with a 0.5 V tolerance. The output of
both voltage regulators stayed well within this range for a variety of different loads which
shows that this specification has been met. As for the voltage into the regulator input, the
specification was -24 V while in our circuit it ranged from -23.6 V to -27.8V with the ripple. With
a maximum ripple of 3.6 V this means that the voltage into the regulator falls in the specified
range. As for the ripple itself, the input at Vc did see a ripple of 4.2 V which was slightly higher
than the maximum allowed. This is most likely caused from using a filter capacitor that was
slightly smaller than the calculated value. This can easily be corrected by using the next larger
size capacitor. Also, the circuit was designed to be able to provide 1 watt of power to the load
resistor at -15 V. Lastly, all of the components that were selected to be used were rated for
conditions well over what would be seen in the circuit.
Summary and Conclusion
Overall this experiment gave an insight on how to convert AC power to DC power. This is a very
useful circuit to know as a lot of electrical appliances have a circuit similar to this built into
them. Since the electricity that is most often seen is 120 V AC, being able to convert that to DC
allows us to work with a much wider range of inputs when designing a circuit. Lastly, the
information we learned about voltage regulators can be extremely helpful when working with
components that are sensitive to fluctuations in their input voltage. It also shows us another
method of creating a buffer circuit other than the op-amp buffer circuit.