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Triple Output Power Supply Lab
Joe O’Connor | 812690496
EE 330 Lab, Fri 1PM
Due: 4/24/2015
(schematic)
(my completed circuit)
This project had us use the concepts we learned of the theoretical rectifier, regulator and filter and to
apply them through calculations and experiments. This is the first lab in this course where we used the
soldering equipment; we soldered many components on a circuit board we were given. This lab is
important because we learned how to solder and how power supplies work. We created the power
supply in the same way many other power supplies are made so the process we used in this lab can be
applied to other power supply designs. The project was divided into 3 parts, completing one per week.
Part 1
Related graphs:
(Figure 1: Block Diagram of a linear power supply)
(Figure 2: Input and Output waveforms of a full wave bridge circuit)
(Figure 3: Full wave bridge rectifier current with positive output voltage)
(Figure 4: Full wave bridge rectifier circuit for vs > 0)
(Figure 5: Full-wave bridge rectifier circuit for vs < 0)
Formulas for calculations:
Before I did any measurements or calculations, I first read about what a DC power supply is and
how they are constructed. I learned that a power supply is an integral part of any electronic circuit
design. It is very important for an electrical engineer to be able to calculate and predict power supply
performance and also to construct one. I also learned that a DC power supply is an electrical energy
source that supplies smooth and robust DC output from and AC input. I also learned how linear power
supplies are constructed using 4 distinct steps. The first is to transform the line level AC to a lower level
AC using a transformer. The second step is to rectify the low level AC by pulsating DC using rectifier
diodes. The third step is to filter the pulsating DC into a relatively steady state DC using capacitors. The
fourth and final step is to regulate the steady state DC into a constant level output using regulating IC’s.
This four step process can be shown in figure 1 above.
The first step of part 1 was to do some preliminary calculations. I first grabbed a 10kΩ and a
4.7uF capacitor from the parts bin. I measured these to be exactly 9.933kΩ and 4.179uF for the resistor
and capacitor. Then I set up the circuit so that the RL was in parallel to the capacitor. Then I found the
value of the transformers secondary peak-to-peak voltage (VSpk-pk) to be 41.4v. I also found the value of
the peak secondary voltage (VSpk) to be 20.7v. The waveform I got is below:
From the calculations I then found RL(C) to equal .415. I then compared that to 10T. Using
RL(C) < 10T, I made the assumption that the equation is accurate. I then calculated the peak-to-peak
ripple voltage (Vrpk-pk) using the accurate equation of the period T of the ripple filtered waveform to
equal 3.15V. Then I found that the period of the ripple (Tripple) equaled 16.2ms. I then used ohms law to
calculate average voltage of the filtered output as well as the average DC current in the resistive load. I
got VDC to be 17.1 and IDC to be 1.71A.
I then grabbed a 10uF capacitor from the parts bin to replace the 4.7uF capacitor. I measured
my new capacitor to equal 9.38uF. Using the same values for RL and VSpk I calculated that RL(C) was
greater than 10t this time so the next steps will be using the approximate equation. I then found that
Vrpk=pk equaled 1.696V and then from there I found VDC to equal 17.8V and IDC to equal 1.78A.
The second step of part 1 was some component installation. I soldered several components to
the circuit board I was given. Parts I soldered to the circuit board were diodes, wires, and a 0Ω resistor
(jumper wire). This was the first time I got to solder in this class and I enjoyed it a lot. I loved the hands
on application. I wasn’t very good at soldering at first but after doing it a few times I made some very
solid connections. After I installed these components I was ready to move on to step 3.
Step 3 of part 1 was some measuring using the oscilloscope connected to the newly soldered
circuit board. I had to connect everything together to resemble the picture below using RL as 10kΩ and C
as 4.7uF (the same parts as before).
I first connected the resistive load and the capacitor like the diagram above. I then connected all
wires appropriately. Then I found the transformers peak secondary voltage (VSpk) to be 19.5V. I also
found Tripple to be 16.2ms and Vr-pk-pk to be 38.2V with a percent difference of 0.5%. This output is
desirable because the percent difference is very small. The ripple waveform I got is in the chart below:
Then I measured the specified values and compared them to the original. I got the average DC
output voltage (VDC) to be 17.28V with a percent difference of 3%. Next I measured the average DC
current through RL, (IDC) to be 1.5A with a percent difference of 6%. I then replaced the 4.7uF capacitor
with the 10uF capacitor from earlier and then recalculated the values. After switching the capacitor I got
Tripple to equal 9.8ms, Vrpk-pk+ to equal 3.3v with a percent difference of 0.78%. This ripple waveform is
below.
I then measured average DC output voltage (VDC) to equal 17.2 with a percent difference of 6%.
Next I measured average DC current in RL (IDC) to be 1.6A with a percent difference of 9%.
For the fourth and final step of part 1, I measured output resistance. I learned the output
resistance of a power supply can be calculated by measuring a change in the output voltage divided by a
change in the output current (Ro - 0Ω if ideal). The formula representing this is below:
𝑂𝑢𝑡𝑝𝑢𝑡 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 =
|∆𝑉𝑜 |
|∆𝐼𝑜 |
I then began to calculate the measurements using the 10uF capacitor I have been using. Using
the DMM I measured the DC output voltage without a load resistor (Vno load) to equal 19.36V. Then I
measured the value for the 10kΩ resistor to be 9.933Ω. I then found the output voltage (V1) to be
19.12V and the output current (I1) to be 2A. Next I replaced the 10kΩ resistor with a 5.1kΩ and redid
some measurements. I got the output voltage with the new RL (V2) to be 17.94V and the output current
(I2) to be 4A. The output resistance of the rectifier (Ro) was 265.3Ω using the formula below.
𝑅𝑂 =
𝑉1 − 𝑉2
𝐼2 − 𝐼1
Modeling my output terminals as a thevenin equivalent circuit I got VTH to be 19.5V and RTH to
be 265.3Ω. Using the thevenins equivalent circuit, I measured the 6.8 load resistor to be 6.62Ω. Then I
calculated the expected Vo if RL is 6.8Ω to be 19V. I then measured the actual value to be 18.1V with a
percent difference of 5%.
Part 2
The second part of the power supply project focused on regulation performance. I learned
about things such as line regulation, ripple rejection ratio, and load regulation. I also learned about
terminal DC regulators. The last thing I learned about is thermal analysis and how temperature can
affect the circuit.
Formulas used this section:
Related Figures:
Before moving to step 1, I learned some crucial definitions:
1. Line regulation – the ability of a circuit to maintain a selected
output under varying line voltage input conditions.
2. Ripple rejection ratio (RRR) – measure of how well a regulator
circuit reduces ripple voltage.
3. Load regulation – measure of circuits ability to maintain prescribed output voltage under
varying load (different RL) conditions.
4. Regulator Input-Output Differential VI/O –the potential difference between the unregulated
input voltage and the regulated DC output voltage for which the regulator will function properly.
5. Regulator Dropout Voltage VDO – minimum input-voltage at which the circuit ceases to regulate.
6. Input Voltage VDCIN – unregulated DC voltage with respect to ground that is applied to the input
terminals of the regulator.
Step 1 of part 2 was all calculations. The answers from these calculations come from the formulas
above as well as measurements from lab materials. I first found values for Vref and IADJ which were 1.25v
and 30uA. Next I was required to measure and record the actual values of two 330Ω, ¼ W resistors. I
grabbed two resistors off the wall and measured them as 324Ω and 325Ω. I then was required to grab a
1.2kΩ, ¼ W and 3.3kΩ, ¼ W resistor. I measured the actual values to be 1.18Ω and 3.21Ω. I then
calculated VO if R2 = 330Ω and R1 = 0Ω to equal 1.25V. Similarly I then calculated VO if R2 = 330Ω and
R1 = 1.2kΩ to equal 5.86V. Finally, I calculated VO if R2 = 330Ω and R1 = 3.3kΩ to equal 13.91V.
Step 2 of part 2 was component installation. I had to solder capacitors, regulators, heat sinks,
resistors, stability capacitors, and wires to the circuit I have been working on which is shown below. It
was important to be very carful with soldering the components in this section. It was necessary to pay
attention to the positive and negative ends of the components as well as the direction they are facing. I
also had to be careful with soldering the middle part of U1 and U2, connecting the solder to the other
ends can cause a short.
Step 3, the last of part 2 was some measuring output voltage and case temperature of LM317. To
do these measurements I had to hook up the circuit to the transformer box as shown below:
I first had to short the wires from T8 to T9 to make RTest = 0Ω. I then measured the DC output
voltage (Vo) of the LM317 with respect to ground to equal 1.25V. Next, I measured the DC voltage
across the 330Ω resistor (Vref) on PCB to be 1.25V. Then I connected RTest = 1.2kΩ between T8 and T9 and
measured VO (H7) to be 5.87V and that’s a percent difference of 0.20%. I then repeated that process but
with a RTest of 3.3kΩ and got a VO to be 14V with a percent difference of 0.75%.
The next several calculations use a RTest of 1.2kΩ and RL of 330Ω. I first measured the actual
value of the 330Ω to be 327Ω. I then connected RL between H7 and H8 on the protoboard and measured
the voltage (VO_330Ω) across it to be 5.872V. I then calculated the current and power in RL (PL_330Ω) to be
0.08808W. Next using the same method as earlier, I measured the DC voltage (VDCin) at the input (TST6)
of LM317 regulator to be 19.5V. Then I calculated the In/Out voltage (VI/O) drop through the regulator to
be 13.83V by (VDCin - Vo) and I got the power dissipated (PD) in the regulator to be 0.216W.
I was next required to measure the temperature of the room (TA), I got it to be 23.5⁰C. I then
measured the temperature of the regulator (TC(calc)) to be 36.2⁰C. By using the thermal ohms law and
thermal equations I was able to get the next set of calculations. I got ѲJA to be 63⁰/W and ѲJC to be
5⁰/W. I next estimated the junction temperature (TJ) to be 37⁰C. Finally, I measured the regulator cases
temperature (TC(meas)) to be 38.8⁰C. Comparing this to TC(calc) I got a percent difference of 0.8%.
Part 3
The last part of the power supply lab consists of measuring output resistance, component
installation, and characterizing the LM7805, LM317, and LM337. All measurements and calculations
come from the formulas throughout this lab as well as DMM and oscilloscope readings.
The first step of this part was to measure the LM317 output resistance. This section, as well as
all sections, has a lot of quick calculations and measurements. Rather than explaining each step in detail
like before, I will just summarize the procedures to avoid going over the page limit. All values and
calculations for this section can be seen attached to this document.
I started off by measuring the values actual values of the resistors. Then I connected the
resistors to the circuit and measured the voltages, currents, and power with no load connected using
the DMM.I repeated the process of finding these values with two different resistors. After these
measurements I created a thevenins equivalent circuit with my values. Adding a regulator the rectifier
circuit caused R0 to equal 0. The advantage of this made it so we can use the regulator to make V0 to
remain constant. I then discovered that a regulated rectifier was a more ideal voltage source in this
situation.
Step 2 of part 3, was component installation. We had to carefully solder trim pots, output wires
as well as hook-up wires. This was simple and didn’t take very long but it was very important to not
solder the middle part of the trim pot with the side connections, this will cause a short circuit.
Steps 3, 4 and 5 of part 3 are characterizing the functionality of the LM317, LM7805 and LM337
regulators. For each of these regulators, we had to find several key calculations/measurements. For
each regulator, we had to find: Open Circuit Output Voltage of the regulator, the ripple rejection ratio, %
load regulation, and the short circuit protection. These were obtained using the DMM, oscilloscope and
formulas shown throughout the project. These values can be found at the end of this project but to
summarize I got the open circuit output voltages (V0) to be 5V, 10.5V, and -10.5V for the 3 regulators. I
got the ripple rejection ratios (RRR) to be 18.5dB, 11dB, and 50dB. I then got the % load regulations
(%reg) to be 0.7%, 0.25%, and 0.5%. Finally the short circuit protection was calculated by removing the
resistive load and shorting the output. The values I obtained were 5V, 10.5V, and -10.5V. This meant
that the circuit was protected. Waveforms from these steps are below:
Observations and answers to thought questions
The filter capacitance makes it so we can transform AC into DC voltage when the capacitors
charge. This is shown from the graphs above. As the line of the graph went down, the capacitors held a
charge that minimized the ripple of the waveforms. After each cycle is completed, the waveform goes
back up to the peak and then goes down again. The wave never goes below 0 because the capacitors
carry the charge that sticks around.
Voltage regulation is designed to maintain a constant voltage level. When maintaining a
constant DC voltage this affects the ripple voltage and ROut so that the values remained constant over
time.
The impact of LM317 on the output resistance of the diode bridge shows us that the regulator
added to the rectifying circuit of RO always was around 0. So, one advantage of using regulators as
output voltage is that the output voltage remains constant.