Content on this page is mostly from the lab report that I submitted for this project. Parts may have been omitted. Like with all content on this site, it may be incorrect, so you should use it at your own risk. Remember to modify and reference the content if you are using it in your own work, as you may be caught by plagiarism checking services such as Turnitin.
This assignment demonstrated the principles behind an AC to DC Power Supply, including different rectifier circuits, the relationship between different circuit components and load performance of the regulator circuit. The assembly of the power supply was split up in to 5 lab sessions, with each session progressing through the understanding and assembly of the PSU. Several tests were carried out at different stages of assembly as well as when the PSU was completed. The maximum percentage error calculated on all tests was 3.8%.
Introduction
This assignment was performed to become familiar with soldering components on to a printed circuit board and the principles behind AC to DC power supplies. The main principles covered in this assignment are different rectifier circuits, and their relationship to ripple and polarity, load performance of the regulator circuit and the general use and relationship between the different components.
The complete power supply will comprise of a variable output of 0V – 15V DC (yellow post), a 5V DC output and a -12V DC output. The 5V DC output can be connected using the red post or the USB connection which can be used for charging a mobile phone, and the -12V DC output can be used for powering operational amplifier circuits.
Tests were performed throughout the construction of the power supply, to test the different stages of converting 12V AC to the various DC voltages. The results of the tests were gathered in a log book. After the construction of the power supply, the efficiency of the unit was tested to ascertain any power losses between the mains AC and the various DC outputs. Regulation and ripple tests were carried out at nominal output voltages of 5V, 10V and 15V.
Results for the tests were gathered and curated into tables. Graphs show the results in a more visual way.
Theory
AC Transformer
This project uses a 240V AC to 12V AC transformer. To make it safer so that mains voltages does not have to be handled, the transformer is housed in a class II (double-insulated) plug.
Ideally, a transformer would transform an input voltage (in this case 240V AC) to an output voltage (in this case 12V AC), without any energy losses during the conversion process (an efficiency of 100%). In reality, winding resistance, iron loss in the core and flux leakage cause all transformers to have a <100% efficiency. [1]
Cathode Ray Oscilloscope
An oscilloscope is a lab instrument with one of its functions being able to read sinusoidal waveforms. The oscilloscope used for the experiment used a cathode ray tube, which fires an electron beam on to a phosphorus coated screen. [2] The oscilloscope traces the voltage on the Y-axis, and time on the X-axis, by using the deflection plates as part of the cathode ray tube. [3] This is what makes it appear as a wave when a sinusoidal waveform is inputted into the oscilloscope. A picture below shows a sine wave on the oscilloscope, during the experiment:
The waveform can be read to determine the peak to peak amplitude of the waveform, and in turn the RMS amplitude can be calculated. The diagram below shows the different properties of a sine wave:
Root Mean Squared (RMS)
RMS stands for root mean squared. This is because the peak amplitude is divided by the mean, being 2, squared. Below is the equation used to calculate the RMS amplitude:
Rectification and Smoothing
Full-wave bridge rectifier
The power supply utilises a full-wave bridge rectifier to convert the AC signal into a DC signal. It does this in stages. The diagram below shows that the power supply includes removable links to analyse the signal when different diodes are connected:
By removing Link 2 from the circuit, single diode rectification (half-wave rectification) takes place. The diagram below shows what half-wave rectification looks like:
![Figure 6: Half-wave rectification [4]](img/figure6.png)
Connecting Link 2 uses all four diodes, which results in full-wave rectification, show below:
![Figure 7: Full-wave rectification [4]](img/figure7.png)
Smoothing Capacitors
Capacitors store charge; this makes them useful for smoothing DC signals with fluctuations as they act as a buffer. As the rectifier signal voltage increases, this charges up the capacitor and when the rectifier signal voltage returns to 0V, the capacitor starts to discharge, but this happens a lot slower than that the raw rectifier signal. The diagram below illustrates this:
![Figure 8: Full-wave Rectifier with smoothing capacitor [5]](img/figure8.png)
Voltage regulation
Two linear voltage regulators are used in the power supply to make sure that a steady voltage of a specified value are provided on the outputs. The voltage regulators are capable of taking an unregulated input voltage, in this case from the smoothing capacitors, that can be fluctuating over time and outputs a regulated voltage. There are capacitors positioned around the voltage regulators and these are there to maximise the stability of the output voltage. The voltage regulators used in this project are capable of regulating loads of up to 100mA, anything greater than 100mA require a heatsink to dissipate the waste heat. The waste heat makes linear voltage regulators very inefficient. The power wasted in a linear regulator can be calculated using the following formula [6]:
This means that if a 12V DC unregulated input is regulated down to 5V DC while drawing 500mA, the voltage regulator will generate 3.5W of wasted heat.
Percentage Error
The percentage error is the different between the actual results and the expected results, as a percentage of the expected results. It can be calculated using the following formula [7]:
Risk Assessment
The experiments were carried out in the lab. There were some risks associated with carrying out the experiment in the lab. A main hazard was the potential trip hazard if students’ bags are left on the floor where people walk. To prevent this the bags should be tucked underneath the desks; clear of any gangways. Soldering irons were used for this project. The tip of a soldering iron can reach temperatures up to 400°C, meaning the tip should never be touch while in use. The soldering iron should be left to cool before storing. A soldering iron should be kept in a stand while not in use. The nature of this project involves the use of electricity. Safe working practices should be observed to avoid shorts or electrocution.
Method
Construction
The power supply was constructed over several lab sessions, soldering components to the PCB in order of the circuit diagram from left to right. See appendix 1 for a complete circuit diagram [8].
Stranded wire was used for all wired connections as this is more flexible and can withstand a greater number of flexes, compared to single core wire.
After soldering all of the diodes to the rectifier part of the circuit and connecting the board up to the DMM and CRO, neither showed any voltage when probing the junction between D3 and D4. It was discovered that there was an error on the PCB. The fix for this can be seen in the photograph below:
As a result of the error, it caused the fuse to blow. This was easily replaced. Electrolytic capacitors are not designed to be connected with the polarity reversed. The polarity of the capacitors were labelled incorrectly on the PCB silkscreen. The error was scratched off with a knife before soldering the capacitors in, so the capacitors or any other components were not damaged.
Throughout construction, different parts of the circuit were tested to make sure they were functioning as expected. The potentiometer was initially wired up incorrectly, causing the variable voltage to increase as the potentiometer was turned down, and for the variable voltage to decrease as the potentiometer was turned up. This was easily rectified by switching the potentiometer cables.
Some of the components inherently produce quite a bit of heat, meaning they required attention to make sure they did not overheat and become damaged. The voltage regulators were electrically insulated and attached to a heatsink, and the 7W resistor was raised away from the board. The method of raising the 7W resistor away from the board was by looking the legs, as shown in this picture:
![Figure 11: Joseph Taylor’s completed PCB [9]](img/figure11.jpg)
Components were double checked on the PCB and the board was mounted into the enclosure, with the external components being mounted into the enclosure. See appendix 2 and appendix 3 for photographs of the power supply in its completed state.
Test procedure
Although tests were carried out throughout the construction of the power supply, tests were also performed once the unit was complete to measure the performance of the power supply as a whole. Firstly, the transformer was connected to mains 240V AC and the switch was set to it’s on position, resulting in the LED illuminating.
The rheostat was connected to the variable 0V DC – 15V DC output of the power supply. Current was measured using a digital multimeter in series with the rheostat. Voltage was monitored with a digital multimeter connected in series with the rheostat converter.
The rheostat simulates a load on the circuit. It can be adjusted so that it draws a specified current.
The power supply’s potentiometer was set to its maximum, so that a voltage of 15V DC was on the output. The value of the rheostat was decreased in increments of 10mA until the output voltage became unstable.
This test was repeated twice more, with the variable output voltages at 10V DC and 5V DC respectively.
Equipment Used
|
ANTEX Soldering Iron | |
|
Digital Multimeter (DMM) | Serial number: MBGK043285 |
|
Hameg HM400 Cathode Ray Oscilloscope (CRO) | Serial Number: 060369128 |
|
HY3003 Variable Bench Power Supply | Serial Number: 263531 |
|
Rapid Toolkit | Screwdrivers and solder sucker used |
|
Rheostat | Variable resistor |
Components
|
Barrel Power Socket | For connecting the AC transformer |
|
Fuse and Fuse Holder | To provide protection to the circuit |
|
Diodes | 4x IN5403 Diodes |
|
Capacitors | 1x 470µF 1x 1000µF 2x 10µF 1x 22µF 4x 100nF |
|
LED and LED Holder | Illuminated when the unit is powered on |
|
Variable Resistor/Potentiometer | 2200 Ω variable, used to vary the resistance for the LM317 voltage regulator |
|
LM317 Voltage Regulator | 1.25V DC – 37V DC variable regulator |
|
LM7805 Voltage Regulator | 5V DC regulator |
|
Resistors | 1x 1000 Ω 1x 6.8 Ω 7W 1x 200 Ω 1x 100 Ω 1x 2200 Ω |
|
Switch | On-On Switch |
|
Voltage Meter | Analogue, capable of displaying 0V – 15V DC |
|
Connection Posts | Yellow (12V DC) Red (5V DC) Black (GND) Blue (-12V DC) |
|
USB Socket | To provide 5V DC |
|
DC to DC Converter | NME0512, used to provide -12V DC |
|
Enclosure | Used to house all of the components |
|
AC Transformer | Input: 230V 50Hz AC Output: 12V 1.6A max AC Model: AD4830-12.0-1600 |
|
PCB | Designed by Ian Watts |
Results and Discussion
Test results when LM317 regulator is set to 15V DC
The table below shows the LM317 output voltage decrease from 15V DC, as the value of the rheostat is decreased, increasing the current draw.
| Current (mA) | Voltage (V) | % Error |
|---|---|---|
| 0 | 15 | 0 |
| 110 | 14.63 | 2.47 |
| 120 | 14.62 | 2.53 |
| 130 | 14.62 | 2.53 |
| 140 | 14.62 | 2.53 |
| 150 | 14.61 | 2.6 |
| 160 | 14.61 | 2.6 |
| 170 | 14.61 | 2.6 |
| 180 | 14.6 | 2.67 |
| 190 | 14.6 | 2.67 |
| 200 | 14.6 | 2.67 |
| 210 | 14.59 | 2.73 |
| 220 | 14.58 | 2.8 |
| 230 | 14.55 | 3 |
| 240 | 14.54 | 3.07 |
| 250 | 14.52 | 3.2 |
| 260 | 14.51 | 3.27 |
| 270 | 14.43 | 3.8 |
The graph below shows the test results in a graphical form
Test results when LM317 regulator is set to 10V DC
The table below shows the LM317 output voltage decrease from 10V DC, as the value of the rheostat is decreased, increasing the current draw.
| Current (mA) | Voltage (V) | % Error |
|---|---|---|
| 0 | 10 | 0 |
| 90 | 9.99 | 0.1 |
| 100 | 9.99 | 0.1 |
| 110 | 9.99 | 0.1 |
| 120 | 9.98 | 0.2 |
| 130 | 9.98 | 0.2 |
| 140 | 9.96 | 0.4 |
| 150 | 9.96 | 0.4 |
| 160 | 9.95 | 0.5 |
| 170 | 9.95 | 0.5 |
| 180 | 9.94 | 0.6 |
| 190 | 9.93 | 0.7 |
| 200 | 9.91 | 0.9 |
| 210 | 9.89 | 1.1 |
| 220 | 9.87 | 1.3 |
| 230 | 9.76 | 2.4 |
The graph below shows the test results in a graphical form
Test results when LM317 regulator is set to 5V DC
The table below shows the LM317 output voltage decrease from 5V DC, as the value of the rheostat is decreased, increasing the current draw.
| Current (mA) | Voltage (V) | % Error |
|---|---|---|
| 0 | 5 | 0 |
| 40 | 4.99 | 0.2 |
| 50 | 4.98 | 0.4 |
| 60 | 4.98 | 0.4 |
| 70 | 4.98 | 0.4 |
| 80 | 4.97 | 0.6 |
| 90 | 4.97 | 0.6 |
| 100 | 4.97 | 0.6 |
| 110 | 4.97 | 0.6 |
| 120 | 4.96 | 0.8 |
| 130 | 4.95 | 1 |
| 140 | 4.94 | 1.2 |
| 150 | 4.93 | 1.4 |
| 160 | 4.91 | 1.8 |
| 170 | 4.9 | 2 |
| 180 | 4.89 | 2.2 |
| 190 | 4.88 | 2.4 |
The graph below shows the test results in a graphical form
Discussion
The results show that for all tests, the voltage decreases slightly as more current is drawn. This is expected, but it proves that the power supply works as it should.
Conclusion
After completing the construction and testing of the power supply, the understanding of how to use a soldering iron and the principle behind a power supply has increased allowing the equipment to be used competently for similar uses, in the future. The calculation used to calculate the oscilloscope’s RMS amplitude was used to compare the voltage against the reading of the digital multimeter and recorded in the log book. It was evident that the digital multimeter has a limit on the frequency it can accept. After further research on the internet, a user guide for the digital multimeter used in the experiment was found. It stated that it is only capable of showing accurate data for sine wave voltages, not half-wave rectified voltages.
The power supply proves to work, but not without a few errors on the PCB. This shows that mistakes are sometimes made, but that it is usually possible to find the fault and rectify the problem with a modification. Further research could be done on power supplies to look into a more efficient design, as the voltage regulators used in this power supply generate a lot of wasted heat energy.
Overall, this has been an enjoyable project to work on.
References
All photographs in this lab report were taken by Joseph Taylor on 8th November 2016 between 10:00 and 11:00, in C411.
All diagrams and graphs were created by Joseph Taylor on 9th November 2016, unless referenced below.
[1] ElectricalEasy, “Ideal transformer and it’s characteristics,” ElectricalEasy, 2014. [Online]. Available: http://www.electricaleasy.com/2014/03/ideal-transformer-characteristics.html. [Accessed 10 November 2016].
[2] C. R. Nave, “The Oscilloscope,” HyperPhysics, 2001. [Online]. Available: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/oscope.html. [Accessed 9 November 2016].
[3] Doctronics, “Using an Oscilloscope,” Doctronics, [Online]. Available: http://www.doctronics.co.uk/scope.htm. [Accessed 8 November 2016].
[4] Jjbeard, “Rectification.svg,” 29 May 2006. [Online]. Available: https://en.wikipedia.org/wiki/File:Rectification.svg. [Accessed 10 November 2016].
[5] AspenCore, “Electronics Tutorials – Full Wave Rectifier Circuit,” 2016. [Online]. Available: http://www.electronics-tutorials.ws/diode/diode18.gif?x98918. [Accessed 10 November 2016].
[6] Afrotechmods, “Voltage regulator tutorial & USB gadget charger circuit,” 23 November 2010. [Online]. Available: https://www.youtube.com/watch?v=GSzVs7_aW-Y. [Accessed 9 November 2016].
[7] MathsIsFun.com, “Percentage Error,” 2014. [Online]. Available: http://www.mathsisfun.com/numbers/percentage-error.html. [Accessed 8 November 2016].
[8] I. Watts, “PSU circuit diagram 2016,” 2016. [Online]. Available: https://studentcentral.brighton.ac.uk/bbcswebdav/pid-2807777-dt-content-rid-5226277_1/courses/EO122_2016/PSU%20circuit%20diagram%202016.jpg. [Accessed 7 November 2016].
[9] I. Watts, “How the PSU should look,” 28 October 2016. [Online]. Available: https://studentcentral.brighton.ac.uk/bbcswebdav/pid-2807775-dt-content-rid-5226275_1/courses/EO122_2016/How%20the%20PSU%20should%20look.jpg. [Accessed 8 November 2016].
Appendices
Appendix 1
![Figure 12: Complete circuit diagram [8]](img/figure12.jpg)
Appendix 2
Appendix 3
