Joseph Taylor

Power Supply

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.

Figure 1: Class II Insulation symbol
Figure 1: Class II Insulation symbol

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]

Figure 2: Transformer circuit diagram symbol
Figure 2: Transformer circuit diagram symbol

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:

Figure 3: Sine wave on an oscilloscope
Figure 3: Sine wave on an oscilloscope

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:

Figure 4: Sine wave diagram showing period and amplitude
Figure 4: Sine wave diagram showing period and amplitude

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:

Figure 5: Diagram of the switch and rectifier part of the PSU
Figure 5: Diagram of the switch and rectifier part of the PSU

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]
Figure 6: Half-wave rectification [4]

Connecting Link 2 uses all four diodes, which results in full-wave rectification, show below:

Figure 7: Full-wave rectification [4]
Figure 7: Full-wave rectification [4]

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]
Figure 8: Full-wave Rectifier with smoothing capacitor [5]

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:

Figure 9: Photograph of the PCB modification
Figure 9: Photograph of the PCB modification

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 10: 7W resistor with looped legs
Figure 10: 7W resistor with looped legs
Figure 11: Joseph Taylor’s completed PCB [9]
Figure 11: Joseph Taylor’s completed PCB [9]

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
0150
11014.632.47
12014.622.53
13014.622.53
14014.622.53
15014.612.6
16014.612.6
17014.612.6
18014.62.67
19014.62.67
20014.62.67
21014.592.73
22014.582.8
23014.553
24014.543.07
25014.523.2
26014.513.27
27014.433.8

The graph below shows the test results in a graphical form

Graph 1: Test results for LM317 set at 15V DC
Graph 1: Test results for LM317 set at 15V DC

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
0100
909.990.1
1009.990.1
1109.990.1
1209.980.2
1309.980.2
1409.960.4
1509.960.4
1609.950.5
1709.950.5
1809.940.6
1909.930.7
2009.910.9
2109.891.1
2209.871.3
2309.762.4

The graph below shows the test results in a graphical form

Graph 2: Test results for LM317 set at 10V DC
Graph 2: Test results for LM317 set at 10V DC

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
050
404.990.2
504.980.4
604.980.4
704.980.4
804.970.6
904.970.6
1004.970.6
1104.970.6
1204.960.8
1304.951
1404.941.2
1504.931.4
1604.911.8
1704.92
1804.892.2
1904.882.4

The graph below shows the test results in a graphical form

Graph 3: Test results for LM317 set at 5V DC
Graph 3: Test results for LM317 set at 5V DC

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]
Figure 12: Complete circuit diagram [8]

Appendix 2

Figure 13: Completed power supply, showing wiring
Figure 13: Completed power supply, showing wiring

Appendix 3

Figure 14: Completed power supply
Figure 14: Completed power supply