Introduction: AC to +15V,-15V 1A Variable and 5V 1A Fixed Bench DC Power Supply
A power supply is an electrical device that supplies electric power to an electrical load. This Model Power Supply features three solid-state DC power supplies. The first supply gives a variable output of positive 1.5 to 15 volts at up to 1 ampere. The second gives a negative 1.5 to -15 volts at 1 ampere. The third has a fixed 5V at 1 ampere. All supplies are fully regulated. A special IC circuit keeps the output voltage within .2V when going from no load to 1 ampere. The output is fully protected from short circuits. This supply is ideal for use in school labs, service shops or anywhere a precise DC voltage is required.
Step 1: How a Supply Works?
Supply consists of two circuits, one is fixed 5v output and other is 0 to+15, and -15 variable supply with each section explained below. It consists of a power transformer, a DC rectifier stage and the regulator stage.
- Stepping down 220V AC using Transformer: As the input of the regulators is supposed to be around anywhere from 1.5 to 40 volts. So 220v AC was stepped down using the transformer. 220v AC from the main is supplied to the transformer secondary coil via fuse and switch, which steps it down to 18 volts. Turn ratio of the transformer was 12:1. When tested, open circuit voltage of transformer turned out to be 22 volts.The transformer serves two purposes. First, it reduces the 220VAC input to 17VAC and 9VAC to allow the proper voltage to enter the rectifier stages. Second, it isolates the power supply output from the 220VACline. This prevents the user from dangerous voltage shock, should the user be standing in a grounded area. A center Tapped Transformer has two secondary windings which are 180 degree out of phase.
- AC to DC Converter: For rectification of the AC (conversion from AC to DC), bridge configuration of diodes was used which clipped off the negative cycle of the AC and converted it to pulsating dc. Each diode works only when it is in forward bias state (when voltage at anode is higher than voltage at cathode). This DC had some ripples involved in it so a capacitor was used to relatively smoothen it before sending it to the regulation circuit.
- Regulator Circuit: The regulator circuit in the PowerSupply consists of a LM-317 and LM-337 integrated circuit. The LM317 supply more than 1.5 A of load current with an output voltage adjustable over a 1.2 to 37 V range. The LM337 series are adjustable 3-terminal negative voltage regulators capable of supplying in excess -1.5 A over a -1.2 to -37 V output voltage range. They are exceptionally easy to use and require only two external resistors to set the output voltage. Further, both line and load regulation are better than standard fixed regulators. The output voltage of the LM317/LM377 is determined by ratio of the two feedback resistors R1 and R2 which form a potential divider network across the output terminal.The voltage across the feedback resistor R1 is a constant 1.25V reference voltage, Vref produced between the “output” and “adjustment” terminal. Then whatever current flows through resistor R1 also flows through resistor R2 (ignoring the very small adjustment terminal current), with the sum of the voltage drops across R1 and R2 being equal to the output voltage, Vout. Obviously the input voltage, Vin must be at least 2.5 volts greater than the required output voltage to power the regulator.
- Filter: Output of the LM317/337 was fed to the capacitor to filter out the pulsating effect. And then it was sent to the output. It should be noticed that capacitor’s polarity should be kept in mind before placing it.
5v fixed DC supply
5v DC works on the same principle, but the regulator used for that is a fixed 7805. Also transformer used was of 220V to 9V AC.
Step 2: Circuit Diagram and Components Required:
Circuit Diagram and Components required are listed in the pictures above.
Step 3: Simulations and Pcb Layout
Proteus Schematic and Simulations:
The schematic circuit was simulated in order to see if the circuit is working correctly and achieves our goal of a ±15V variable and 5V fixed power supply. Which was verified by measuring the output voltage with the help of multi-meter.
Proteus PCB layout:
The schematic circuit after testing was then converted into its PCB layout. The components are first placed and the routing is done through auto routing. The power wire’s width is T80 whereas the rest of the wire has the width T70.The board length was selected to be 6 by 8 inches. A 3d layout was also checked for the expected PCB design. The layout on completion and testing whether the paths don’t cross is exported as PDF. Only the board edge and bottom layer is selected to be on the PDF file and the rest is unselected. It gives us a print of the track of the whole PCB.
Step 4: PCB Printing
Printing on Butter Paper:
The track which got as a PDF file was printed on the butter paper. Printer used for this purpose was the one with toner rather than the liquid ink as it cannot be transferred on the butter paper. For that purpose butter paper is cut so as to match the size of an A4 paper for easy printing and then cut so as to fit the PCB size.
Transferring the print from Butter paper to PCB board:
The butter paper is placed on top of the PCB board. A hot iron is used to press the butter paper resulting in the track photocopying itself on the PCB board due to heating of the toner ink. After that track corrections are made using the permanent marker.
Transferring the track on the PCB board, in next step the board is dipped in container filled with Ferric Chloride placed in the oven which results in the removal of copper from all across the PCB board except the track which was printed resulting in a plastic sheet with copper only present on the track.
After preparation of the PCB, the holes are drilled using a Pcb drill by keeping it mid to hold the drill at 90 degree to the PCB and not applying extra pressure otherwise drill bit will break. The holes for transistors, connectors, regulators Diodes are made bigger than that of regular resistors, capacitors etc
Cleaning using Thinner/Petrol:
The PCB board is washed with a few drops of thinner or petrol according to the availability so that the ink gets removed from the track for perfect soldering of the component on PCB. PCB is ready to be soldered with components.
Soldering of components:
The components are then soldered on the PCB board according to the Proteus PCB layout. The components are soldered with caution by not shorting the tracks or points. Polarities of components like capacitors/transistors is kept in mind. Heat sinks are attached with the regulators using the paste for better conductivity and soldered with the PCB. Similarly
One last time, PCB is tested for any short while soldering the components on the board. After that, PCB was powered up and output was noted which was according to the desired output. PCB is ready to be placed in the casing.
Step 5: Casing Preparation
A premade casing with basic layout was bought from the market and was modified according to the desired requirement. It came with two holes for two binding posts, so additional of 4 holes for binding post and 2 for potentiometers were drilled in the casing. A female 3 pin socket was placed as well for easy connectivity of AC supply cable. A switch was also placed outside to turn the power supply ON or OFF. In addition to that a VOLTMETER was installed in the supply for easy readability/selection for the user.
Step 6: Setting Up the Supply
Transformers and circuit was placed in the casing with the help of a wood/insulating sheet to avoid any short with the body. Bolts and cable ties were used for holding the components together. Binding posts, fuse holder potentiometers and button were installed on the casing. Jumper wire was used to connect and was soldered to secure the connection. shrink wrap was used to secure the connections and to avoid any short. Supply was tested.
Step 7: Load Regulation
Load was connected to the supply output and output voltage drop was faced which was due to the drop across the resistances of the wires/pcb tracks/ connection points. So to cater with that, values of the resistors across the LM317/LM337 was changed so as to provide load voltage of 15 volts. As the voltage that was at output was open circuit voltage.
Step 8: Final Testing/Observations
Voltmeter used in the supply only worked for the voltage levels above 7v (other not available in the market). So by using a better voltmeter, lower voltage values could also be measured. Preferably using a bidirectional analogue voltmeter and using a switch to change the value to be measured (+ve supply or –ve supply voltage), it could be made more practical.
Overall it was an interesting project. A lot was learned as I was familiarized with the manufacturing of PCB, problems in making a supply and variable voltage regulators.
Also please visit https://easyeeprojects.blogspot.com/ for upcoming projects. :)