Dual-Polarity-Variable Linear Power Supply [DPV-LPS]

Disclaimers:

1. This tutorial is for educational purpose only and neither the author nor this website makes no warranty, representation, or guarantee regarding the information contained herein or any product based on this tutorial or use of any components or circuit design. 

2. --- This project was submitted to EE458 - Design of Power System Components - class at California State University, Long beach.--- So, make sure to cite!

APA Citation:

Dual Polarity Variable Linear Power Supply. (2019, December 27). Retrieved MM/DD/YYYY, from https://www.fwdskillzone.com/variable-power-supply.html. 

Dual-Polarity-Variable Linear Power Supply​ 

Project Overview

The Dual-Polarity-Variable Linear Power Supply (DPV-LPS) is a versatile and precise power supply designed to provide both positive and negative voltages with adjustable output levels. This type of power supply is essential for various applications, including electronics testing, circuit development, and research in fields requiring stable and reliable power sources.

Key Features


Objectives:

The goal of this project is to design and build a Dual-Polarity-Variable Linear Power Supply with an external 5-volt Universal Serial Bus (USB) port, fixed ±12V, and a variable 1.25V to 10V output at a maximum current of 1A.

Theory:

The theory behind designing this DPV-LPS consists of using a transformer to step down the 120 VAC input to a 24 VAC, followed by a full-bridge rectifier composed of four diodes that convert the AC voltage into a pulsating DC. Then, using a capacitor(s) the pulsating Direct Current is converted into a pure Direct Current. The output filter capacitors smooth out the ripple in the DC output. Finally, voltage regulators are used to obtain different voltage outputs and to automatically maintain a constant voltage level at the output.

DPV-LPS top level architecture

Major components: 

Transformer: To step down 120V to 24V

The transformer plays a key role in the design of the DC power supply. This design uses a center-tap, step-down transformer that decreases a 120V coming from a wall outlet down to a 24V with a maximum current of 1A. This type of transformer is called center-tap because the two outer taps (total winding) of the transformer give the total voltage across the transformer, and half of the total voltage is measured from each outer tap to the center terminal. For example, the voltage across the two outer taps (winding as a whole) of the transformer that is used for this project is 24 VAC, and it provides 12 VAC from each outer tap to the center tap (half winding). These two 12VAC supplies are 180 degrees out of phase with each other; therefore, it is easy to derive the positive and the negative 12-volt requirements. 

Center Tap XFMR and output waveforms 

For the center-tapped transformer shown above, the output is a sine wave centered around zero volts. The peak voltage Vpk is 1.414 (square root of 2) times the RMS output of the transformer. Then, for a 120V:24V transformer, the peak voltage will be 1.414 times 24 = 33.94V across the outer ends of the transformer. The transformer will also have losses at the windings; however, a transformer rated at 120V: 24V at 1A will usually provide more than 30V RMS output. Therefore, this transformer is ideal for this project as the voltage output will be higher than what we need. 

Rectifier: To convert AC to DC

AC to DC conversion requires the use of a transformer and rectifier as shown in the figure below. The transformer steps the voltage up or down based on the requirements; and the rectifier removes the negative cycle of the input signal, resulting in only positive voltage output. The diagram below shows a bridge rectifier fed from a center tap transformer. 

A rectifier converts the AC voltage from the transformer into DC voltage. In this context, we focus on the bridge rectifier, which is a common and efficient rectification method.

ge drop across the rectifier diodes.

Center Tap XFMR and Bridge Rectifier (unfiltered) and output waveforms 

Bridge Rectifier Configuration:

Center-Tap Transformer:

Working Principle

AC Input: The secondary coil of the transformer provides an AC voltage, typically with a center tap connected to the ground, resulting in two equal but opposite AC voltages.

Positive Half-Cycle:

Negative Half-Cycle:

The result is a pulsating DC voltage, where both halves of the AC input contribute to the DC output, significantly improving the efficiency of the rectification process.

Filter or Smoothing capacitor(s): 

As discussed above, the output from the transformer/rectifier circuit is a pulsating DC voltage that contains a large unwanted AC component. A rectifier circuit without a filter produces pulsating output. This fluctuation can be reduced if some of the energy can be stored in a capacitor while the rectifier is producing pulses and if it is allowed to discharge the capacitor between pulses. This is where the use of the filter capacitor comes into the picture. The capacitor that is installed in parallel with the load resistor reduces the pulsating action of the rectifier output wave. Then, the filter capacitor charges rapidly and discharges slowly; as a result, it will smooth the waveform.

Bridge Rectifier (filtered) and output waveforms 

Function of the Filter Capacitor

The primary role of the filter capacitor is to smooth out the fluctuations in the rectified DC output. This is achieved by storing energy during the peaks of the rectified waveform and releasing it during the troughs. Here’s how it works:

Energy Storage: When the rectifier produces a voltage pulse, the capacitor charges quickly to the peak voltage level of the pulse.

Energy Release: Between pulses, the capacitor discharges slowly, providing a continuous DC output to the load. This discharge helps to fill in the gaps between the pulses, effectively smoothing the waveform.

How the Filter Capacitor Works

Rapid Charging:

Slow Discharging:

Smoothing Effect:

Voltage regulators:

The regulator section of the power supply controls the output voltage level to a constant value irrespective of the input voltage, load, or temperature variations. The voltage regulators maintain a constant voltage level regardless of the fluctuation of the input voltage. Depending on the design, a regulator may be used to provide the desired output voltage. This voltage regulator can use either a simple forward or a negative feedback design. For this project, the following three types of voltage regulators are used: LM7912, LM7812, and LM317. 

​The LM78XX and LM79XX are three-terminal regulators with TO-220 through-hole package. These regulators have fixed output voltages, making them useful in a wide range of applications. According to the datasheet, these voltage regulators have internal current limiting, thermal shut-down, and safe area protection circuits. Besides, with an appropriate heat sink, they can provide regulated voltages with over 1A output current. Similarly, the LM317 voltage regulator comes in TO-220 packages and it is intended for use as a positive adjustable voltage regulator. LM317 can provide more than 1.5 A of load current with an output voltage adjustable over a range of 1.2V to 37 V. The desired output voltage is determined by means of a resistive voltage divider circuit. This makes LM317 a remarkably easy-to-use device for any output in the range of 1.25 to 37V. To get the required output voltage, we need only two resistors or a resistor and a potentiometer.  

​Note: refer to the respective datasheets for detailed information.

According to the datasheet, the output of the LM317 voltage regulator is determined by the following formula: 

Note:

I_adj is too small and it can be neglected. Then, the formula will be reduced to:

Then, the following values are collected from the datasheet.


Circuit Diagram

After collecting all the required information and calculation results, the power supply circuit is designed as shown below. This circuit diagram has additional components that were not discussed above. These components are not mandatory to the design, but they will enhance the fit-form function of the DPV-LPS. Besides, the reverse current protection diodes and the STSP switches are also added as additional safety features. The circuit diagram of the Dual-Polarity-Variable Linear Power Supply (DPV-LPS) incorporates LM7812 and LM7912 regulators for generating +5V and -5V outputs respectively, LM317 for providing variable output voltage ranging from 1.25V to 10V, and an additional LM317 for fixed 5V/500mA USB charging.

Circuit diagram of the DPV-LPS 

DC Analysis of +12V and -12V Outputs

The DC analysis of the +12V and -12V outputs of the power supply is conducted using the OrCAD Capture simulation program. This analysis is performed without a load to examine the performance of the LM7912 and LM7812 voltage regulators. The results indicate that regulator #1 provides a +12.801V output, while regulator #2 provides a -12.428V output.


Testing Setup

The simulation setup involves the following components:

OrCAD simulation of the DPV-LPS at no-load. 

Analysis Results

+12V Output (Regulator #1):

The LM7812 regulator provides a +12.801V output at no load. This indicates that the regulator is functioning correctly and is capable of maintaining a slightly higher voltage than the specified +12V output.

-12V Output (Regulator #2):

The LM7912 regulator provides a -12.428V output at no load. Similar to the +12V output, this voltage is slightly lower than the specified -12V output but still within an acceptable range.

Waveform and cursor values of the no-load simulation 

Voltage Regulation in Power Supplies

Voltage regulation is a crucial aspect of power supply performance, as it determines how effectively the supply can maintain a constant output voltage despite changes in load or input voltage. It is a measure of the power supply's ability to provide a stable output voltage over a range of operating conditions.

Importance of Voltage Regulation

Stable voltage output is essential for the proper functioning of electronic devices and circuits. Variations in voltage can lead to malfunctions or damage to sensitive components. Voltage regulation ensures that the output voltage remains within acceptable limits, providing consistent power to connected devices.

Practical Application

Voltage regulation is an important specification to consider when selecting a power supply for a particular application. Devices requiring precise and stable voltage levels, such as microcontrollers, sensors, and communication equipment, demand power supplies with tight voltage regulation tolerances.

Calculation of Voltage Regulation

Voltage regulation is typically expressed as a percentage and is calculated using the following formula:

Where: 

V_{NL} = No-load voltage 

V_{FL} = Full-load voltage 

Printed Circuit Board

The Printed Circuit Board (PCB) of the DPV-LPS is designed in the Eagle CAD program.

Note: EagleCAD can be downloaded from the Autodesk website with a student e-mail address. The program helps the designer connect schematic diagrams, component layout, PCB circuit routing, and component library. The free version of Eagle CAD has a limitation of only two schematic sheets, double layers, and an 80 cm^2 board area. However, for a small project like this one, these limitations are not big deals.

The PCB design is started in the “schematic” workbench of the Eagle CAD program and all the symbols for the components are placed and oriented on a blank schematic sheet as shown in the figure below.

EagleCAD PCB schematic design 

PCB Layout

After the schematic is done, electrical connectivity is made between each part with the “nets” layer and assigned values and names to our parts accordingly. Finally, we make sure everything is connected as per the Eagle CAD design criteria by running an Electrical Rule Check (ERC). Then, the board layout is done on the “board” workbench of Eagle CAD. Then we used a “PCB Trace Width Calculator” tool to calculate the race width and routed the corresponding wires with an appropriate trace width of 65 mils. Then, a copper pour of the ground plane is created on both the top & bottom layers of the PCB and connected these ground planes with multiple “vias” to connect the two opposite layers of the board. These “vias” also help reduce the heat resistance of the copper pour across the PCB.

Next, perform a Design Rule Check (DRC) to validate the “board” design meets the industry standards. Then, fix all the DRC errors until you get the “DRC: No error” message. Finally, convert the file to a Gerber file format using the CAM Processor of the Eagle CAD and then send the Gerber file to a PCB fabrication company such as:

- PCB way - https://www.pcbway.com/

- OSH park - https://oshpark.com/

- JLC PCB - https://jlcpcb.com

Eagle CAD PCB layout

Once the PCB is fabricated, solder all the components onto the PCB as shown in the figure below. 


Next, design and manufacture an enclosure that is approximately is 150mm x 94mm x 100mm (L*W*H) and it should have four PCB mounting holes or standoffs with a hole diameter of 3mm and which are 70.23mm apart (center to center) horizontally and 70.10mm apart vertically. Then, connect all electrical wirings from the PCB to the enclosure. 

​In our case, we used a 3D printer to manufacture the enclosure as shown below. 

Functional Test

1. No-load test

Finally, Perform successive output tests as shown in the following table and calculate the percent error.


Percent error is calculated using the formula: 

2. Output load test

The following output test validates the maximum output current at different loads. While charging a tablet that has a fully drained battery, the maximum load current of 900 mA is obtained from the DPV-LPS as shown in the figure below.


USB charging max output current test 

Conclusion

In summary, this project has detailed the design and construction of a Dual-Polarity-Variable Linear Power Supply (DPV-LPS) capable of delivering fixed 12V and -12V, 5V (USB), and a variable 1.25V to 10V outputs. Beginning with the transformation of 120VAC to 24VAC through a transformer, followed by rectification using a full-bridge rectifier, the AC voltage was efficiently converted to DC. Subsequent filtering with capacitor(s) ensured the output achieved a smooth and consistent DC voltage by minimizing ripple. The integration of voltage regulators facilitated the generation of various output voltages while maintaining stability. This comprehensive conclusion encapsulates the key stages of the DPV-LPS construction process, illustrating its functionality and importance in diverse electronic applications.


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