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. 

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.

Major components: 

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. 

Bridge Rectifier Configuration:

• Consists of four diodes arranged in a bridge topology.

• Each diode allows current to pass in only one direction, ensuring that both halves of the AC signal contribute to the DC output.

Center-Tap Transformer:

• Often used with bridge rectifiers to provide two equal AC voltages relative to a common center tap (ground).

• This configuration improves efficiency and reduces the volta

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:

• • • During the positive half-cycle of the AC input, diodes D1 and D2 conduct, allowing current to pass through the load in one direction.

    • Diodes D3 and D4 remain non-conductive, blocking the current flow in the opposite direction.

Negative Half-Cycle:

• • • During the negative half-cycle, diodes D3 and D4 conduct, allowing current to pass through the load in the same direction as during the positive half-cycle.

    • Diodes D1 and D2 remain non-conductive.

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.

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:

• During the positive half-cycle of the AC input, the diodes in the bridge rectifier conduct, allowing current to charge the capacitor.

• The capacitor charges up to the peak voltage of the rectified output.

Slow Discharging:

• Between the peaks, when the rectifier output drops, the capacitor begins to discharge through the load.

• This discharge provides a continuous current to the load, maintaining the voltage level.

Smoothing Effect:

• By rapidly charging and slowly discharging, the capacitor smooths the pulsating DC, reducing the ripple.

• The result is a more stable DC voltage with reduced AC components.

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.

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.

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:

• Voltage Regulators (LM7912 and LM7812): These regulators are responsible for providing the +12V and -12V outputs respectively. They regulate the input voltage to ensure a stable output voltage regardless of variations in the input voltage or load conditions.

• No Load: The analysis is conducted without any load connected to the regulators. This allows for a pure assessment of the output voltage without any external factors influencing the results.

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.

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.

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.

 »»» END of  DPV-LPS Design »»» 

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