Overview

In this DIY project, we will design, assemble, and test a breadboard power supply (BBPS) that provides regulated and selectable DC voltage. This BBPS will power breadboard projects with either 3.3 volts or 5 volts of regulated power. The BBPS is designed to accept input power through a DC barrel jack as the primary input or through stripped wires on secondary input terminals. It can handle input voltages ranging from 6 to 30V DC from a DC wall wart and outputs either 5V or 3.3V regulated voltage, selectable via a slide switch.

Design Procedures

Select a Voltage Regulator

First, select a voltage regulator based on the design requirements. For this project, we will use the LM317 voltage regulator. The design is based on the Texas Instruments application note and datasheet of the LM317-N. A voltage regulator is a constant voltage source that adjusts its internal resistance to any occurring changes in load resistance. It provides a constant voltage at the regulator output. Below is the link to the datasheet of the voltage regulator: LM317 Datasheet.

Understand the LM317-N Voltage Regulator

The LM317-N is an adjustable positive voltage regulator capable of supplying in excess of 1.5A over a 1.25V to 37V output range. As per the datasheet, the LM317 has the following design features:

• Typical 0.1% Load Regulation

• Typical 0.01%/V Line Regulation

• 1.5-A Output Current

• Adjustable Output Down to 1.25V

• Current Limit Constant with Temperature

• 80-dB Ripple Rejection

• Short-Circuit Protected Output

• 0°C to 125°C Temperature Range

Why 3.3V and 5V Selectable Power Supply?

The BBPS's selectable 3.3V and 5V outputs are particularly useful for various electronic projects:

Microcontroller Compatibility: 

Many microcontrollers, such as Arduino, operate at 5V for input voltage, while their GPIO (General Purpose Input/Output) pins function at 3.3V. Having both voltage options available allows for flexible interfacing with different types of microcontrollers and boards.

Voltage Level: 

When working with microcontrollers that use a 3.3V logic level, such as the Arduino MKR 1000, it is crucial to match voltage levels to avoid damage and ensure proper operation. While the BBPS provides the necessary voltage levels, it is also recommended to use a voltage level shifter when interfacing 3.3V logic level microcontrollers with 5V devices.

Typical Low-Dropout Voltage Regulator Circuit

The application note of the LM317-N provides a typical low-dropout voltage regulator application circuit diagram as shown in Figure 2. If you want a variable voltage output with a potentiometer, you can use the circuit shown in Figure 2 without any modification.

• Vin: 6 to 30V DC

• C1: Filter capacitor

• C2: Optional capacitor to improve transient response

• R1: Current set resistor

• R2: Variable resistor to get different output voltages

• Vout: 1.25 to 37V

However, our design requires two voltage outputs, 5V, and 3.3V instead of variable output voltages using a potentiometer. Therefore, we need to modify the circuit in Figure 2. Using the potentiometer or the variable resistor, R2, a variety of output voltages can be made possible. Similarly, we can create a voltage division circuit and a switch to replace the variable resistor functions in the above example.

Based on the calculation, we need 330 and 390 ohm resistors for the voltage division and 240 ohm for the current limiter resistor, R2. Next, using the recommended values from the datasheet, the following capacitors were selected as shown in figure 3.

- Filter capacitor, C1 = 100uF to filter-out input transient

- Output capacitor, C2 = 10uF to improve output impedance

- Bypass capacitor, C3 = 0.1uF to prevent ripple from being amplified.

•R1: current set resistor

•R2 and R3: voltage division to get different output voltages.

Finally, a voltage selector switch is added to the circuit and finalized the design of the regulated section as shown in figure 3.

Note:

My initial design did not include capacitor C2, resulting in a significantly higher percent error in the output voltage compared to this design. Therefore, I recommend adding the optional capacitor, C2.

Circuit Diagram Explanation

In this circuit, we utilize the LM317 voltage regulator to provide a stable and adjustable DC output voltage. The circuit components include an input filter capacitor (C1), an output capacitor (C2), a bypass capacitor (C3), resistors to set the output voltage, and a slide switch to select between different output voltages.

Components and Their Functions:

LM317 Voltage Regulator: 

The central component of the circuit, capable of regulating the output voltage. It has three pins: input (Vin), output (Vout), and adjust (Adj).

• C1: Input Filter Capacitor (100µF):

• Placed between the input voltage (Vin) and ground. Filters out noise and smoothens the input voltage to the LM317.

• C2: Output Capacitor (10µF): Connected between the output (Vout) and ground. Stabilizes the output voltage and reduces ripple.

• C3: Bypass Capacitor (0.1µF): Connected between the adjust pin (Adj) and ground. Improves the transient response and further stabilizes the output.

• Resistors (390 ohm and 330 ohm): These resistors are used to set the output voltage.

Circuit Operation:

Input Voltage: A DC input voltage ranging from 6V to 30V is applied to the circuit through the Vin pin of the LM317.

Voltage Regulation: The LM317 regulates this input voltage down to a lower, stable output voltage.

The output voltage is determined by the resistor network connected to the adjust pin (Adj) of the LM317.

Capacitor Filtering:

• C1 (100µF) smoothens the input voltage, filtering out any noise.

• C2 (10µF) stabilizes the output voltage, reducing any remaining ripple.

• C3 (0.1µF) connected to the adjust pin, helps improve the transient response of the voltage regulator, ensuring stability during rapid load changes.

Output Voltage Selection:

The Output Selector Sliding Switch facilitates the selection between different sets of resistors to determine the desired output voltage. By toggling between resistor configurations of 390 ohms and 330 ohms, the switch alters the regulated voltage, providing options such as 3.3V and 5V outputs. This feature enables users to choose preset voltage levels based on the specific needs of the circuit or device being powered, enhancing flexibility and usability.

Final Design

The final Breadboard Power Supply (BBPS) combines the regulated and unregulated sections, with a n additional power-on indicator LED with a series resistor (330 ohms), as shown in Figure 5.

Components summary:

Unregulated Section:

1. DC Barrel Connector: Primary input.

2. Header Pins Terminals: Secondary input.

3. On/Off Switch: Controls power.

4. Reverse Protection Diode: Prevents reverse polarity.

5. PTC Resettable Fuse (500mA): Overcurrent protection.

Regulated Section:

1. LM317 Voltage Regulator: Provides stable output voltage.

2. Capacitors: Filter and stabilize (C1: 100µF, C2: 10µF, C3: 0.1µF).

3. Resistors: Set output voltage (390 ohm for 3.3V, 330 ohm for 5V).

Output Selector Switch: 

• Switches between 3.3V and 5V.

Power-On Indicator:

• LED with Series Resistor (330 ohms): Indicates power status.

PCB Finalization and Manufacturing

Steps After Completing the PCB Layout

Run ERC (Electrical Rule Check): Ensure all electrical connections are correct and there are no unintentional shorts or opens.

Note: Ignore or approve the ERC errors related to "unconnected pin NC_1 and NC_2" as these pins are for breadboard mounting only and do not require electrical connections.

Run DRC (Design Rule Check): Verify that the layout adheres to the design rules set for the PCB manufacturing process. Ensure there are no design rule violations. The DRC check should return "No errors" for the design to be considered ready for manufacturing.

Generate Gerber Files:

Export the Gerber files from Eagle CAD. These files contain all the necessary information for PCB manufacturing, including copper layers, solder mask, silkscreen, and drill files. Zip the Gerber files for easy upload to the PCB manufacturer's website.

PCB Manufacturing Options

1. OSH Park:

• Location: USA

• Turnaround Time: Quick

• Price: Moderate

• Quality: High

• Preferred for fast turnaround times.

2. JLCPCB:

• Location: China

• Turnaround Time: Moderate

• Price: Cheapest

• Quality: High

• Best option for the lowest price.

3. PCBWAY:

• Location: China

• Turnaround Time: Moderate

• Price: Moderate

• Quality: High

• Ideal for larger quantities (10 PCBs for the same price).

Component Assembly

Required Tools: 

To assemble the PCB, the following tools are necessary:

• Soldering Iron: For soldering the components onto the PCB.

• Solder: The material used to create electrical connections between components and the PCB.

• Wire Cutters: For trimming excess leads from components after soldering.

• Multimeter (Optional): For testing electrical connections and verifying the functionality of the assembled PCB.

Required Parts

The following parts are needed to assemble the PCB, except for items 15, 16, and 17 which are optional:

• DC Barrel Connector (Primary Input)

• Header Pins Terminals (Secondary Input)

• On/Off Switch

• Reverse Protection Diode

• PTC Resettable Fuse (500mA)

• LM317 Voltage Regulator

• Capacitors:

  • C1: 100µF

  • C2: 10µF

  • C3: 0.1µF

• Resistors:

  • R1: 240 ohm

  • R2: 390 ohm

  • R3: 330 ohm

• Series Resistor for LED: 330 ohm

• Output Selector Sliding Switch

• LED (Power-On Indicator)

Soldering Instructions

All components listed in the parts list will be soldered onto the PCB, except for the heat sink, which will be mounted on the regulator using a screw and a nut. 

Follow these instructions carefully to ensure proper assembly.

Preparation:

• Ensure your soldering iron is heated to the appropriate temperature.

• Have all your components and tools ready and organized.

• Component Placement:

• Insert the components into their designated positions on the PCB according to the schematic.

• Ensure the components are placed on the top face of the PCB.

Soldering Process:

• Start with Small Components: Begin with the smallest components, such as resistors and small capacitors.

• Move to Larger Components: Once the small components are soldered, proceed with larger ones like connectors and switches.

• Capacitor Polarity: Pay special attention to capacitors C1 and C2, which are polarized. Ensure the minus sign on the capacitor aligns with the correct polarity on the PCB. The positive terminal must always be connected to a positive point on the board.

Soldering Technique:

• Place the soldering iron tip on the joint where the component lead meets the PCB pad.

• Feed solder into the joint until it flows smoothly and forms a solid connection.

• Remove the soldering iron and let the joint cool.

• Ensure all solder joints are shiny and well-formed.

Header Pins:

• The 0.1" header pins will be assembled on the bottom of the PCB.

• Insert the header pins from the bottom and solder them from the top face of the PCB.

Testing the Breadboard Power Supply (BBPS)

Now that the assembly is complete, it's time to test the BBPS to ensure it provides the correct output voltages. Follow these steps to verify the functionality of the power supply.

Testing Procedure

Power Up: Connect the DC power source (6-30VDC) to the BBPS using either the DC barrel connector or the header pins terminals.

Turn on the power supply using the On/Off switch.

Safety Check: Ensure there are no visible issues such as smoke or burning smells. If anything, unusual is noticed, immediately turn off the power and recheck the connections.

Voltage Measurement:

• Use a multimeter to measure the output voltage.

• Set the multimeter to the DC voltage measurement mode.

Measure 5V Output:

• Slide the output selector switch to the 5V position.

• Place the multimeter probes on the output terminals (positive probe on the 5V output pin and negative probe on the ground pin).

• Verify that the multimeter reads approximately 5V.

Measure 3.3V Output:

• Slide the output selector switch to the 3.3V position.

• Place the multimeter probes on the output terminals (positive probe on the 3.3V output pin and negative probe on the ground pin).

• Verify that the multimeter reads approximately 3.3V.

Results

In my case, the output voltage tests were performed successfully. Both 3.3V and 5V outputs were accurately obtained from the BBPS, as shown in the following figures: