05 linear regulator

Diode Power Entry and 5 V Linear Regulator

A 9 V battery or 7.6 V to 12 V DC supply turned into a clean 5 V rail: a Schottky diode against reverse connection, an L7805 linear regulator, input and output capacitors, a power LED and four test points. Project 5 of my KiCad learning path: reverse-polarity protection, the linear regulator, capacitors that keep it stable, and heat.

Table of contents

• Overview
• Key specifications
• Gallery
• Operating area and power budget
• How the circuit works
• Design calculations (summary)
• Purpose of each component
• Bill of materials
• Netlist
• Repository structure
• Getting started
• Testing
• Verification status
• Future improvements
• License

Overview

Every earlier board assumed a clean 5 V supply. This board makes one. Real power sources are not clean: batteries sag, bench supplies are set too high, and people plug them in backwards. This board handles all three.

New ideas compared with Project 4:

1. Reverse-polarity protection and the cost of a diode drop
2. Linear regulation, dropout voltage and efficiency
3. Heat: power dissipation, thermal resistance, junction temperature and heatsinks
4. Electrolytic and ceramic capacitors working together, and electrolytic polarity
5. The regulator's minimum load current
6. A wider net class (1.0 mm) for supply tracks
7. A voltage and power budget in the documentation

The full engineering write-up (theory, derivations, thermal analysis, KiCad configuration, layout rules, test procedure) is in docs/DESIGN_NOTES.md.

Key specifications

Gallery

Schematic

Full-size PDF: docs/schematic.pdf

PCB layout

3D views

Top	Bottom

Operating area and power budget

Dropout and thermal limits

Where the power goes

A linear regulator wastes the difference between input and output as heat. From 9 V to 5 V it can never be better than 56 % efficient, and from 12 V to 5 V never better than 42 %. At 9 V and 300 mA the regulator burns 1.17 W, while the load receives 1.55 W.

How the circuit works

 J1+ ── TP1 ── D1 (1N5819) ── TP2 ──┬──────────┬── U1 IN ─── U1 OUT ──┬─────────┬────────┬── TP3 / J2+
                                   C1         C2                    C3        C4       R1 ─ D2 (LED)
                                 100 µF       1 µF                 100 nF    100 µF       │
 J1− ── TP4 ───────────────────────┴──────────┴────── U1 GND ───────┴─────────┴────────┴── J2−

1. Reverse polarity. With the supply connected correctly, D1 conducts and drops about 0.45 V. If the supply is reversed, D1 is reverse-biased and blocks it, so nothing downstream sees a negative voltage.
2. Input capacitors. C1 (100 µF) is a reservoir that supplies sudden current demands. C2 (1 µF ceramic) sits at the regulator's input pin and handles fast changes. The datasheet asks for at least 0.33 µF at the input, with the shortest possible leads.
3. Regulation. U1 compares its output with an internal reference and adjusts an internal pass transistor to hold 5 V. The pass transistor drops the surplus voltage multiplied by the current, and that power becomes heat.
4. Output capacitors. C3 (100 nF) and C4 (100 µF) improve the response to load changes. The L7805 is stable without an output capacitor, but the datasheet notes that one improves transient response.
5. Minimum load. The regulator needs at least 5 mA of load to regulate properly. R1 and the power LED draw 9.1 mA, so it is always satisfied.

The metal tab of the TO-220 package is connected to ground (pin 2). It is the part that carries heat to a heatsink.

Design calculations (summary)

Minimum input voltage. The regulator needs its input to be at least 7 V (the datasheet's lowest test condition, equal to 5 V plus the 2 V dropout), and the diode takes up to 0.6 V:

$$V_{in,min} = 7.0\ \text{V} + 0.6\ \text{V} = 7.6\ \text{V}$$

Regulator power:

$$P_{reg} = (V_{raw} - V_{out}) \times I_{out} + V_{raw} \times I_q \qquad V_{raw} = V_{in} - V_F$$

For 9 V in, 300 mA load (plus 9.1 mA for the LED), 0.45 V diode drop and 8 mA quiescent current:

$$P_{reg} = (8.55 - 5.0) \times 0.309 + 8.55 \times 0.008 = 1.17\ \text{W}$$

Junction temperature (TO-220 in free air, 50 °C/W, 25 °C ambient):

$$T_J = T_A + P_{reg} \times R_{\theta JA} = 25 + 1.17 \times 50 = 83\ ^\circ\text{C}$$

The datasheet's operating limit is 125 °C.

The full derivation, tolerance analysis, heatsink selection and capacitor reasoning are in docs/DESIGN_NOTES.md.

Purpose of each component

Bill of materials

The same list is available as CSV in fabrication/bom.csv.

Netlist

Repository structure

This project lives in the 05-linear-regulator/ folder of the kicad-learning-path repository.

05-linear-regulator/
├── README.md
├── 05_linear_regulator.kicad_pro     KiCad project file
├── 05_linear_regulator.kicad_sch     Schematic
├── 05_linear_regulator.kicad_pcb     PCB layout
├── fabrication/
│   ├── gerbers/                      Gerber and drill files
│   ├── 05_linear_regulator_gerbers.zip
│   └── bom.csv
└── docs/
    ├── DESIGN_NOTES.md               Full engineering documentation
    ├── schematic.pdf
    ├── images/                       Operating area, power budget, screenshots, renders
    └── reports/                      ERC and DRC reports

Getting started

Open the project

1. Install KiCad 10.
2. Clone or download this repository.
3. Open 05-linear-regulator/05_linear_regulator.kicad_pro in KiCad. The project uses only the standard KiCad libraries.

Rebuild the manufacturing files

1. Open the PCB editor and run Inspect → Design Rules Checker. It should report 0 errors.
2. Open File → Fabrication Outputs → Gerbers and Generate Drill Files. The exact settings are listed in docs/DESIGN_NOTES.md.
3. Zip the output folder.

Order the PCB

Upload fabrication/05_linear_regulator_gerbers.zip to a PCB fabricator. Suggested options for a low-cost fab: 2 layers, 1.6 mm, 1 oz copper, HASL finish.

Testing

Safety first: the regulator gets hot (about 77 °C at its case when delivering 300 mA from 9 V) and the test load resistors get hot too. Step-by-step measurements, expected values, test loads and a troubleshooting table are in docs/DESIGN_NOTES.md. In short: test the reversed connection first with a current-limited supply, then apply 9 V and check TP2 (about 8.5 V) and TP3 (about 5.0 V), then load the output step by step.

Verification status

Reports: docs/reports/

Future improvements

• Add a protection diode across the regulator if you attach very large output capacitors
• Replace the series diode with a P-channel MOSFET for a much smaller voltage drop (see Project 12)
• Try a low-dropout regulator and compare its efficiency at 7.6 V
• Add a fuse or resettable fuse at the input
• Add mounting holes and a bolt-on heatsink position

License

Released under the MIT License.

Author: Nitish Ghadage · GitHub