kicad-learning-path

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.

Section	Parts	What it demonstrates
Power entry	J1, D1	A series Schottky diode blocks a reversed connection, at the cost of a small voltage drop
Regulation	U1	A linear regulator turns any input from 7.6 V to 12 V into a steady 5 V, burning the difference as heat
Capacitors	C1 to C4	Bulk and high-frequency capacitors on the input and output
Indicator	R1, D2	A power LED that also provides the regulator’s minimum load
Measurement	TP1 to TP4	Probe points before the diode, after the diode, at the output, and ground

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

Parameter	Value
Input voltage	7.6 V to 12 V DC (9 V battery nominal), at screw terminal J1
Reverse-polarity protection	1N5819 Schottky, blocks up to 40 V in reverse
Output voltage	5 V, within 4.75 V to 5.25 V over all line, load and temperature conditions (datasheet)
Output current, free air	300 mA with up to 10 V input at 40 °C ambient
Output current, with heatsink	500 mA with up to 12 V input and a clip-on heatsink of 18 °C/W or better
Minimum load	5 mA, met by the power LED (9.1 mA)
Regulator heat	1.17 W at 9 V in and 300 mA out (junction 83 °C at 25 °C ambient)
Efficiency	54 % at 9 V in and 300 mA out, 41 % at 12 V in and 300 mA out
Board layers	2
Board thickness	1.6 mm
Copper weight	1 oz (35 µm)
Board size	70 × 45 mm
Components	14 (10 circuit parts and 4 test points)
Pads / nets	25 / 5
Design tool	KiCad 10.0.x
ERC / DRC	0 errors / 0 errors

Gallery

Schematic

Full-size PDF: docs/schematic.pdf

PCB layout

3D views

Top	Bottom

Operating area and power budget

Dropout and thermal limits

Plot	Reading
A. Output against input	The output stays at 5 V while the input is high enough. Below about 7.6 V at J1 the regulator is in dropout and the output falls with the input
B. Maximum load against input	The higher the input voltage, the more heat the regulator must burn for the same current, so the allowed current falls. A heatsink raises the limit

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.

Operating point	P_reg	T_J at 25 °C ambient	T_J at 40 °C ambient	Efficiency
9 V in, 100 mA	0.46 W	48 °C	63 °C	52 %
9 V in, 300 mA	1.17 W	83 °C	98 °C	54 %
9 V in, 500 mA	1.88 W	119 °C	134 °C (over the limit)	55 %
12 V in, 300 mA	2.12 W	131 °C (over the limit)	146 °C (over the limit)	41 %
12 V in, 500 mA	3.43 W	needs a heatsink of 18 °C/W or better		41 %

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

Purpose of each component

Ref	Value	Purpose
J1	2-pin screw terminal, 5.08 mm	Input from a 9 V battery clip or a bench supply (pin 1 +, pin 2 −)
D1	1N5819 Schottky, 40 V, 1 A	Blocks a reversed supply. Chosen over a silicon diode for its lower drop (0.6 V maximum at 1 A)
U1	L7805CV, TO-220	Fixed 5 V linear regulator, up to 1.5 A, with thermal shutdown and current limiting
C1	100 µF, 25 V electrolytic	Input reservoir: supplies sudden current demands and smooths the supply. Polarised
C2	1 µF, 50 V X7R ceramic	Input high-frequency capacitor at the regulator pin. Meets the datasheet’s 0.33 µF minimum even after ceramic DC-bias loss
C3	100 nF, 50 V X7R ceramic	Output high-frequency capacitor (the datasheet’s test value)
C4	100 µF, 16 V electrolytic	Output reservoir: improves response to load steps. Polarised
R1	330 Ω	Sets the power LED current to 9.1 mA
D2	Red LED, 0805	Power indicator, and the regulator’s minimum load
J2	2-pin header	5 V output (pin 1 +5V, pin 2 GND)
TP1	Test point	VIN: input before the diode
TP2	Test point	VRAW: regulator input, after the diode
TP3	Test point	+5V: regulated output
TP4	Test point	GND: reference

Bill of materials

Ref	Qty	Value	Footprint	Description
J1	1	Screw_Terminal_01x02	TerminalBlock_bornier-2_P5.08mm	2-pin screw terminal, supply in
J2	1	Conn_01x02_Pin	PinHeader_1x02_P2.54mm_Vertical	2-pin header, 5 V out
D1	1	1N5819	D_DO-41_SOD81_P10.16mm_Horizontal	Schottky diode, 40 V, 1 A
U1	1	L7805CV	TO-220-3_Vertical	5 V linear regulator
C1	1	100 µF, 25 V	CP_Radial_D6.3mm_P2.50mm	Electrolytic capacitor
C2	1	1 µF, 50 V, X7R	C_0805_2012Metric	Ceramic capacitor
C3	1	100 nF, 50 V, X7R	C_0805_2012Metric	Ceramic capacitor
C4	1	100 µF, 16 V	CP_Radial_D6.3mm_P2.50mm	Electrolytic capacitor
R1	1	330 Ω	R_0805_2012Metric	LED resistor, 1 %
D2	1	RED	LED_0805_2012Metric	Red LED
TP1 to TP4	4	TestPoint	TestPoint_THTPad_D2.0mm_Drill1.0mm	Through-hole test points
Optional	1	Clip-on TO-220 heatsink, 18 °C/W or better	n/a	For loads above 300 mA or inputs above 10 V

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

Netlist

Net	Connections
VIN	J1-1, D1 anode, TP1
VRAW	D1 cathode, C1 +, C2-1, U1-1 (IN), TP2
+5V	U1-3 (OUT), C3-1, C4 +, R1-1, J2-1, TP3
GND	J1-2, U1-2 (GND), C1 −, C2-2, C3-2, C4 −, D2 cathode, J2-2, TP4
LEDA	R1-2, D2 anode

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

Check	Result
Electrical Rules Check (ERC)	0 errors
Design Rules Check (DRC)	0 errors, 0 unrouted nets
Gerber viewer check	Passed
Physical board built and tested	Not yet

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

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