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Design Notes: Diode Power Entry and 5 V Linear Regulator

Revision 1.0 · KiCad 10.0.x · Last updated: 2026-10-24

This document records every engineering decision behind the board: the theory of the linear regulator and of heat, the calculations and tolerance analysis, the operating-area and power-budget diagrams, the KiCad configuration, the layout rules, the verification results, the manufacturing settings and the test procedure.

Contents

  1. Requirements
  2. Electronics background
  3. Calculations
  4. Operating area and power budget
  5. Schematic
  6. KiCad configuration
  7. Layout
  8. Verification
  9. Manufacturing outputs
  10. Bring-up and testing
  11. Lessons learned
  12. References

1. Requirements

# Requirement
R1 Accept a 9 V battery or a DC supply from 7.6 V to 12 V through a screw terminal
R2 Survive a reversed supply connection without damage
R3 Deliver a regulated 5 V within 4.75 V to 5.25 V
R4 Deliver 300 mA in free air (input up to 10 V, 40 °C ambient), and 500 mA with a heatsink (input up to 12 V)
R5 Keep the regulator’s junction temperature below 125 °C in every rated condition
R6 Always satisfy the regulator’s minimum load current
R7 Show that 5 V is present, with an indicator LED
R8 Let the input, the protected input, the output and ground be measured at test points
R9 Hand-solderable parts (through-hole and 0805 SMD)
R10 2-layer board that a low-cost fab can build without special options
R11 Zero ERC errors and zero DRC errors

2. Electronics background

2.1 Why regulate

Batteries sag as they discharge, bench supplies are set to whatever the user dials, and wall adapters vary with load. Electronics usually needs a fixed voltage, so a regulator sits between the source and the circuit and holds the output constant while the input and the load change.

2.2 How a linear regulator works

A linear regulator contains four blocks:

Block Function
Voltage reference A stable internal voltage that does not depend on the input
Error amplifier Compares a fraction of the output with the reference
Pass transistor A transistor in series with the load, controlled by the amplifier
Protection Current limiting, safe-area protection and thermal shutdown

If the output rises, the amplifier turns the pass transistor down. If it falls, the amplifier turns it up. The pass transistor behaves like an automatic variable resistor in series with the load, and it drops the surplus voltage:

\[V_{drop} = V_{in} - V_{out} \qquad P_{reg} = (V_{in} - V_{out}) \times I_{out} + V_{in} \times I_q\]

The second term is the quiescent current $I_q$ the regulator itself consumes. All of this power turns into heat inside the package.

2.3 Dropout voltage

The pass transistor needs some voltage across it to work. The minimum difference between input and output is the dropout voltage. For the L7805C the datasheet gives 2 V at 1 A and 25 °C, and the dropout is smaller at lower currents. Below this, the regulator can no longer hold 5 V and the output follows the input down.

\[V_{in,regulator} \ge V_{out} + V_{dropout} = 5\ \text{V} + 2\ \text{V} = 7\ \text{V}\]

2.4 Efficiency

The current into a linear regulator equals the current out plus the quiescent current. Ignoring the latter, the best possible efficiency is simply the voltage ratio:

\[\eta \le \frac{V_{out}}{V_{in}} \qquad \frac{5}{9} = 55.6\ \% \qquad \frac{5}{12} = 41.7\ \%\]

Efficiency does not improve with load current. It is a fixed penalty of the topology. Switching regulators (Project 13) avoid it.

2.5 Heat, thermal resistance and junction temperature

Heat flows from the silicon (the junction) to the surrounding air through a chain of thermal resistances, measured in °C per watt:

Symbol Path L78 TO-220 value
$R_{\theta JC}$ Junction to case 5 °C/W
$R_{\theta CS}$ Case to heatsink (thermal pad or paste) about 1 °C/W (assumed)
$R_{\theta SA}$ Heatsink to ambient depends on the heatsink
$R_{\theta JA}$ Junction to ambient, no heatsink 50 °C/W

The junction temperature is:

\[T_J = T_A + P \times R_{\theta JA}\]

With a heatsink, the total resistance is $R_{\theta JC} + R_{\theta CS} + R_{\theta SA}$. To find the heatsink needed for a given power:

\[R_{\theta SA} \le \frac{T_{J,max} - T_A}{P} - R_{\theta JC} - R_{\theta CS}\]

The L78C operating junction range is 0 °C to 125 °C. The regulator also has internal thermal shutdown, which turns the output off if it overheats, but a design must never rely on it.

2.6 Reverse-polarity protection and the cost of a diode drop

Plugging a supply in backwards would push current the wrong way through the regulator and capacitors, destroying electrolytic capacitors in particular. A series diode passes current in the correct direction and blocks it in the reverse direction. Its cost is a forward voltage drop and the power it wastes:

\[P_{diode} = V_F \times I\]
Diode type Forward drop Consequence
Silicon (for example 1N4007) about 1 V Higher loss and a higher minimum input voltage
Schottky (1N5819) 0.6 V maximum at 1 A, lower at lower currents Less heat and more headroom

A Schottky diode has a metal-semiconductor junction with a lower forward drop, at the price of a higher reverse leakage (up to 1 mA at 25 °C for the 1N5819 at its full rated reverse voltage).

2.7 Capacitors around a regulator

Part Type Role
C1, input 100 µF electrolytic A reservoir that supplies sudden current demands, so the input does not sag through the supply’s wiring resistance and inductance
C2, input 1 µF ceramic X7R Handles fast changes. It must be right at the regulator’s input pin
C3, output 100 nF ceramic X7R Handles fast load changes and keeps the output quiet
C4, output 100 µF electrolytic A reservoir that improves the response to load steps

Why two types in parallel? An electrolytic capacitor stores a lot of charge but responds slowly, because of its internal resistance and inductance. A ceramic capacitor stores little but responds quickly. Together they cover both timescales.

Things to remember:

The L78 datasheet advises bypassing the regulator input when it is far from the supply filter, with a capacitor of 0.33 µF or larger that has low impedance at high frequency, mounted with the shortest possible leads directly across the input terminals. It also notes that no output capacitor is needed for stability, but that one improves the transient response.

2.8 Minimum load current

A fixed regulator such as the L7805 needs a small minimum load to regulate properly. The datasheet states that the minimum load current for regulation is 5 mA. With no load, the output could drift upwards. The power LED branch (9.1 mA) guarantees this minimum.

2.9 Datasheet limits

L7805C (STMicroelectronics L78 series, DS0422, TO-220):

Parameter Value
Output voltage at 25 °C ($V_I$ = 10 V, $I_O$ = 500 mA) 4.8 V / 5 V / 5.2 V (min / typ / max)
Output voltage over line, load and temperature ($I_O$ 5 mA to 1 A, $V_I$ 7 V to 18 V) 4.75 V to 5.25 V
Line regulation, $V_I$ 7 V to 25 V 3 mV typical, 100 mV maximum
Line regulation, $V_I$ 8 V to 12 V 1 mV typical, 50 mV maximum
Load regulation, 5 mA to 1.5 A 100 mV maximum
Load regulation, 250 mA to 750 mA 50 mV maximum
Quiescent current 8 mA maximum
Quiescent current change 0.5 mA (load 5 mA to 1 A), 0.8 mA ($V_I$ 7 V to 23 V)
Output noise (10 Hz to 100 kHz) 40 µV per volt of output
Supply voltage rejection ($V_I$ 8 V to 18 V, 120 Hz) 62 dB minimum
Dropout voltage ($I_O$ = 1 A, 25 °C) 2 V
Output resistance (1 kHz) 17 mΩ
Short circuit current ($V_I$ = 35 V) 0.75 A
Short circuit peak current 2.2 A
Output voltage drift −1.1 mV/°C
Maximum input voltage (output 5 V to 18 V) 35 V
Operating junction temperature (L78C) 0 °C to 125 °C
Thermal resistance, TO-220: junction to case / junction to ambient 5 °C/W / 50 °C/W
Minimum load current for regulation 5 mA

1N5819 (Diodes Incorporated DS23001 and equivalents):

Parameter Value
Repetitive peak reverse voltage 40 V
Average forward current 1 A
Peak forward surge (8.3 ms) 25 A
Forward voltage at 1 A 0.60 V maximum (0.90 V at 3 A)
Reverse current at 25 °C, rated reverse voltage 1 mA maximum
Package DO-41, the stripe marks the cathode

3. Calculations

3.1 Inputs

Symbol Meaning Value Source
$V_{in}$ Supply voltage at J1 7.6 V to 12 V, 9 V nominal Requirement R1
$V_{out}$ Regulated output 5.0 V L7805C
$V_F$ Diode drop 0.45 V typical (assumed between the datasheet points), 0.6 V maximum 1N5819 datasheet
$I_q$ Regulator quiescent current 8 mA (datasheet maximum, used for a conservative power figure) L7805C
$I_{LED}$ Power LED branch 9.09 mA (see 3.7) Design
$R_{\theta JA}$ Junction to ambient, no heatsink 50 °C/W L78 datasheet
$R_{\theta JC}$ Junction to case 5 °C/W L78 datasheet
$T_{J,max}$ Maximum junction temperature 125 °C L78C datasheet
$T_A$ Ambient temperature 25 °C (lab) and 40 °C (warm enclosure) Requirement

3.2 Minimum input voltage

The regulator’s input must be at least 7 V (the lowest input in the datasheet’s regulation specification, equal to 5 V plus the 2 V dropout). The diode takes up to 0.6 V:

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

With a typical drop of 0.45 V the limit is 7.45 V. A 9 V battery is considered flat at 7.6 V.

3.3 Regulator power and junction temperature

The load current in the table is the current into J2. The total output current is that plus the 9.09 mA LED branch.

\[I_{out} = I_{load} + I_{LED} \qquad V_{raw} = V_{in} - 0.45\ \text{V} \qquad P_{reg} = (V_{raw} - 5.0)\,I_{out} + V_{raw} \times 0.008\ \text{A}\]

Worked example, 9 V in and 300 mA load:

\[V_{raw} = 8.55\ \text{V} \qquad I_{out} = 0.3091\ \text{A} \qquad P_{reg} = 3.55 \times 0.3091 + 8.55 \times 0.008 = 1.166\ \text{W}\] \[T_J = 25 + 1.166 \times 50 = 83\ ^\circ\text{C} \qquad T_{case} = T_J - P \times R_{\theta JC} = 83 - 1.166 \times 5 = 77\ ^\circ\text{C}\]
Vin Load Vraw P_reg ΔT (50 °C/W) T_J at 25 °C T_J at 40 °C Verdict
9 V 0 mA 8.55 V 0.10 W 5 °C 30 °C 45 °C Fine
9 V 100 mA 8.55 V 0.46 W 23 °C 48 °C 63 °C Fine
9 V 300 mA 8.55 V 1.17 W 58 °C 83 °C 98 °C Fine (design rating)
9 V 500 mA 8.55 V 1.88 W 94 °C 119 °C 134 °C Heatsink needed at 40 °C
12 V 100 mA 11.55 V 0.81 W 40 °C 65 °C 80 °C Fine
12 V 300 mA 11.55 V 2.12 W 106 °C 131 °C 146 °C Heatsink needed
12 V 500 mA 11.55 V 3.43 W 171 °C 196 °C 211 °C Heatsink needed
7.6 V 300 mA 7.15 V 0.72 W 36 °C 61 °C 76 °C Fine

3.4 Maximum load against input voltage

Rearranging the power equation for the maximum current at a given power limit $P_{max} = (T_{J,max} - T_A) / R_{\theta total}$:

\[I_{load,max} = \frac{P_{max} - V_{raw} \times 0.008}{V_{raw} - 5.0} - 0.00909\]
Vin Free air, 25 °C ($P_{max}$ = 2.00 W) Free air, 40 °C ($P_{max}$ = 1.70 W) Heatsink 18 °C/W, 40 °C ($P_{max}$ = 3.54 W)
7.6 V 895 mA 755 mA above 1 A
8 V 752 mA 634 mA  
9 V 535 mA 451 mA 969 mA
10 V 414 mA 348 mA  
11 V 336 mA 282 mA  
12 V 282 mA 236 mA 518 mA

The design rating follows from this table: 300 mA in free air up to 10 V input at 40 °C ambient (the limit there is 348 mA), and 500 mA up to 12 V with a heatsink of 18 °C/W or better. The plot in section 4 shows the same data.

3.5 Heatsink selection

\[R_{\theta SA} \le \frac{125 - 40}{P} - 5 - 1\]
Condition P_reg Required $R_{\theta SA}$
9 V in, 500 mA out 1.88 W 39 °C/W or better (a very small clip-on heatsink)
12 V in, 500 mA out 3.43 W 18.8 °C/W or better, so choose 18 °C/W or better

With an 18 °C/W heatsink at 12 V and 500 mA: $T_J = 40 + 3.43 \times (5 + 1 + 18) = 122\ ^\circ\text{C}$, just under the limit. For more margin, use a lower input voltage or a larger heatsink.

3.6 Efficiency and the power budget

\[\eta = \frac{V_{out} \times I_{out}}{V_{in} \times (I_{out} + I_q)}\]
Operating point Input power Load power Regulator heat Diode heat Efficiency
9 V, 100 mA 1.05 W 0.55 W 0.46 W 0.05 W 52 %
9 V, 300 mA 2.85 W 1.55 W 1.17 W 0.14 W 54 %
9 V, 500 mA 4.65 W 2.55 W 1.88 W 0.23 W 55 %
12 V, 300 mA 3.81 W 1.55 W 2.12 W 0.14 W 41 %

More than half of the input power is wasted in every case. That is the price of a linear regulator’s simplicity.

3.7 Power LED and the minimum load

\[I_{LED} = \frac{V_{out} - V_F}{R_1} = \frac{5.0\ \text{V} - 2.0\ \text{V}}{330\ \Omega} = 9.09\ \text{mA}\]
Quantity Value
R1 power 27.3 mW (about 125 mW rating for 0805)
D2 power 18.2 mW
Minimum load required by U1 5 mA
Margin 9.09 mA is 1.8 times the minimum

3.8 Reverse-polarity diode

Quantity Value
Power at 300 mA (typical drop 0.45 V) 0.14 W
Power at 500 mA (typical drop 0.45 V) 0.23 W
Worst case, 0.6 V at 0.5 A 0.30 W
Current rating margin 0.5 A against 1 A average and 25 A surge
Reverse voltage margin 12 V maximum input against 40 V rating
Reverse leakage at a reversed connection 1 mA maximum at 25 °C at the full 40 V
Compared with a silicon diode (about 1 V) Silicon would lose about 0.32 W at 300 mA and raise the minimum input to about 8 V

3.9 Capacitor selection

Part Value Reasoning
C1 100 µF, 25 V A 300 mA demand lasting 100 µs takes 30 µC, which droops 100 µF by $Q/C = 0.3$ V. The 25 V rating is 2.2 times the highest working voltage (11.55 V)
C2 1 µF, 50 V X7R The datasheet asks for 0.33 µF or larger. A 1 µF part keeps well above that after DC-bias loss at 8.5 V
C3 100 nF, 50 V X7R The datasheet’s test value for the output capacitor
C4 100 µF, 16 V The 16 V rating is 3.2 times the 5 V working voltage

3.10 Regulation accuracy

Effect Datasheet limit At this design
Output voltage tolerance 4.75 V to 5.25 V over all conditions (4.8 V to 5.2 V at 25 °C) 5 V ±5 %
Line regulation, 8 V to 12 V input 50 mV maximum 1 % of 5 V at most
Load regulation, 250 mA to 750 mA 50 mV maximum 1 % of 5 V at most
Supply voltage rejection, 120 Hz 62 dB minimum 1 V of ripple on the input appears as 0.79 mV on the output
Output resistance, 1 kHz 17 mΩ A 300 mA load step changes the output by about 5 mV
Output noise 40 µV per volt of output about 200 µV for 5 V
Temperature drift −1.1 mV/°C about −0.11 V over a 100 °C swing

3.11 Test loads

Resistors that draw current from the 5 V output generate real heat. Choose a power rating of at least twice the dissipation.

Load resistor Current at 5 V Dissipation Minimum power rating
100 Ω 50 mA 0.25 W 0.5 W
47 Ω 106 mA 0.53 W 1 W
33 Ω 152 mA 0.76 W 2 W
22 Ω 227 mA 1.14 W 3 W
15 Ω 333 mA 1.67 W 5 W
10 Ω 500 mA 2.50 W 5 W (10 W for long tests)

3.12 Component purposes

Ref Value Purpose
J1 Screw terminal Supply input
D1 1N5819 Reverse-polarity protection
U1 L7805CV 5 V regulation
C1 100 µF, 25 V Input reservoir
C2 1 µF, 50 V Input high-frequency bypass
C3 100 nF, 50 V Output high-frequency bypass
C4 100 µF, 16 V Output reservoir
R1 330 Ω LED current limit
D2 Red LED Power indicator and minimum load
J2 Header 5 V output
TP1 to TP4 Test points VIN, VRAW, +5V, GND

4. Operating area and power budget

4.1 Dropout and thermal limits

Operating area

Plot Reading
A Output voltage against input voltage at J1. The curve flattens at 5 V once the input is high enough (the shaded band is the 4.75 V to 5.25 V tolerance). To the left of 7.6 V the regulator is in dropout and the output follows the input down. The 2 V dropout figure is for 1 A, so the real knee at lower currents is a little further left
B Maximum load current against input voltage for three cooling conditions. The curves fall as the input rises, because the same current produces more heat. The marked points are the design ratings

4.2 Where the power goes

Power budget

Each bar is the input power split into three parts: useful load power (green), heat in the regulator (red) and heat in the diode (orange). The red segment grows quickly with input voltage: compare 9 V and 12 V at the same 300 mA.

4.3 No timing diagram

This board is a DC circuit with no periodic signals, so the roadmap’s extra for this project is the voltage and power budget above rather than a timing diagram. The only time-dependent effects are the capacitors’ response to load steps (a few millivolts, see 3.10) and the slow warming of the regulator, which takes minutes.

5. Schematic

5.1 Symbols and footprints

Ref Library symbol Value Footprint
J1 Connector:Screw_Terminal_01x02 VIN TerminalBlock:TerminalBlock_bornier-2_P5.08mm
J2 Connector:Conn_01x02_Pin 5V_OUT Connector_PinHeader_2.54mm:PinHeader_1x02_P2.54mm_Vertical
D1 Diode:1N5819 1N5819 Diode_THT:D_DO-41_SOD81_P10.16mm_Horizontal
U1 Regulator_Linear:L7805 L7805CV Package_TO_SOT_THT:TO-220-3_Vertical
C1 Device:CP 100uF 25V Capacitor_THT:CP_Radial_D6.3mm_P2.50mm
C2 Device:C 1uF Capacitor_SMD:C_0805_2012Metric
C3 Device:C 100nF Capacitor_SMD:C_0805_2012Metric
C4 Device:CP 100uF 16V Capacitor_THT:CP_Radial_D6.3mm_P2.50mm
R1 Device:R 330 Resistor_SMD:R_0805_2012Metric
D2 Device:LED RED LED_SMD:LED_0805_2012Metric
TP1 to TP4 Connector:TestPoint TP1 to TP4 TestPoint:TestPoint_THTPad_D2.0mm_Drill1.0mm
Power power:+5V, power:GND, power:PWR_FLAG ×2 n/a n/a

The L7805 symbol has pin 1 as input, pin 2 as ground and pin 3 as output, which matches the TO-220 package. Check the diameter and lead spacing of your electrolytic capacitors against the footprint.

5.2 Nets

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

5.3 Design decisions

5.4 Schematic conventions

6. KiCad configuration

6.1 Project creation

  1. Create the folder 05-linear-regulator inside the repository.
  2. In KiCad, choose File → New Project, browse to that folder, name the project 05_linear_regulator, and untick “Create a new folder for the project”.

6.2 Board setup

Import the settings from the previous board: in the PCB editor, open File → Board Setup, click Import Settings from Another Board…, select 04_npn_transistor_switch.kicad_pcb, tick the stackup, design rules, net classes and text defaults, and confirm.

Setting Value
Copper layers 2, board thickness 1.6 mm, 1 oz copper
Minimum clearance 0.2 mm
Minimum track width 0.2 mm
Minimum connection width 0.2 mm
Minimum annular width 0.15 mm
Minimum via diameter 0.6 mm
Copper to hole clearance 0.3 mm
Copper to edge clearance 0.5 mm
Minimum drill size 0.3 mm
Hole to hole clearance 0.5 mm
Silkscreen: minimum clearance / text height / text thickness 0.15 mm / 1.0 mm / 0.15 mm

6.3 Net classes (new in this project)

This board carries up to 0.5 A on its supply path, so it gets a wider class. Open Board Setup → Design Rules → Net Classes, add a class called Supply, and edit the patterns so that no net matches two classes.

Net class Clearance Track width Via diameter / drill Net patterns
Default 0.2 mm 0.25 mm 0.6 mm / 0.3 mm all others (LEDA)
Power 0.2 mm 0.5 mm 0.8 mm / 0.4 mm GND (remove +5V and COLL from this class)
Supply 0.2 mm 1.0 mm 0.8 mm / 0.4 mm VIN, VRAW, +5V

Under Pre-defined Sizes, add a track width of 1.0 mm.

Why 1.0 mm. The IPC-2221 estimate for a 1 oz outer-layer track at a 10 °C rise is about 0.9 A for 0.25 mm, 1.4 A for 0.5 mm and 2.4 A for 1.0 mm. The supply path carries at most about 0.5 A, so 0.5 mm would be enough electrically. The wider track costs nothing, lowers the voltage drop, and is easier to connect to the large pads of the terminal block and regulator. Ground is mostly a copper pour, so its tracks only need to reach the pour.

6.4 Grid

PCB grid: 0.5 mm for placement, 0.25 mm or 0.1 mm for routing. For the test point row use 5.08 mm.

7. Layout

7.1 Outline

A rectangle on Edge.Cuts, 70 × 45 mm, with 3 mm corner radius. The outline is a single closed shape with no gaps.

7.2 Placement

Rule Reason
Power flows left to right: J1, D1, C1, U1, C4, J2 Short supply path, matching the schematic
J1 at the left edge, J2 at the right edge Short, separate input and output paths
C2 touching U1 pin 1, C3 touching U1 pin 3 The datasheet asks for the shortest possible leads at the input, and the output capacitor works best close to the pin
U1 near the top edge, with the tab facing outwards Room for a clip-on heatsink and airflow
Keep a 20 × 15 mm area around U1’s tab free of parts Heatsink and airflow clearance
C1, C4 and the LED at least 10 mm from U1 and D1 Electrolytic capacitors age faster when hot
R1 and D2 at the bottom of the board Away from the heat
TP1 to TP4 in a row along the bottom edge on a 5.08 mm pitch, in the order VIN, VRAW, +5V, GND Probe-friendly, with GND last

7.3 Routing

Rule Value
VIN, VRAW, +5V track width 1.0 mm (the Supply net class applies this)
GND 0.5 mm tracks to the pour (the Power net class)
LEDA track width 0.25 mm
Corner angles 45° or curved, never 90°
Copper to board edge at least 0.5 mm
Via size 0.6 mm pad / 0.3 mm drill (0.8 mm / 0.4 mm in the Supply and Power classes)

TO-220 pads. The TO-220-3_Vertical footprint has its pads on a 2.54 mm pitch with a gap of about 0.64 mm between them. A 1.0 mm track cannot pass between the pads, so route the supply tracks to the pads from outside. Only a 0.25 mm track fits between them.

The input path. Run VIN, then D1, then VRAW in a straight line, so that the large input current does not mix with the sensitive output path.

7.4 Ground pour and stitching vias

7.5 Silkscreen

8. Verification

Check How Result
ERC Schematic editor → Inspect → Electrical Rules Checker 0 errors
DRC PCB editor → Inspect → Design Rules Checker 0 errors, 0 unrouted
Board statistics PCB editor status bar 25 pads, 5 nets
3D view PCB editor → View → 3D Viewer (Alt+3) Orientation of D1, C1, C4 and U1 correct, space for a heatsink
Gerber check Third-party viewer (KiCad Gerber Viewer, Gerbv, tracespace) Outline closed, holes aligned, silkscreen clear of pads

Saved reports are in reports/.

9. Manufacturing outputs

9.1 Gerber plot settings (File → Fabrication Outputs → Gerbers)

Setting Value
Output directory fabrication/gerbers
Layers F.Cu, B.Cu, F.Paste, B.Paste, F.Silkscreen, B.Silkscreen, F.Mask, B.Mask, Edge.Cuts
Use Protel filename extensions On
Subtract soldermask from silkscreen On
Check zone fills before plotting On
Coordinate format 4.6, unit mm
Use extended X2 format On
Include netlist attributes On
Disable aperture macros Off
Use drill/place file origin Off
Plot drawing sheet Off

9.2 Drill file settings

Setting Value
Drill file format Excellon
Oval holes drill mode Use alternate drill mode
Drill origin Absolute
Drill units Millimetres
Zeros format Decimal format
Map file format (optional) Gerber

9.3 Suggested fab order options

Option Value
Layers 2
Thickness 1.6 mm
Outer copper weight 1 oz
Surface finish HASL (lead-free if available)
Quantity Fab minimum

Verify the generated files in a Gerber viewer, zip the gerbers folder, and upload the zip.

10. Bring-up and testing

10.1 Safety

10.2 Before powering

  1. Solder the 0805 parts first, then D1, the headers, test points and terminal block, then the electrolytic capacitors and finally U1 (with its heatsink, if used).
  2. Check the orientation of D1 (stripe towards U1), D2 (cathode towards GND), C1 and C4 (stripe side to GND) and U1 (tab towards the board edge).
  3. With a multimeter in continuity mode, confirm there is no short between +5V and GND, and none between VIN and GND.

10.3 Reverse-polarity test (do this first)

  1. Set a bench supply to 9 V with a current limit of 50 mA.
  2. Connect it reversed to J1.
  3. Measure TP3: it should be about 0 V. The supply current should be well below 1 mA (the diode’s leakage).
  4. Disconnect, then reconnect correctly.

10.4 Powered test

Set the multimeter to DC volts. Keep the black probe on TP4 (GND).

  1. Apply 9 V to J1. Measure TP1 (VIN) and TP2 (VRAW). The difference is the diode drop. It must be 0.6 V or less.
  2. Measure TP3: about 5 V, within 4.8 V to 5.2 V. The LED lights.
  3. Connect a test load from the table in 3.11 between the pins of J2. Start with 100 Ω, then 47 Ω, then 22 Ω. At each step, wait a minute and record TP3 and TP2.
  4. With 22 Ω (227 mA), TP3 should change by 50 mV or less compared with no load.
  5. Dropout test (optional): with a 47 Ω load and an adjustable supply, slowly reduce TP1 from 9 V. The output should hold up to about 7.6 V, then begin to fall.
  6. Thermal test: run at 9 V and 300 mA (15 Ω, 5 W load) for 5 minutes and measure the regulator’s case temperature with an infrared thermometer. Expect about 75 °C at 25 °C ambient.
Measurement Expected Measured
Reverse connection: TP3 and supply current about 0 V, well below 1 mA  
TP1 (VIN) 9.0 V (or your supply setting)  
TP1 − TP2 (diode drop) 0.6 V or less (about 0.2 V to 0.5 V depending on load)  
TP3, no load 4.8 V to 5.2 V  
TP3, 22 Ω load within 50 mV of the no-load value  
LED On  
Supply current, no load about 9 mA (LED) plus up to 8 mA quiescent, so about 15 mA  
Regulator case temperature, 9 V and 300 mA about 75 °C at 25 °C ambient  
Regulation starts to fail at TP1 of about 7.6 V (or lower at light load)  

10.5 Troubleshooting

Symptom Likely cause
No output, no LED D1 fitted backwards (stripe towards J1), no input, or U1 fitted wrongly
Output stuck near 0 V and U1 gets hot A short on the output, or C3/C4 reversed or shorted
Output low (about 4 V) at heavy load The input is too low: the regulator is in dropout. Check TP2
Output drifts up with no load Load below the 5 mA minimum: the LED branch is missing or open (R1, D2)
Regulator too hot to touch at 300 mA Normal at 9 V (about 77 °C). Add a heatsink, lower the input voltage or reduce the load
Regulator shuts down repeatedly Overheating: thermal shutdown. Reduce the load or add a heatsink
Output noisy or oscillating C2 missing, or too far from U1’s input pin
An electrolytic capacitor bulges or smells Reversed polarity or too high a voltage: switch off immediately and replace
Input supply collapses when the load is connected Supply or battery cannot deliver the current, or the wiring is too thin

11. Lessons learned

12. References

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