PCB Design
Design a USB LED desk lamp board: safe, quiet and cool
A USB LED desk lamp is a small board with three classic PCB jobs: a fuse that blows in a short without starving the LEDs, a light sensor on a quiet ground joined at one point, and a driver whose heat goes out through thermal vias. Get each one right on paper before you order boards. Here's the check, with the numbers.
The board
USB gives 5 V through about 0.25 Ω of port and cable. A fuse by the connector, then the main trace, feeds a 1 A LED panel and an ON light. A light sensor for auto-brightness has its own ground, A-GND, and the panel's driver sheds 1.5 W.
1. Safe: size the fuse
The fuse must stay under its 0.25 W at about 1 A, so under about 0.25 Ω. It must also leave the panel at least 4.5 V. With 0.1 Ω, the panel gets about 4.6 V once the cable, the fuse, the main trace and the 0.05 Ω ground return have each taken their share. In a short, the loop is about 0.46 Ω:
The fuse blows, and the 10 mΩ main trace sheds about 1.2 W, inside its 1.5 W.
2. Quiet: one bridge
The sensor's ground should join the main ground once, at the supply. Close a second bridge by the panel and you make a loop: part of the panel's amp comes home through A-GND and lifts the sensor's ground about 26 mV. Open it, and the lift falls to a few hundredths of a millivolt.
3. Cool: join the vias
The driver's 1.5 W crosses θJC, 5 °C/W, then the top copper at 50 °C/W. Alone that's 25 + 1.5 × 55 ≈ 108 °C, over the 100 °C target. Thermal vias into copper on the other side add a second 50 °C/W path in parallel, 25 °C/W in all, and the junction runs at 70 °C. Join the driver's pad to that copper solid, not through a thermal relief: reliefs slow heat into a plane so ordinary pins can be soldered, and here you want the heat to leave.
Live experiment
Check the lamp board
USB power through a fuse to a 1 A LED panel, a light sensor on its own ground, and a driver shedding 1.5 W. Make the three choices, the fuse, the second ground bridge and the thermal vias, then run the seven checks: only a 0.1 Ω fuse, one bridge and joined vias pass them all.
“Project: USB desk lamp” is the project that ends Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Before you order
The fuse sits first, right after the USB connector. Every return current has one road home, with bridges only where you meant them. Every hot part has a planned heat path, vias included. And remember the port: a plain USB 2.0 port promises only 500 mA, so a 1 A lamp should run from a charger or a USB-C port that offers more.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Project: USB desk lamp” is the project that ends Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: Every amp the board draws passes through it, so a short anywhere blows it. Upstream of everything, it protects everything. A fuse halfway along only guards what comes after it.
- Why not “It lowers the fault current”? The fault current is set by the loop's resistance, the same wherever in the loop the fuse sits.
- Why not “It's easier to solder”? Solder is the same everywhere on the board. The position is about what it protects.
- Why not “It stays cooler there”? Where it sits doesn't make it cooler. It goes first so every amp passes through it.
Show the answer
Answer: No: two bridges make a loop, and noisy current flows round it. That's the Try it board again: two joins invite the digital current into the quiet plane. One bridge.
- Why not “Only with a resistor”? A resistor in the second bridge still leaves a loop for part of the noisy current. One bridge is the rule.
- Why not “Yes, more is safer”? Two bridges make a loop, and the noisy current flows round it through the quiet plane.
- Why not “Only at DC”? Even at DC a second bridge shares the noisy current with the quiet ground, and fast signals make it worse.
Show the answer
Answer: Easier to solder, by slowing the heat into the plane. Thin spokes slow the heat, so the iron can bring the pad past 217 °C, while still joining it to the plane.
- Why not “Cooler under the iron”? It makes the pad hotter under the iron, which is the point: hot enough to melt the solder.
- Why not “Free of the plane”? The spokes still join the pad to the plane, for current and for its ground.
- Why not “Carry more current”? The spokes carry less current than solid copper would. A relief trades some current capacity for easy soldering.
“Project: USB desk lamp” is the project that ends Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Questions people ask
How much current can a USB port supply?
A USB 2.0 port guarantees 500 mA at 5 V and USB 3.x 900 mA. USB-C ports can advertise 1.5 A or 3 A, and chargers that follow the battery-charging spec supply 1.5 A. A device drawing more than its port offers may be cut off or brown out.
How many thermal vias do I need?
Enough that, in parallel, they bring the junction under its limit. A single small via is very roughly 100 °C/W or more on its own, so arrays of 5 to 20 under a power pad are common; the real figure depends on drill size, plating and board thickness.
Where should the fuse go on a PCB?
Right after the power connector, before anything else, so every amp the board draws passes through it and a short anywhere downstream blows it.
Related topics
- How to choose a fuse so the right thing burnsCool at the working current, first to fail in a fault, and in front of everything it protects.
- Should you split a ground plane? AGND, DGND and one bridgeNoisy and quiet grounds, joined once: two bridges make a loop, and none leaves the sensor stranded.
- How to calculate junction temperatureHeat flows like current: watts through °C/W give a temperature rise, so size the copper before it cooks.
- PCB trace width for currentCount the squares: how a trace's width sets its ohms, its voltage drop and its heat, and how wide to draw it.