PCB Design
How to choose a fuse so the right thing burns
A fuse has to be the weakest link: cool at the working current, the first thing to fail in a fault, and able to hold the fault current down until it does, so that nothing else cooks first. To size one, work out the fault current from the supply's own resistance, then check the heat, I² × R, in the fuse and in every other part of the path.
Fault current
A 12 V adapter with 0.8 Ω of its own runs a heater at 0.9 A. Then a screw rolls across the heater's pins: 0.1 Ω. With a 0.3 Ω fuse trace in the loop:
That's ten times the working current.
Heat goes as I²
Ten times the current is a hundred times the heat. In normal use the 0.3 Ω fuse sheds 0.9² × 0.3 ≈ 0.24 W, under its 0.5 W. In the short it takes 10² × 0.3 = 30 W, and it blows. The board's main 10 mΩ trace takes 10² × 0.01 = 1 W, inside its 1.5 W.
The window
- Too fat, under 0.07 Ω: the short's current climbs past 12.2 A, the most the main trace can carry, √(1.5 W ÷ 0.01 Ω). At 2 mΩ the short draws 13.2 A, the main trace burns 1.7 W, and the fuse barely warms.
- Too thin, over 0.6 Ω: the fuse sheds more than its 0.5 W at the working 0.9 A, and cooks every time the heater runs.
Live experiment
Choose the weakest link
A 12 V adapter runs a 0.9 A heater through a fuse trace and the main trace. Pick the fuse and drop a screw across the heater: 2 mΩ barely warms while MAIN cooks, 0.05 Ω blows but still lets 12.5 A through, 0.3 Ω runs cool and blows in the short, and 1 Ω cooks every time.
“The weakest link” is lesson 4 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 17 before it first.
Put it first
The fuse goes right after the power connector, so every amp the board draws passes through it, and a short anywhere downstream blows it. A fuse halfway along only guards what comes after it.
Real fuses
A deliberately thin trace works in cheap gadgets, but it can't be replaced. Real boards fit a fuse part in that spot: a cartridge or SMD fuse rated somewhat above the working current, or a resettable PTC like the ones behind many USB ports. Check its I²t, the energy it takes to blow, against what your parts can survive and the brief surges at switch-on it must ignore, and check its voltage rating too.
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.
“The weakest link” is lesson 4 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 17 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: It shrugs, and the current cooks something else instead. A fat trace sheds the fault's heat easily, so the 13 A goes on to burn MAIN, deep in the board. A fuse has to be the weakest link.
- Why not “Nothing overheats”? Something always takes the fault's heat. If the fuse doesn't, the next weakest part does.
- Why not “The short stops by itself”? A short doesn't clear itself. The current flows until something burns open.
- Why not “It blows sooner”? A fat trace has less resistance, so it sheds less of the fault's heat and blows later, if at all.
Show the answer
Answer: Cool at the working current and the first thing to cook in a fault. Its width is a window: narrow enough to blow in a short, wide enough not to in normal use.
- Why not “Placed after the load”? After the load it guards nothing upstream. It belongs first, right at the connector.
- Why not “As thin as possible”? As thin as possible, it would cook in normal use too.
- Why not “As wide as possible”? As wide as possible, it would never blow, and the fault would cook something else.
Show the answer
Answer: Half. Half as many squares end to end.
- Why not “Double”? A wider trace gives the current more room, so its resistance falls, not rises.
- Why not “The same”? Width matters: twice as wide is half as many squares end to end.
- Why not “A quarter”? A quarter would need the width doubled and the length halved too. Width alone halves it.
“The weakest link” is lesson 4 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 17 before it first.
Questions people ask
How do I choose a fuse rating?
Start from the normal current and pick a fuse rated about 1.25 to 1.5 times it, so it never runs near its limit. Then check the fault current: the fuse must blow fast enough at that current for the wires, traces and parts in the path to survive, which its I²t and time-current curve tell you.
Can a PCB trace be used as a fuse?
It can, as a last-resort fuse in low-cost products: a short, narrow section that runs cool in normal use and burns open in a hard short. It isn't precise, can't be replaced and may scorch the board, so a proper fuse part is better wherever space and cost allow.
What is the difference between a fuse and a PTC?
A fuse melts open and has to be replaced. A PTC resettable fuse heats up in a fault, its resistance rises sharply and limits the current, and it recovers once the fault is gone. PTCs are slower, and let a small current flow while tripped.
Related topics
- 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.
- Electrical power explained: P = V × IP = V × I: how fast energy turns into heat, and why a resistor has a power rating.
- How to calculate voltage drop in a wireA wire is a resistor: how many volts a cable steals, the heat it makes, and why the grid runs at high voltage.
- How to calculate junction temperatureHeat flows like current: watts through °C/W give a temperature rise, so size the copper before it cooks.