PCB Design
Lay out a motor PCB: three copper fixes
A motor board fails in three classic ways: a narrow trace that cooks, a supply that sags when the motor switches on, and a sensor that reads wrong because the motor's current shares its ground. Each has a copper fix: widen the neck, put the bulk capacitor at the motor end of the trace, and give the sensor its own return to the supply.
The board
- A 12 V supply
- A motor of about 4 Ω, drawing 3 A through a switch
- A sensor drawing 12 mA
- A power trace with a narrow neck, and about 1 µH of inductance on its way to the motor
Fix 1: widen the neck
At 0.2 Ω the neck holds the motor to about 2.8 A and still makes 2.8² × 0.2 ≈ 1.6 W, over three times the half watt it can shed. Widened to 0.02 Ω it makes under 0.2 W and the motor gets its 3 A. Heat goes as current squared, so the neck on a motor trace needs real width.
Fix 2: a capacitor at the motor
A trace's inductance resists any sudden change in current: V = L × di/dt. When the switch closes, the motor wants 3 A at once, but the current through 1 µH has to build up from zero, so for that first fraction of a microsecond the rail at the motor collapses. A 10 µF capacitor right at the motor end, from the node after the trace to ground, supplies those first microseconds itself, and the rail dips by only about a volt, staying above 10.5 V. The same capacitor at the supply end is useless here: the inductance is still between it and the motor.
Live experiment
Make the three copper decisions
12 V feeds a 3 A motor through a narrow neck and 1 µH of trace, beside a 12 mA sensor. Choose the neck, where the 10 µF capacitor goes and the sensor's return, then test. A thin neck makes 1.6 W; the capacitor at the supply lets the rail at the motor dip at switch-on; a shared return lifts the sensor's ground.
“Project: Motor board” is the project that ends Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 before it first.
Fix 3: the sensor's own return
If the sensor's ground joins the motor's return partway along, the shared copper carries the motor's 3 A. Even 0.05 Ω of it lifts the sensor's ground by 3 × 0.05 = 150 mV. Give the sensor its own return trace to the supply's ground: it carries only the sensor's 12 mA, and through 0.5 Ω that's a lift of just 6 mV. That's a star ground: every load's return meets at one point.
Check it
- The motor gets about 3 A and nothing overheats.
- The sensor draws its 12 mA, with its ground lifted under 20 mV.
- The rail at the motor stays above 10.5 V through the switch-on.
Real motor boards add a flyback diode across the motor and a small ceramic capacitor beside the bulk one, but these three copper decisions come first.
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: Motor board” is the project that ends Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 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: Is lifted by the motor's 3 A through the shared part. Any copper the two share carries the motor's current and lifts the sensor. Separate all the way to the star point.
- Why not “Floats”? It's still connected, so it doesn't float. It's lifted by the motor's current through the shared copper.
- Why not “Is fine”? Any copper the two share carries the motor's 3 A, and that current times its resistance lifts the sensor's ground.
- Why not “Is at 12 V”? The lift is the motor current times a small resistance: a fraction of a volt, not the whole supply.
Show the answer
Answer: The rail at the chip sags as if there were no capacitor. The trace's inductance sits between the capacitor and the chip, so the capacitor's current has the same problem the supply's did. It helps the supply, not the chip.
- Why not “The trace shorts”? Nothing shorts. The trace's inductance just slows the current reaching the chip, so the rail there dips.
- Why not “The rail holds”? The trace's inductance sits between the capacitor and the chip, so the capacitor can't deliver the sudden current in time.
- Why not “The capacitor explodes”? Nothing here stresses the capacitor. It's simply in the wrong place to help.
Show the answer
Answer: The motor's current no longer flows through the sensor's ground path. Each load gets its own return to one point. The motor's current lifts only its own wire.
- Why not “It cancels the current”? The motor's current still flows, just through its own return instead of the sensor's.
- Why not “It uses thicker copper”? Thicker copper would shrink the lift but not remove it. The star works by separating the paths.
- Why not “It adds a capacitor”? A star ground is a way of routing the returns. No capacitor is involved.
“Project: Motor board” is the project that ends Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 before it first.
Questions people ask
How big a capacitor does a motor board need?
Start with what the motor driver's datasheet asks for, usually a bulk electrolytic of tens to hundreds of microfarads right at the driver's supply pins, plus a 100 nF ceramic beside it for fast spikes. Bigger motors and longer supply wires need more.
Why does my sensor read wrong when the motor runs?
Usually a shared ground. The motor's amps flow back through copper the sensor also uses, and that current times the copper's resistance lifts the sensor's ground. A tenth of a volt of lift is a big error to a sensor reading millivolts. Give it its own return.
Should motor and logic grounds be separate?
Their return currents should take separate paths, joined at one point near the supply: a star. They still need to be connected, or nothing has a common reference.
Related topics
- Why a narrow neck in a PCB trace gets hotEvery amp goes through the narrow bit, so that's where the ohms are and where the heat goes.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.
- Star grounding explainedGive each load its own return to one point, so a big current never flows through a sensitive circuit's ground.
- MOSFET as a switch, explainedA gate voltage, not a current, opens the channel: threshold, R_DS(on) and the flyback diode.