PCB Design
Star grounding explained
Star grounding means wiring each circuit's ground return separately to one common point, usually the supply, instead of chaining them along one shared trace. Each return then carries only its own current, so a motor's amps can never flow through a sensor's ground and shift its readings.
Ground is a wire too
On a schematic, ground is a symbol. On a board or in a box, it's copper, with resistance, and every load's current flows back along it. When two loads share a return, the big current lifts the ground of the small one:
Example: a motor drawing 0.91 A through a 0.5 Ω shared return lifts the far end by about 0.46 V. A temperature sensor that outputs 10 mV per degree, measured from that lifted point, now reads about 46 °C too hot, and the error jumps every time the motor switches.
Live experiment
Stop the motor fooling the sensor
5 V runs a motor drawing nearly an amp and a temperature sensor drawing 10 mA. Sharing a thin return, the motor lifts the sensor's ground and it reads tens of degrees hot. Widen the return, or give the sensor its own path to the supply: a star.
“The return path” is lesson 4 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 before it first.
Fix it
- Widen the shared copper. At 0.05 Ω the lift falls to about 50 mV. Better, but it never goes away.
- Don't share at all. Give the sensor its own return straight to the supply's −. It carries only the sensor's 10 mA, so through even a thin 0.5 Ω it lifts just 5 mV, and the motor's current never touches it.
That second fix is the star: returns radiating from one point, meeting nowhere else. A thin return is fine as long as only a small current flows in it.
Power rails, too
The same applies to the + side. Feed three loads by daisy-chaining them along one trace, and the first segment carries everyone's current: its drop lands on every later load, and a neighbour switching on or off moves them all. Give each load its own trace from the supply, a star, and each one drops only its own current.
Ground loops and hum
Connect two pieces of gear with two ground paths, through a mains earth and a signal cable's shield, and you've made a loop. Stray mains-frequency fields drive current round it, and that current drops voltage along the shield: the hum from a badly wired amplifier or stereo. Good amplifiers wire their grounds as a star for exactly this reason.
On fast digital boards, a solid ground plane does the same job better, giving every return a short, low-inductance path. The idea is the same: keep heavy and noisy currents out of quiet circuits' ground.
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 return path” is lesson 4 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 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: 0.45 V high: its ground is lifted by the motor. 0.9 A × 0.5 Ω is 0.45 V between the shared node and the real ground. The sensor measures from the shared node, so everything it reads is off by that.
- Why not “Correctly”? The motor's current through the shared copper lifts the sensor's ground, so it can't read correctly.
- Why not “Nothing”? The sensor still works, but it measures from a ground that's been lifted, so its reading is wrong.
- Why not “0.9 V high”? The lift is current times resistance: 0.9 A × 0.5 Ω = 0.45 V, not 0.9 V.
Show the answer
Answer: Each load's return meets the supply at one point, so no load's current flows through another's copper. Separate returns to one point: the motor's amp goes down the motor's trace and never touches the sensor's.
- Why not “No ground at all”? A star ground is still a ground. It just keeps each load's return separate until one point.
- Why not “A ground shaped like a star”? It's named for the wiring pattern, returns radiating from one point, not the copper's shape.
- Why not “All grounds joined at the load”? Joining every ground at the load is the shared ground that causes the trouble.
Show the answer
Answer: The motor's current times the shared resistance. Ohm's law on the shared path.
- Why not “Nothing”? Copper has resistance, so a big current through a shared return always lifts the ground a little.
- Why not “The sensor's voltage”? The lift has nothing to do with the sensor's own voltage. It's the motor's current flowing through copper the two share.
- Why not “The battery voltage”? Only a short would put the whole battery voltage there. The lift is the motor's current times the shared resistance.
“The return path” is lesson 4 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 before it first.
Questions people ask
What is star grounding?
Wiring each circuit's ground return separately to one common point, usually at the power supply, instead of chaining them together. Each return then carries only its own circuit's current, so a heavy load can't shift a sensitive circuit's ground.
Why does a motor make my sensor readings jump?
If the sensor's ground shares copper with the motor's return, the motor's current through that copper's resistance lifts the sensor's ground: lift = I_motor × R_shared. The sensor measures from the lifted point, so its reading shifts whenever the motor runs.
What is a ground loop?
Two or more ground paths between the same pieces of equipment, forming a loop that current can flow around. That current, often from mains-frequency fields, drops voltage along the ground wires and shows up as hum in audio gear. Star grounding avoids the loop.
Is a ground plane better than star grounding?
For most modern boards with fast signals, yes: a plane gives every return a short, low-inductance path. Star grounding still suits low-frequency, high-current systems like power supplies and amplifiers, and the ideas combine: keep heavy currents' return paths away from sensitive circuits.
Related topics
- What is a ground plane on a PCB?A whole layer of copper for ground: milliohms for steady current, about a nanohenry for fast current.
- Kelvin (4-wire) sensing explainedMeasure a shunt's voltage right at its own pads, so the copper either side can't add to the reading.
- 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.
- Ohm's law explainedV = I × R: set any two of voltage, current and resistance and the third follows.