PCB Design
Kelvin (4-wire) sensing explained
Kelvin sensing, or four-wire sensing, measures a voltage with separate wires that connect right at a part, apart from the wires carrying its current. It's how you measure current through a shunt resistor accurately: sense at the shunt's own pads, and the copper either side, which carries the same current, can't add its drop to the reading.
Measuring current with a shunt
To know how much current a motor or a battery draws, put a shunt in its path: a small, exact resistor. Its voltage is the current, by Ohm's law:
200 mA through 0.1 Ω reads as 20 mV. An ADC or a current-sense amplifier reads the millivolts, and firmware turns them back into amps.
Where the meter touches
The traces leading to the shunt have resistance too, say 0.05 Ω each, and the same 200 mA flows through them. Measure across the shunt's own pads and you read 20 mV. Measure outside the traces and you read the shunt plus both traces:
Same current, double the reading. Widening the traces shrinks the error but never removes it, and it drifts with temperature and with how much solder is on each joint.
Live experiment
Move the sense wires
5 V pushes about 200 mA through a 0.1 Ω shunt between two traces. Move the sense wires: at the shunt's own pads they read 20 mV; outside the thin traces, about 40 mV, double. Widen the traces and that error shrinks, but only the pads remove it.
“Measure at the pads” is lesson 2 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
Four wires
The fix is two thin sense traces that leave straight from the shunt's pads to the amplifier. They carry almost no current, so nothing along them drops a volt. Two connections carry the current; two others measure it. That's four connections to one part: four-wire, or Kelvin, sensing.
On a board, take the sense traces from the inside edges of the shunt's pads and run them side by side to the amplifier, so noise reaches both equally. Many precision shunts have four terminals for exactly this.
Choosing the shunt
A bigger shunt gives a bigger, easier signal but steals more volts from the load and wastes more power, I² × R. A smaller one disturbs the circuit less but gives a tinier signal. Typical choices:
- 0.1 Ω for a few hundred milliamps: 20 mV at 200 mA, 4 mW.
- 10 mΩ for around 10 A: 0.1 V and 1 W, so check its power rating.
Phone battery gauges use shunts of a few milliohms, and multimeters measuring very low resistances use four probes for the same reason.
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.
“Measure at the pads” is lesson 2 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 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: Reads the shunt alone, not the traces. Kelvin sensing: measure right at the part.
- Why not “Reads the traces too”? That's what measuring outside the traces does. At the pads, only the shunt's own drop is included.
- Why not “Doubles the reading”? Measuring at the pads removes the traces' drops. It can't double anything.
- Why not “Reads nothing”? The shunt's voltage is right there at its pads. The meter reads it perfectly well.
Show the answer
Answer: So they steal almost no voltage from the circuit they measure. Everything the shunt drops is lost to the load. A small shunt costs little, and an amplifier makes its tiny voltage readable.
- Why not “Smaller is more accurate in every way”? Smaller isn't better in every way: the signal gets tiny and harder to measure accurately.
- Why not “To make the reading bigger”? A smaller shunt gives a smaller reading. That's the price paid for not stealing voltage.
- Why not “They're cheaper”? Small, precise shunts aren't especially cheap. They're small so they disturb the circuit as little as possible.
Show the answer
Answer: 40 mV: the shunt plus two traces' drops. The same 200 mA drops 10 mV on each trace. Outside them the meter sees 20 + 10 + 10 = 40 mV, and the board thinks it's drawing 400 mA.
- Why not “0”? Current is flowing through all three, so each drops a voltage the meter sees.
- Why not “20 mV”? 20 mV is the shunt alone. Wired outside the traces, the meter also reads both traces' drops.
- Why not “10 mV”? 10 mV is a single trace's drop, 200 mA × 0.05 Ω. The meter reads the shunt and both traces.
“Measure at the pads” is lesson 2 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
Questions people ask
What is Kelvin sensing?
Measuring a voltage with separate wires that connect right at the part, apart from the wires carrying its current. The sense wires carry almost no current, so nothing along them drops a volt, and the reading is the part's voltage alone. It's also called four-wire sensing.
How do you measure current with a shunt resistor?
Put a small, accurate resistor in the current's path and measure the voltage across it: I = V ÷ R. 200 mA through 0.1 Ω gives 20 mV. An amplifier or ADC reads the millivolts, and firmware turns them back into amps.
How do I choose a shunt resistor value?
Big enough to give a readable voltage at your smallest current, small enough not to steal volts or waste power at your largest. At 10 A, a 10 mΩ shunt drops 0.1 V and dissipates 1 W. Check its power rating and temperature coefficient.
Why do low-resistance meters use four probes?
With two probes, the reading includes the probe leads and contact resistance, which can be bigger than a milliohm-sized part. Four probes, two to carry a test current and two to sense the voltage, measure the part alone. Lord Kelvin's 1861 bridge used the same idea.
Related topics
- Star grounding explainedGive each load its own return to one point, so a big current never flows through a sensitive circuit's ground.
- How to use a multimeterVolts across, amps in the loop: where the probes go, and the one mistake that blows the fuse.
- Ohm's law explainedV = I × R: set any two of voltage, current and resistance and the third follows.
- 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.