Sense Resistor
A current-sense resistor is a precise, low-value resistor (typically 1 mΩ – 1 Ω) whose voltage drop is measured by a current-sense amplifier or ADC to report the current flowing through it.
Current flowing through the resistor produces a small voltage (V = I × R) that an op-amp differentially measures and amplifies. To keep self-heating from changing the value, the element is usually a bulk metal foil, manganin, or copper-nickel alloy with a temperature coefficient near zero (often <50 ppm/°C). Four-terminal Kelvin sensing eliminates the trace and solder-joint resistance from the measurement.
In plain terms
A precision speed-bump in the path of current. The voltage across the bump tells the rest of the circuit how fast cars are going.
Why designers use it
- Measure current in battery management ICs and motor drives.
- Set a regulated current level for an LED constant-current driver.
- Form the feedback element in an op-amp current sensor.
Best for
- Battery gauges
- Motor controllers
- LED drivers
Key specifications
- Resistance: 0.5 mΩ – 1 Ω
- Tolerance: ±0.1 % – ±2 %
- Power: 0.25 W – 10 W
- Tempco: ±15 – ±100 ppm/°C
- Construction: Manganin foil, metal-strip, wirewound
When not to use it
- When the load doesn't tolerate even the few millivolts the sense resistor steals — a Hall-effect sensor is lossless.
- At very high currents where the I²R loss in the sense element exceeds the system's heat budget.
- When the bandwidth required exceeds what a sense amplifier can resolve from the small voltage drop.
Common mistakes
- Routing two-terminal sense resistors instead of four-terminal Kelvin pads — solder joint resistance dominates and the reading drifts with temperature.
- Picking a 25 mΩ part and forgetting the I²R rises to several watts at full load — the case temperature soars and the value drifts up.
- Putting the sense resistor on the high side of a 48 V rail and feeding it to a low-side amp without a proper differential current-sense amp.
Where you will find it
- An electric bicycle's battery management system uses a 5 mΩ shunt resistor in the main current path: the BMS IC reads the millivolt drop across it 1000 times per second to track amp-hours into and out of the pack, calculating the state of charge that the display shows the rider.
- A USB-C power delivery charger uses a 10 mΩ current-sense resistor on its output rail: the controller monitors the voltage drop to detect when the connected device draws above the negotiated current limit and shuts down within microseconds to protect both the cable and the device.
- A 3D printer's heated-bed controller uses a 0.1 Ω sense resistor in the FET's source path: the voltage tells the firmware the exact heater current, which it cross-checks against the thermistor reading — a mismatch triggers an emergency shutdown to prevent a fire.
A short history
Current-sense resistors — precision resistors with very low resistance values (typically 1 mΩ to 1 Ω) — provide a safe, non-intrusive method of measuring current by converting it to a small voltage via Ohm's law. Isabellenhütte in Germany pioneered the use of Manganin and similar low-temperature-coefficient alloys for precision shunt resistors in industrial measurement applications from the early twentieth century, with the modern surface-mount chip shunt format emerging in the 1960s–1970s. Vishay, founded in 1962, became a leading producer of bulk-metal-foil current-sense resistors offering temperature coefficients below 5 ppm/°C and tolerances of ±0.1 %. Current-sense resistors are now standard in battery-management systems, motor drives, and power converters, where accurate current feedback is essential for efficiency and protection.
Good to know
- A 1 mΩ shunt at 100 A drops 100 mV — the kelvin-sense leads measure that drop without the heavy load currents corrupting the reading.
- Manganin (87 Cu / 13 Mn / 4 Ni) was developed in 1889 specifically for low-tempco shunts; it's still the gold standard 135 years later.
- Modern metal-strip shunts replace the wirewound cylinder with a flat L-shape that's easier to mount and has better thermal coupling to the PCB.