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Power Electronics8–10 min read

Shunt Current Sensing in Power Circuits

Measure load and switch current using a shunt while accounting for power loss, common-mode voltage and switching transients.

Mental model

A shunt is a very small known restriction. The tiny voltage it develops reveals the current, but the measurement circuit must separate that signal from a much larger moving voltage.

Rshuntload currentsense amplifierADCKelvin traces start at the shunt terminals
Kelvin traces carry the small shunt voltage to an amplifier and ADC without including power-trace voltage drops.
UNDERSTAND THE BEHAVIOR
01

A shunt resistor produces Vsense = I × R. A larger resistance creates an easier signal to measure but wastes more power and changes the circuit more. Choose resistance, power rating, tolerance and temperature coefficient from the maximum continuous and transient current.

02

Low-side sensing is ground-referenced and simple, but the shunt lifts the load return above system ground. High-side sensing preserves a ground-connected load and can detect some shorts, but the amplifier must tolerate a large common-mode voltage while measuring millivolts of differential signal.

03

In an H-bridge or switching converter, common-mode voltage and current can change rapidly. Amplifier bandwidth, common-mode transient rejection, input filtering, Kelvin connections and sample timing determine whether the reading represents real current or switching disturbance.

04

Protection and measurement are different timing problems. A slowly filtered ADC reading may be excellent for monitoring yet too slow to protect a MOSFET from a short circuit. Fast hardware current limiting may need a comparator or the driver's dedicated protection path.

Why it matters

Measure load and switch current using a shunt while accounting for power loss, common-mode voltage and switching transients.

Core ideas

  • Shunt voltage is proportional to current.
  • Signal size trades against power loss.
  • Common-mode transients and layout can dominate a millivolt measurement.
Key equationsVsense = I × RshuntPshunt = I²RshuntVADC = gain × Vsense
EXAMPLE

Work through a practical example

1Choose a 10 mΩ shunt for a nominal 8 A current.

2Vsense = 8 × 0.010 = 80 mV.

3Shunt dissipation is 8² × 0.010 = 0.64 W, before overload margin.

4With amplifier gain 25, the ADC sees 2.0 V. Check peak current, amplifier output range, common-mode limits and shunt pulse rating.

Common mistake

Routing the sense traces through the high-current copper path instead of using Kelvin connections directly at the shunt terminals.

RememberShunt voltage is proportional to current.
TEST YOUR UNDERSTANDING

Three quick checks

Choose one answer per question, then check your work. Each answer includes an explanation.

1. What voltage appears across a 10 mΩ shunt at 6 A?
2. What is the shunt dissipation at 6 A and 10 mΩ?
3. Why use Kelvin sense connections?

Practice and check your understanding

Try each question before opening the worked answer.

1. What voltage appears across 20 mΩ at 3 A?

Show answer and reasoning

V = IR = 3 × 0.020 = 0.060 V, or 60 mV.

2. Why can a heavily filtered current measurement be unsuitable for short-circuit protection?

Show answer and reasoning

The filter delays the response. Monitoring and fast device protection can require separate signal paths and time scales.

Continue learning

  • H-Bridge Motor Drives — Reverse and regulate current through an inductive load using two coordinated half-bridges.
  • Synchronous Buck Converters — Replace the freewheel diode with a controlled MOSFET to reduce loss while managing dead time and reverse current.
  • Differential Amplifiers — Understand how a circuit amplifies the difference between two signals while rejecting what they share.

Technical references and further reading

References are for further study, not endorsements. Report an error or suggest an improvement.

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