Knowing how much current flows through a circuit is fundamental to power monitoring, overcurrent protection, and closed-loop motor control. The most common method is deceptively simple: place a small resistor in the current path and measure the voltage across it. This is shunt-based current sensing.

Choosing the shunt resistor. You want a resistor small enough that it doesn't waste significant power or disturb the circuit, but large enough to produce a measurable voltage. A practical rule of thumb: keep the shunt voltage drop under 1–2% of your supply voltage. For a 12 V system measuring up to 5 A, a 10 mΩ resistor drops only 50 mV at full scale and dissipates just 250 mW — manageable without a heatsink.

High-side vs. low-side sensing. The shunt can sit between the supply and the load (high-side) or between the load and ground (low-side):

• Low-side: Easy — the shunt voltage is ground-referenced, so a standard op-amp can amplify it. The downside: the load's ground reference shifts by the shunt voltage, and you lose the ability to detect faults that short the load directly to ground.
• High-side: Preserves the load's ground connection and catches ground faults, but the shunt sits at a high common-mode voltage. You need a differential amplifier or a dedicated current-sense amplifier (like the INA219 or INA181) designed to reject that common-mode.

Real-world example. Suppose you're building a battery-powered robot and want to monitor motor current on a 12 V bus. Place a 10 mΩ shunt on the high side. Use an INA181 current-sense amplifier with a gain of 100 V/V. At 3 A, the shunt drops 30 mV, and the amplifier outputs 3.0 V — a clean signal you can feed straight into a microcontroller's ADC.

Practical concerns to watch for:

• Kelvin connection: Use four-terminal (Kelvin) sensing on the shunt. Run separate trace pairs for current flow and voltage measurement. If your sense traces carry load current, PCB trace resistance corrupts the reading.
• Resistor tolerance and tempco: Standard thick-film resistors drift with temperature. For precision work, use dedicated metal-element shunt resistors (±1% or better, <50 ppm/°C).
• Layout: Keep the amplifier close to the shunt. Long sense traces pick up noise, especially near switching regulators or motor drivers.

Quick calculation: For a 20 A max current with a 100 mV full-scale drop: R_shunt = 0.1 V ÷ 20 A = 5 mΩ. Power dissipated: P = I²R = 400 × 0.005 = 2 W — you'll want a resistor rated for at least 3 W with adequate PCB copper for heat sinking.