A sensing resistor (often called a shunt) converts load current into a measurable voltage drop via Ohm's Law (V = I × R). For a 5A DC load, a 10mΩ (0.010Ω) sensing resistor drops exactly 50mV. By feeding this differential voltage into a dedicated current-sense amplifier with a gain of 50V/V, you get a clean 2.5V analog signal that perfectly interfaces with a 3.3V microcontroller ADC. This direct, ground-referenced low-side topology is the most reliable way to monitor power consumption, implement overcurrent protection, or perform Coulomb counting in battery systems.
The following design guide assumes a standard 2-layer FR4 copper PCB, 25°C ambient temperature, and 3.3V logic levels. While the math scales to any current, the 5A threshold is where thermal management and trace widths transition from trivial to critical.
The Low-Side Topology: Node Mapping and Behavior
In a low-side configuration, the sensing resistor is placed between the load's return path and the system ground. This topology is defined by three critical nodes:
- Node A (LOAD-): The return path from your load (motor, LED array, heater).
- Node B (SHUNT-HI): The junction between the load and the sensing resistor. This connects to the non-inverting input (IN+) of your differential amplifier.
- Node C (SHUNT-LO / GND): The junction between the sensing resistor and system ground. This connects to the inverting input (IN-) of the amplifier.
The amplifier measures the differential voltage (V_B - V_C) and rejects the common-mode voltage. Because Node C is tied directly to ground, the common-mode voltage is near 0V, allowing you to use standard, low-cost operational amplifiers or dedicated low-side current sense ICs without worrying about high-voltage common-mode rejection limits.
Circuit Behavior and Extreme Failure Modes
Understanding what happens when components fail or drift is just as important as nominal operation. The table below maps the circuit behavior across normal and extreme fault conditions for a 5A system using a 10mΩ shunt and a 50V/V amplifier.
| State / Fault Condition | Shunt Voltage (V_B - V_C) | Amplifier Output (V_OUT) | System Consequence & Hardware Risk |
|---|---|---|---|
| Nominal 5A Load | 50 mV | 2.50 V | Normal operation; ADC reads mid-scale. |
| Load Open Circuit | 0 mV | 0 V (or slight offset) | System detects 0A; safe state. |
| Shunt Resistor Opens | Floats to Load Supply Voltage | Rails to VCC (e.g., 3.3V) | Load loses ground (turns off). Amp inputs may exceed absolute max ratings if V_SUPPLY > 5V, risking silicon damage. |
| Shunt Resistor Shorts | 0 mV | 0 V | Load gets direct ground (runs unchecked). System loses current visibility and overcurrent protection. |
| Amp Input Overvoltage | > 100 mV (>10A) | Saturates at ~3.2V | ADC reads max value. Shunt dissipates >1W, risking thermal pad delamination if not properly heatsinked. |
Low-Side vs. High-Side: Why Ground-Referenced Wins for Prototyping
While high-side sensing (placing the shunt between the power supply and the load) is mandatory for detecting load-to-ground short circuits, low-side sensing remains the superior choice for general prototyping, battery monitoring, and cost-sensitive designs. The primary differentiator is the common-mode voltage.
| Design Criterion | Low-Side Sensing (Ground-Referenced) | High-Side Sensing (Supply-Referenced) |
|---|---|---|
| Common-Mode Voltage | Near 0V (Ground potential) | Equal to Supply Voltage (e.g., 12V, 24V, 48V) |
| Amplifier Requirements | Standard op-amps (LM358) or low-cost sense ICs | Requires high CMRR, high-voltage tolerant dedicated ICs (e.g., INA219, INA226) |
| Short Circuit Detection | Cannot detect if load shorts directly to ground | Detects all load faults, including ground shorts |
| Ground Bounce / Noise | Load current flows through system ground, causing noise for sensitive analog sensors | Load current bypasses system ground; keeps ground plane clean |
| Component Cost (2026) | ~$0.15 (Shunt + LM358) | ~$0.85 (Shunt + INA226 or equivalent) |
Choose low-side when you are monitoring battery discharge, driving isolated loads, or keeping BOM costs under $0.50. Choose high-side when you are driving grounded loads (like a car chassis or an industrial frame) or when your microcontroller shares a ground plane with highly sensitive RF or audio circuitry that cannot tolerate the millivolt-level ground bounce caused by a 5A return current.
Design Walkthrough: Sizing a 5A Shunt with the INA180
Let's build a concrete 5A low-side monitor using real component values. Our target is a 3.3V microcontroller ADC, and we want to maximize resolution without clipping at peak loads.
1. Selecting the Sensing Resistor
We need a voltage drop large enough to overcome amplifier offset voltage, but small enough to avoid wasting power and altering the load voltage. A 50mV drop at full scale is the industry sweet spot.
- Resistance: R = V / I = 0.050V / 5A = 10mΩ (0.010Ω).
- Power Dissipation: P = I² × R = 25 × 0.010 = 0.25W.
While a 0.25W dissipation seems low, surface mount resistors suffer from severe thermal derating on standard 1oz copper PCBs. If you use a standard 0.5W 1206 package, the solder joints will run hot and drift. According to Bourns thermal derating curves, you should select a resistor rated for at least 4x your calculated dissipation to maintain a low temperature coefficient of resistance (TCR). We will use the Bourns CSS2H-2512R-L010F, a 2512-package 10mΩ resistor rated for 2W with a tight 50ppm/°C TCR.
2. Selecting the Current Sense Amplifier
We need to amplify 50mV up to roughly 2.5V to leave headroom below the 3.3V ADC ceiling. Gain = V_OUT / V_IN = 2.5V / 0.050V = 50V/V.
The Texas Instruments INA180A2 is a dedicated low-side current sense amplifier with a fixed gain of 50V/V, a typical input offset voltage of just 100µV, and a bandwidth of 350kHz. It operates from a 2.7V to 5.5V supply, making it perfect for 3.3V logic systems.
| Component | Part Number | Key Specification | Package / Footprint |
|---|---|---|---|
| Sensing Resistor | Bourns CSS2H-2512R-L010F | 10mΩ, 2W, 50ppm/°C, 1% Tolerance | 2512 SMD |
| Sense Amplifier | TI INA180A2IDBVR | Gain = 50V/V, V_OS = 100µV, 350kHz BW | SOT-23-5 |
| Bypass Capacitor | Murata GRM155R71C104KA88D | 100nF, 16V, X7R Dielectric | 0402 SMD |
Breadboard Testing and Calibration Steps
Sensing resistors in the milliohm range are notoriously difficult to test accurately on a solderless breadboard due to the high and variable contact resistance of the breadboard's internal spring clips (often 50mΩ to 100mΩ per clip). To breadboard-test this topology, you must use a through-hole equivalent shunt (like the Vishay WSN01\2 10mΩ axial wirewound) or mount the SMD shunt on a SOIC-8 to DIP adapter board.
Follow these steps to verify your circuit before committing to a PCB layout:
- De-energize and Wire the Load Path: Connect your DC power supply positive terminal to the load. Connect the load return to one leg of the through-hole 10mΩ shunt resistor. Connect the other leg of the shunt to the power supply ground. Do not power on yet.
- Establish Kelvin Sense Lines: Take two separate, thin jumper wires. Solder or clip them directly to the physical legs of the shunt resistor, as close to the resistor body as possible. Connect these sense wires to the IN+ and IN- pins of your INA180 amplifier circuit on the breadboard.
- Power the Amplifier: Supply 3.3V to the VCC pin of the INA180 and connect its GND pin to the main system ground. Place the 100nF bypass capacitor directly across the VCC and GND pins of the IC.
- Apply a Known Test Load: Instead of a variable load, use a high-wattage power resistor to draw a known current. For example, a 2.4Ω, 25W chassis-mount resistor connected to a 12V supply will draw exactly 5A (I = 12V / 2.4Ω). Warning: The 2.4Ω resistor will dissipate 60W and become extremely hot within seconds. Mount it to a heatsink and limit the test duration to 3 seconds.
- Measure and Verify: Power the 12V load supply. Immediately measure the voltage at the INA180 OUT pin with your multimeter. You should read 2.50V ± 25mV. If you read significantly higher (e.g., 2.8V), your sense wires are picking up the voltage drop of the breadboard clips, proving why PCB Kelvin routing is mandatory for final designs.
- Check the Zero-Current Offset: Disconnect the 12V load supply. The amplifier output should drop to near 0V. A reading of 5mV to 10mV is normal and represents the amplifier's input offset voltage multiplied by the gain (100µV × 50 = 5mV). Subtract this offset in your microcontroller firmware for maximum accuracy.
