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Resistor Supplies > Resource > Milliohm Resistor > How to Choose a Milliohm Resistor for High-Accuracy Current Sensing

How to Choose a Milliohm Resistor for High-Accuracy Current Sensing

How to Choose a Milliohm Resistor for High-Accuracy Current Sensing

In precision current sensing applications - from battery management systems (BMS) to motor control - milliohm shunt resistors play a critical role. Selecting the wrong resistor can lead to measurement errors exceeding 10%, compromising system safety and efficiency. This guide breaks down key selection criteria with test data comparisons to help engineers optimize high-accuracy current measurement circuits.

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    The Accuracy Challenge: Why Standard Resistors Fail

    A common problem in high-current sensing applications is thermal drift. When a 5mΩ resistor carries 20A:

    Resistor TypePower DissipationTemp RiseResistance Change
    Thick Film (200ppm/°C)2W85°C+1.7mΩ (+34%)
    Metal Strip (50ppm/°C)2W65°C+0.25mΩ (+5%)

    This drift severely impacts current measurement accuracy in applications like EV battery monitoring where ±1% error matters.

    5 Critical Parameters for Current Sensing Resistors

    For stable low-resistance current measurement, prioritize these specs:

    ParameterTarget ValueWhy It Matters
    TCR (Temperature Coefficient)<50 ppm/°CMinimizes thermal drift
    Power Rating2× Calculated PdissPrevents overheating
    Resistance Tolerance±1% or betterInitial accuracy
    TerminationCopper alloyReduces contact resistance
    Inductance<5nHFast transient response

    Resistor Material Showdown: Metal Strip vs. Thick Film

    Comparative data for 10mΩ resistors at 15A continuous current:

    PropertyThick FilmMetal Strip
    Cost (USD/1k)$0.20$0.80
    TCR (ppm/°C)20035
    ΔR after 100hrs @15A+3.2%+0.5%
    Recommended ForNon-critical sensingBMS/Motor control

    For precision current measurement in harsh environments, metal strip resistors deliver 5-8× better stability despite higher cost.

    3 Proven Solutions to Reduce Measurement Error

    Practical fixes for common current sensing accuracy issues:

    ProblemSolutionImprovement
    Thermal driftUse metal foil resistors + thermal padsΔR < 1% @50°C
    Contact resistance4-wire Kelvin connectionEliminates 0.5-2mΩ error
    Noise pickupGuard rings + differential measurementSNR +12dB

    PCB Implementation Tips for Milliohm Resistors

    Layout considerations for high-accuracy current sensing circuits:

    • Copper area: Minimum 10mm² copper pour per amp for heat dissipation

    • Trace matching: Keep sense traces equal length (±5mm) to cancel thermal EMF

    • Placement: Position resistors away from heat sources (MOSFETs, inductors)

    • Calibration: Always include test points for in-circuit resistance verification

    Proper implementation can improve current measurement precision by 3-5× compared to basic layouts.

    Key Takeaways

    1. Prioritize low TCR (<50ppm/°C) over initial tolerance for dynamic systems

    2. Metal strip resistors outperform thick film in high-current sensing applications

    3. Always use 4-wire Kelvin connections for resistances below 10mΩ

    4. Budget 2-3× resistor cost for proper thermal management

    By following these guidelines, engineers can achieve ±0.5% current measurement accuracy even in demanding power electronics applications.


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