TheAlgorithms/Python · error · ValueError

Resistance must be positive.

Error message

Resistance must be positive.

What it means

Thrown by charging_inductor() when resistance <= 0. Resistance sets the RL time constant L/R and the steady-state current Vs/R; zero or negative R makes the current infinite/undefined, so the library guards against it before computing.

Source

Thrown at electronics/charging_inductor.py:86

    Traceback (most recent call last):
        ...
    ValueError: Source voltage must be positive.

    >>> charging_inductor(source_voltage=10,resistance=0,inductance=20,time=5)
    Traceback (most recent call last):
        ...
    ValueError: Resistance must be positive.

    >>> charging_inductor(source_voltage=15, resistance=25, inductance=0, time=5)
    Traceback (most recent call last):
        ...
    ValueError: Inductance must be positive.
    """

    if source_voltage <= 0:
        raise ValueError("Source voltage must be positive.")
    if resistance <= 0:
        raise ValueError("Resistance must be positive.")
    if inductance <= 0:
        raise ValueError("Inductance must be positive.")
    return round(
        source_voltage / resistance * (1 - exp((-time * resistance) / inductance)), 3
    )


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Use the real series/winding resistance (even milliohms) instead of 0.
2. If you truly need R=0, this step-response model does not apply — the current ramps linearly as V*t/L; compute that directly.
3. Check unit conversions and keyword spelling.

Example fix

# before
charging_inductor(source_voltage=10, resistance=0, inductance=20, time=5)

# after
charging_inductor(source_voltage=10, resistance=50, inductance=20, time=5)

if resistance <= 0:
    raise ValueError("resistance must be positive; use winding resistance")
i = charging_inductor(source_voltage, resistance, inductance, time)

When it happens

Trigger: charging_inductor(source_voltage=10, resistance=0, inductance=20, time=5) or a negative resistance value. Runs after the source-voltage check.

Common situations: Ideal (lossless) inductor assumptions of R=0 — physically real inductors always have winding resistance; unit mix-ups (kOhm vs Ohm); negative-resistance models (tunnel diodes) which this API does not support.

Related errors

• Source voltage must be positive.
• Inductance must be positive.
• Capacitor at index {index} has a negative or zero value!
• You cannot supply more or less than 2 values
• Electron concentration cannot be negative in a semiconductor

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/902d9f7db12270b4. Report an issue: GitHub.