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Circuit lab 01 · Beginner

LED and series resistor

The first circuit most people build. Change the supply, the resistor and the LED, and watch every voltage, current and power in the loop update together. Every number on this page comes from the same three equations shown under the meters.

The bench

Try a setup:
Bench · ideal DC source + resistor + LEDSimplified model
Series LED circuit A DC supply on the left, a resistor on the top wire and an LED on the right side, connected in one loop.
Current I—
Across resistor—
Across LED—
Resistor power—
LED power—
V
Ω

The slider is logarithmic: 1 Ω to 100 kΩ. Type any value up to 1 MΩ.

V

Typical values only. Real forward voltage depends on the part and the current; check your LED’s datasheet.

Limits used for warnings
mA

20 mA is a common continuous rating for small 5 mm indicator LEDs, but ratings vary. Use the value from your datasheet.

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    How the numbers were worked out

    Design it: choose the resistor

    Usually you know the supply and the LED and need the resistor. Pick a target current and the lab works out the exact value and the next standard values above it. Rounding up keeps the current at or below your target.

    Exact R—
    Next E12—
    Next E24—

    The model behind this lab

    Simplified model, not a SPICE simulation

    The supply is an ideal DC source: its voltage never sags. The resistor is exact: no tolerance, no change with temperature. The LED is treated as an ideal switch with a fixed forward voltage Vf.

    I = (Vs − Vf) ÷ R when Vs > Vf, otherwise I = 0

    When the LED conducts, VLED = Vf, VR = I × R, and the resistor dissipates PR = VR × I = I² × R. When it does not conduct, no current flows, VR = 0, and the whole supply appears across the LED.

    Real LEDs start conducting gradually, and Vf rises a little with current, so a real circuit will be close to these numbers but not identical. Near Vs ≈ Vf the real current is very sensitive to the exact part.

    Safety

    Build this with a low-voltage supply such as a USB power bank, a 9 V battery or a bench supply set to 12 V or less, and always include the resistor. Never connect anything to mains. A shorted battery can get hot enough to burn; lithium cells can catch fire.

    Practical challenge

    Use the bench above to find or check each answer.