LED Series Resistor

An LED needs a current limit: R = (Vsupply − Vf) / If. Check the resistor's power too — it burns the leftover voltage as heat.

Resistor
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Resistor power
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Next E12 value
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If the supply is below the LED forward voltage the LED will not light — you need a boost driver, not a resistor.

A helper, not a substitute for engineering judgment. Verify against the governing standard.