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TOOL IDB-ELE-008
LED series resistor

Desktop only

Engineering tools are available only on desktop screens.

LED series resistor

Current-limiting resistor for an LED string. Ideal value, nearest E12 standard, and wattage to spec on the BOM.

Circuit drop — V
Vs 5 V R = — LED Vf = 2.1 V If = 20 mA
LED I-V curve & load line operating point
— mA 0 0 — V V across LED → I → Vf load line · slope = −1/R Q (Vf, If)
Engineering notes

Formulas

  • R = (Vs − n · Vf) / If
  • PR = (Vs − n · Vf) · If

Sizing the resistor

  • Round up to the nearest E12 value. The current drops slightly.
  • Spec a wattage at least 2× the dissipated power.
  • If Vs is below n · Vf, the resistor cannot drop enough voltage.

Result

Live