WOBBLled

Will a blue LED on 5 V with a 47 ohm resistor fry?

Circuit report
2026-10-13 10:56
FAILS

The LED fries: 36.9 mA through it, above its 30 mA rating (1.2x).

LED current
36.9 mA
rating
30 mA
resistor power
63.9 mW
LED power
120 mW

Confidence high

  • passed hand calculation, power balance

Checks

statuscheckresult
passhand calculationdiode equation solved by hand gives 36.9 mA, ngspice 36.9 mA (5.4e-05% apart); the quick estimate (Vs - Vf)/R is 38.3 mA
passpower balancesupply gives 184 mW = resistor 63.9 mW + LED 120 mW (5.4e-08% apart)
infooperating limitsLED 36.9 mA of 30 mA; resistor 63.9 mW of 250 mW

How to fix it

Use a 91 ohm resistor
Gives about 19.8 mA; it dissipates 35.6 mW, so a 0.25 W part is enough.

Assumptions

  1. blue LED modeled with the diode equation: 3.2 V at 20 mA, ideality 2.7, series resistance 1.5 ohm.
  2. Resistor 47 ohm exact, rated 0.25 W.
  3. Room temperature, steady DC.