Led Series Resistor
Calculator
Results
- Series resistor (Ω)
- 145
- Resistor power (mW)
- 57.999999
- LED power (mW)
- 42
Electrical results
| Series resistor (Ω) | 145 |
| Resistor power (mW) | 57.999999 |
| LED power (mW) | 42 |
formula-map diagram
- Series resistor (Ω)
- 145
- Resistor power (mW)
- 57.999999
- LED power (mW)
- 42
Electrical relationship
Formula
R = (Vsupply − Vforward) / Iled= 145
Note
This is a simplified model: it applies the textbook relationship to the numbers you entered and assumes ideal components, steady-state sinusoidal conditions, balanced loads and copper resistivity of 0.0172 Ω·mm²/m at 20 °C. It ignores component tolerances, temperature drift, skin effect, harmonics, inrush, transformer and battery losses, and it is not a substitute for the wiring code that applies where you are. Have any installation sized and verified by a licensed electrician or engineer.
More in Electrical engineering
See all →Frequently asked questions
Why does an LED need a series resistor at all?+
An LED's voltage stays nearly constant once it's conducting, so without a resistor to limit current, even a tiny increase in supply voltage causes a runaway current spike that destroys the LED almost instantly.
How is the resistor value calculated?+
R = (Vsupply - Vf) / If, where Vf is the LED's forward voltage drop and If is your target current. The resistor absorbs whatever voltage is left over after the LED's own drop.
What happens if I use a resistor that's too small?+
Too little resistance lets too much current flow, which overheats the LED's semiconductor junction and shortens its life dramatically, or burns it out immediately. Always round up to a slightly larger standard resistor value rather than down.
Where do I find the forward voltage and current values for my LED?+
Check the LED's datasheet: typical forward voltages range from about 1.8V for red to 3.2V for blue or white, and typical currents run 10-20mA for standard indicator LEDs. Using a rough estimate is fine for simple projects.
Does the series resistor value change if I power the LED from a different voltage?+
Yes, the resistor value depends directly on the supply voltage in the formula, so switching from a 5V source to a 9V source requires recalculating (and typically increasing) the resistor to keep the same target current.