Free LED Series Resistor Calculator
Determine exact current-limiting resistance, nearest standard E-series resistor, and power dissipation rating to prevent LED burnout.
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How to Calculate Current Limiting Resistors for LEDs
Light Emitting Diodes are non-linear semiconductor devices. Unlike incandescent bulbs, an LED's current spikes exponentially once its forward threshold voltage ($V_f$) is exceeded. Without a series resistor or constant current driver, the diode enters thermal runaway and burns out instantaneously.
The Ohm's Law Formula
R = (V_supply - (N × V_forward)) / I_forward
Where $N$ is the number of series LEDs and $I_{forward}$ is in Amperes ($mA / 1000$).
Resistor Wattage & Thermal Margin
Resistors convert excess voltage directly into heat: $P = V_{drop} imes I$. In engineering practice, always apply a 2x safety derating factor. If a resistor dissipates 0.18W, choose at least a 1/2W (0.5W) resistor to keep surface temperatures safe.
Frequently Asked Questions
What happens if I wire multiple LEDs in parallel with one single resistor?
Wiring LEDs in parallel with a shared single resistor causes 'current hogging'. Due to microscopic manufacturing tolerances, one LED will have a slightly lower forward voltage, drawing more current, heating up, dropping its resistance further, and burning out, followed by a cascade failure of the remaining LEDs. Always provide each parallel branch with its own dedicated resistor.
Why can't my supply voltage be lower than the LED forward voltage?
If the DC supply voltage is less than the LED's forward threshold voltage (e.g. attempting to power a 3.4V white LED with a 3.0V coin cell without a boost circuit), current cannot overcome the PN junction barrier and the LED will not illuminate.
What are E12 and E24 standard resistor values?
Electronic resistors are manufactured in logarithmic standard preferred series (E12 has 12 values per decade, E24 has 24). Our calculator rounds up to the closest commercially available standard resistor so you can immediately pick parts from your bin.