Series Resistor for 3 LEDs on a 12V Supply
Find the series resistor for one or more LEDs from the supply, the forward drop, the count, and the current.
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About this tool
This page opens the LED resistor calculator with a 12V supply, a 3V LED forward voltage, an LED count of 3 and a 20mA target current already filled in — the resistor value below is computed live on load.
12V is the standard rail for LED strips and many project power supplies, and the usual way to use it efficiently is a segment of several LEDs in series with one resistor rather than one resistor per LED. Three LEDs at roughly 3V each (typical for the LEDs used in 12V strip segments) use 9V of the 12V supply, leaving 3V across the resistor at 20mA — 150Ω, dissipating 0.06W.
This is exactly the series-segment math printed (or not printed) on the back of commercial LED strips: change the LED count and forward voltage above to match your actual LEDs, and remember the resistor voltage must stay comfortably positive — cramming too many LEDs in one series segment on a given supply is the most common reason this kind of calculation fails.
Example. The defaults, 5 V, a 2 V drop, one LED, and 20 mA, return 150 Ω and 60 mW. The nearest E24 values are 150 Ω, 160 Ω, and 130 Ω. 9 V with the same LED and current returns 350 Ω and 140 mW.
Limit. The LEDs are in series. The calculation stops when the combined forward drop is not strictly below the supply. The current unit menu opens on milliamps.