LED Resistor Value Calculator

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ガイド

LED Resistor Value Calculator

Pick the right current-limiting resistor for any LED circuit. Enter your supply voltage, the LED’s forward voltage and forward current, and this calculator returns the exact resistance, the nearest E12 and E24 standard values, the power dissipation, the recommended wattage rating, and a 4-band color-code drawing of the resistor you need to buy.

使用方法

そこで登場するのが supply voltage (e.g. 9 V battery, 5 V USB, 12 V car).
[表示モード]を選択してください LED preset (red, green, blue, white, etc.) or type a custom forward voltage.
[名前]を設定してください forward current in mA — most indicator LEDs run at 20 mA.
そこで登場するのが number of LEDs を有効にし、 series または parallel.
Read the calculated resistance, the nearest standard value, and the color bands.

機能

Ohm’s Law math – computes R = (Vsupply − Vforward) / Iforward automatically.
E12 and E24 lookup – snaps the calculated resistance to the nearest off-the-shelf value.
Color-band diagram – renders the resistor with the four color bands you need to look for.
Power dissipation – calculates P = I² × R and recommends a wattage rating with a 2× safety margin.
Series and parallel support – handles single LEDs, multiple LEDs in series, and one resistor per LED in parallel.
LED color presets – red 1.8 V, orange 2.0 V, yellow 2.1 V, green 2.2 V, blue 3.4 V, white 3.2 V, UV 3.6 V, IR 1.2 V.
Safety warnings – flags low headroom, excessive current, and overheating resistors.

 よくある質問

Why does an LED need a current-limiting resistor?

An LED's current rises sharply once the supply exceeds its forward voltage, and without a resistor the current is limited only by the LED's tiny internal resistance. Even a small over-voltage will push current past the rated maximum and burn out the junction. A series resistor sets a fixed current by absorbing the difference between the supply voltage and the LED's forward voltage.
What are E12 and E24 resistor series?

E12 and E24 are standardized geometric sequences of preferred resistor values that manufacturers actually stock. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) with ±10% tolerance, and E24 doubles that to 24 values per decade with ±5% tolerance. You round your calculated value to the nearest one in the series so you can buy the part.
Why does power dissipation matter for the resistor?

The resistor turns the voltage drop across itself into heat at a rate of P = I² × R. A 470 Ω resistor at 20 mA dissipates only 0.19 W and is fine on a 1/4 W part, but the same resistor at 100 mA dissipates 4.7 W and will scorch a 1/4 W carbon film. Sizing the resistor's wattage rating with a 2× safety margin keeps it cool and reliable.
Why are parallel LEDs sharing one resistor a bad idea?

LEDs of the same nominal color have small variations in forward voltage from the manufacturing process. Wired in parallel through one resistor, the LED with the lowest Vf hogs most of the current while the others stay dim. As the brightest LED heats up, its Vf drops further, the imbalance gets worse, and that LED can fail prematurely. Each parallel LED branch needs its own series resistor.