LED抵抗計算機

LED回路の直列抵抗値と定格電力を計算します。電源電圧、LED順方向電圧、電流、LED個数を入力すると、抵抗値、消費電力、最も近い標準E12抵抗値を即座に表示します。

Supply Voltage
V
LED Forward Voltage
V
LED Current
mA
LEDs in Series

How to Calculate LED Resistors

Ohm's Law

Ohm's Law: R = (V_supply − V_led) ÷ I_led

Formula

Formula: R = (V_supply − (V_led × N)) ÷ I_led

LEDs require a current-limiting resistor to prevent damage. The resistor drops the excess voltage from the power supply while limiting current to the desired value. Always use a resistor with a higher power rating than calculated.

Common LED Forward Voltages

Resistor Power Rating

Use a resistor with at least 2× the calculated power dissipation for safety. Common ratings: ¼W (0.25W), ½W (0.5W), 1W, 2W.

LED抵抗計算機について

This free online calculator helps you find the ideal current-limiting resistor for any LED circuit. Whether you are building an Arduino project, repairing electronics, or designing custom lighting, choosing the correct resistor is essential to protect your LEDs from excessive current and ensure long lifespan. Simply enter your supply voltage, LED forward voltage, desired current, and number of LEDs in series to get the resistor value, nearest standard resistor, power dissipation, and color code. All calculations happen entirely in your browser — no data is sent to any server.

Features

How to Use the LED Resistor Calculator

  1. Enter your power supply voltage (e.g., 5V for USB, 12V for automotive).
  2. Enter the LED forward voltage (Vf) or use a preset button for common colors (Red 2.0V, Green 2.2V, Blue 3.2V, etc.).
  3. Enter the desired LED current (typically 10–20 mA) or use a preset.
  4. Specify how many LEDs are connected in series (default is 1).
  5. Click Calculate to get the resistor value, standard resistor, and color code.
  6. Use the Copy button to save the results to your clipboard for your project documentation.

Common Use Cases

Frequently Asked Questions

LED順方向電圧とは何ですか？

順方向電圧（Vf）は、LEDが導通して光を発するために必要な最小電圧です。代表的な値：赤（1.8〜2.2V）、緑（2.0〜2.4V）、青/白（3.0〜3.3V）、黄（2.0〜2.2V）。

どのくらいの電流を使用すべきですか？

標準的なLEDは、通常10〜20mAでフル輝度で動作します。電流を上げると明るくなりますが、寿命が短くなり、熱も増えます。最大定格はデータシートで確認してください。

E12抵抗シリーズとは何ですか？

E12は1桁あたり12値（1.0、1.2、1.5、1.8、2.2、2.7、3.3、3.9、4.7、5.6、6.8、8.2）の標準抵抗シリーズです。一般的に入手可能で、ほとんどのLED用途に十分です。

複数のLEDを直列に接続できますか？

はい、LEDを直列に接続できます。すべてのLEDの電圧降下の合計が電源電圧より小さい必要があります。残りの電圧は抵抗器で降下します。

抵抗値を間違えるとどうなりますか？

抵抗が小さすぎる → 電流が過大 → LEDがすぐに焼損。抵抗が大きすぎる → 電流が不足 → LEDが暗いか点灯しない。

Can I connect multiple LEDs in series?

Yes, you can connect multiple LEDs in series. The total voltage drop across all LEDs must be less than the supply voltage. The resistor drops the remaining voltage. The calculator validates this automatically and warns you if the supply voltage is insufficient.

What happens if I use the wrong resistor value?

A resistor that is too small allows excessive current, causing the LED to be very bright but shortening its lifespan or burning it out. A resistor that is too large limits current too much, making the LED dim or not light up at all. Always use the calculated value or the nearest standard resistor.

What is the resistor color code?

The resistor color code uses colored bands to indicate the resistance value and tolerance. For example, brown-black-brown = 100Ω ±1%. The calculator shows the color code for the nearest standard resistor, making it easy to find the right component in your parts bin.

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