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Ohm's Law: R = (V_supply − V_led) ÷ I_led
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.
Use a resistor with at least 2× the calculated power dissipation for safety. Common ratings: ¼W (0.25W), ½W (0.5W), 1W, 2W.
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.
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E12 is a standard resistor series with 12 values per decade (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2). These are commonly available and sufficient for most LED applications.
Yes, you can connect LEDs in series. The total voltage drop across all LEDs must be less than your supply voltage. The remaining voltage is dropped across the resistor.
Too low resistance → excessive current → LED burns out quickly. Too high resistance → insufficient current → LED is dim or does not light.
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.
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.
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.