LED Current or Resistance Calculator

How to use LEDs calculator

To use LED calculator enter three values. Blank fields will be calculated automatically. If you fill in all four fields, the last calculated value will be recalculated. Dissipated power fields are read only.

How to use LEDs in a circuit

To use LEDs you have to know the color (voltage drop) and maximum current. It is important to get those right since circuits with LEDs are very common. They are a way we have of obtaining information about what is happening in the circuit. For example, a green LED can be used to indicate that something is ON or a red one to indicate if something has broken. You can also use them for artistic reasons or simply to illuminate.

A circuit with LEDs has three elements: the input voltage (it can be a node of your circuit, a source, a battery...), a resistor, which serves to limit the current, and the LED. The key is to understand that the input voltage has to be shared between the resistor and the LED. On one hand, the LED has a fixed voltage (approximately fixed, since it actually depends a bit on the current) that depends on its color. For example, a red LED has a voltage drop of about 1.8V, while a blue one about 3.5V. What remains of voltage has to fall into the resistance. Since the voltage across the LED is fixed, and so is the resistor, you could ideally put any resistor. However, in real circuits, low resistances will cause a lot of current to flow through the circuit and the LED could burn out. So that you have a number in your head, think that a 'reasonable' brightness will be achieved with 10 or 20 mA. The equation you need is:

$$V_i = I \cdot R + V_{LED}$$

The voltage of the LED is something known that you will find in the datasheet. Surely it will also indicate how much current is needed for a certain brightness. The voltage is something from your circuit. You would only have to solve for the 'R' and calculate the necessary value.

Example

You have an Arduino that outputs a 5V signal to actuate a fan switch and you want a green LED to light up when that happens. How do you do it?

• You know the voltage (5V). The LED you know is green and needs 3V drop to drive. In the datasheet it is indicated that with 10mA it already shines well. What resistor do you use?
• Using the slightly manipulated equation to isolate the 'R' we are left with:$$R = \frac{V_i-V_{LED}}{I} = \frac{5-3}{0.01} = 200[Ω]$$

Frequently Asked Questions

• Do I always need a resistor with an LED?
  Yes, LEDs are current-driven devices. Without a resistor to limit the current, the LED can draw too much current and burn out immediately.
• How do I choose the resistor value?
  Use Ohm's Law: R = (Source Voltage - LED Voltage) / Desired Current. Common currents are 10-20mA. Always check the LED datasheet for maximum ratings.
• What happens if I connect an LED backwards?
  It will not light up because it is a diode and blocks current in the reverse direction. If the voltage is too high and it exceeds the maximum reverse voltage, it might also get damaged.
• Can I power multiple LEDs in series with a single resistor?
  Yes, you can connect multiple LEDs in series with a single resistor, but make sure to account for the total voltage drop of all LEDs when calculating the resistor value. Also note that if one LED fails, the entire string will go out.
• Can I use one resistor for multiple parallel LEDs?
  No, it is not recommended to use a single resistor for multiple parallel LEDs because each LED may draw different currents (not all LEDs are exactly equal), leading to uneven brightness and potential damage. It is better to use a separate resistor for each LED.