About

This tool finds an optimal series resistor for an LED. While many tools assume a constant LED forward voltage, this one does not. Instead, it estimates the change in the LED's forward voltage as a function of current. For instance, at low currents / brightness, the LED forward voltage is somewhat lower than the typical forward voltage specification. This is useful when using LEDs at currents above or below the LED's "test current" that the typical forward voltage spec is based on.

This tool can also solve directly for luminous intensity, which is a level or brightness perceived by the human eye. This makes it useful for matching perceived brightness of LEDs of different colors. For instance, if your application utilized both red and green LED indicators, it can be used to select resistors that match their brightness.

Power dissipation of the LED, resistor, and total circuit is also provided.

Instructions

To get started, select an LED from the table at the top of the calculator. The selected row will be highlighted in yellow. Next, use the Solve for menu to choose either current (default) or luminous intensity, and enter the target value for the selected parameter below. Finally, enter the source voltage. If desired, you can also change the standard resistor series from its default of E24/5%.

The resistor value and other outputs will appear on the schematic at the bottom of the calculator. The values appear once all inputs are provided. If an input is missing or invalid, its background will appear red.

Output Format

All outputs are shown on a schematic. The schematic depicts the voltage source, series resistor, and LED in a series circuit. The outputs include:

• Resistor value: Closest standard resistor value to achieve the selected current or luminous intensity.
• Current: Current through the LED, voltage source, and resistor.
• Luminous Intensity: LED's luminous intensity. It is shown to the right of the LED in units of millicandelas (mcd).
• LED forward voltage: The voltage across the LED resulting from the circuit current. It is shown to the right of the LED.
• Total power: Power consumed by both the LED and series resistor. It is shown next to the voltage source.
• Resistor power: Power consumed by the resistor. It is shown below the resistor.
• LED power: Power consumed by the LED. It is shown to the right of the LED.

How it Works

The LED's current vs. voltage relationship is calculated using the Shockley equation. The Shockley equation coefficients are estimated based on the manufacturer LED data. This LED model is used to build a transcendental equation describing a series circuit with an LED, voltage source, and resistor. The equation is solved with a numerical method, yielding the various circuit parameters.

The LED's brightness is estimated based on a curve fit of manufacturer's data on current vs. luminous intensity.

The LED forward voltage vs. current characteristics for Typical parts are based on figure 1c127 in The Art of Electronics, The X Chapters.

Luminous Intensity

Luminous intensity is measure of human perceived brightness at a specific angle - typically looking straight on at the LED. It accounts for differences in perceived brightness between different colors.