Created by
Created November 13, 2011
Last modified January 28, 2012
Tags bias-point   current-limiting   led  

Summary

LEDs are usually found with a resistor in series to select the desired current (and therefore brightness and power consumption).


Description

We're using four LED part models which mimic actual LEDs: LTL-307EE for a red LED, LTL-307GE for a green LED, LTL-307Y for a yellow LED, and QED-123 for an infrared LED.

How does the current vary with voltage and our choice of resistor? We need to make sure the current is high enough that the light is visible, but not so high that the LED overheats and is destroyed.

A few things to try in CircuitLab

This circuit highlights several cool features of CircuitLab simulations that you can quickly try. Load this circuit with the "Open in editor" button above, and then click the "Simulate" tab to get started.

DC

Run the DC simulation to quickly get a sense of the current through each LED when the power supply is really at 5V.

DC Sweep

See how the circuit varies of our power supply isn't really at 5V.

You can also try doing a sweep over resistor value. Example: enter "R1.R" as a sweep variable, and sweep it between 220 * 0.95 and 220 * 1.05 to see how a 5% resistor tolerance might affect your circuit.


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