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Light Emitting Diodes

Preventing over-current

Usually, when protecting a single LED from over-current, we place a resistor in series with the LED that will lower the current for a given voltage, taking the voltage drop across the LED into account. However, if we desire multiple LEDs but not multiple resistors, we can use an adjustable current regulator to limit the current through a string of LEDs in series. Most LEDs limit current to 20mA, so an LM317T with a 63Ω resistor, hooked to a power supply that was at least 2V higher than the sum of the voltage drop across the string, but less than the regulator's 40V limit, would safely light the string. [1]