5 “Power good” LED

It is convenient to see that at least an embedded controller is getting power. This is typically done by an LED that is connected in series with a current limiting resistor. The anode (positive side) of the LED connects to the output of a regulator. The cathode (negative side) of the LED connects to one end of the current limiting resistor. The other end of the current limiting resistor connects to the ground (0V) of the system.

The current limiting resistor is necessary to make sure the amount of current passing through the LED is “reasonable”. Most high efficiency LEDs only require 5mA to light up brightly. The calculation goes as follows:

• the voltage drop across an LED about 1.7V (white and blue ones are about 4V). If the output of a regulator is 5V, then there is a remainder of 3.3V.
• In physics, I = , or R = . Because the remaining voltage is 3.3V (V = 3.3), and only 5mA (I = 0.005) is needed, the resistor value is R = = 660 ohms.