Wiring LED to Driverstation

[Alan Anderson]

Originally Posted by sircedric4

Is this 330 Ohm resistor necessary if I spec'd my panel mount LED to not draw more current than the driverstation can handle? I used Digi-Key part L50141-ND (LED IND LIGHT 6V RED), does that require the resistor you mentioned?

That part includes a 150 ohm resistor. It is perfect for this application as it is.

However, if I'm reading things correctly, it costs more than five dollars per piece from Digi-Key! A discrete LED and separate resistor are much, much less expensive.

If it does, can you point me to some documentation or website that explains why I need resistors with LEDs and why different resistances to different colors. I saw no documentation on the Driver Station that would send me down this path. I want to understand why I wire LED's up this way to the DriverStation.

LEDs, like all semiconductor diodes, have a voltage drop across them that is essentially independent of how much current is flowing. If you supply more voltage than that, they will pull as much current as the supply is capable of delivering, and will very likely overheat and die. The series resistor is chosen to drop the rest of the supply voltage across it at the current you wish to flow through the LED.

Different LEDs have different forward voltage drops, and work best at different currents. That's why you will need different resistor values for different colors.

Section 3.1.6.2 Driver Station Digital Outputs of the 2009 FRC Control System manual says: "The DS digital output pins are driven directly by 5V transceiver (SN74LVC4245A device). Be sure to use caution as each pin can source/sink more than 40mA. It is possible to damage the device if too much current is driven on a single pin or the total current supplied by the device on all pins is excessive. Best practice should be to use a limiting resistor to prevent external loads from drawing excessive current, especially for LEDs, be sure to use a series resistor to limit the current." The overview of Driver Station ports earlier in the manual also mentions them: "Digital Output pins 5V output level (8) – Provides 5V TTL for interfacing to digital devices and to drive LEDs (with current limiting resistors)."