VEX LED's In Motor port

We are wondering if anyone has tried using the Vex LED’s in the ~8.4v motor ports rather than the 5v Digital ports.

Is it just too much power for the LED’s.

Are the LED’s bright?


Yes, don’t do it :slight_smile:

I was surprised when Karthik ruled this legal.

Don’t do it !

An LED is a form of diode, here is the characteristic curve for a diode.

When the diode is in the forward operating region the current flowing needs to be limited or the diode will be damaged. A VEX motor port does not provide this current limiting and the addition of a current limit resistor is not legal for competition use.

The old VEX flashlight included the necessary resistors to allow that to work in a motor port.

Thanks for the insight!

what if you wired 2 LED indicators in series (rather than parallel) to a motor port so that the voltage would be split between the 2 LED’s and they would both only draw the amount of current needed for full brightness.

here is my thought process: A motor port sends out a theoretical voltage of 7.2v, but is normally more around 8.4v.

So in series if the voltage is split between the two Led’s, each would get about 4.2v which would be in the operating voltage of a 5v Led (or are they 3.3?)

As far as I know, a series circuit would be possible by using non altered vex extension cables and y’s.

Any reason this would be illegal or not work?
Do the vex LED’s have Resistors in them to slow the current?

The VEX Leds do not have series resistors, the digital ports do (a 1k IIRC).

The forward voltage drop across leds is dependent on manufacturer and color. A typical red led may have a drop of 1.7V, the 1k series resistor allows about 3.3mA of current ( (5-1.7)/1000, not really enough which is why they are rather dim). Two leds in series will have a drop of 3.4V, not enough for a motor port. As we said before, don’t do this without a series resistor.