Relay I/O Current

On the digital sidecar, what’s the maximum current output on the Relay I/O ?

One Spike.

You’re not supposed to connect it to anything else.

I see. Thank you.

So if we were to put an LED on our robot, where would it get power from? Like if you wanted it to indicate something through it.

You will find the information you seek here.


I see nothing in the rules that stipulate that a relay output can not be connected to a custom circuit.

Can you point out the rule?


I thought <R57> restricted PWM outputs to connect to speed controllers or servos, and relay outputs to connect to Spike relay modules, but now that I look at it again I was probably reading it backwards.

<R42> Each power-regulating device (speed controller or relay module) shall control one and only one electrical load (motor, actuator, light or compressor).
Exception: Multiple low-load, pneumatic solenoid valves or lights may be connected to a single relay module. This would allow one relay module to drive multiple pneumatic actions or multiple lights. No other electrical load can be connected to a relay module used in this manner.

Does this help?


Wouldn’t <R39-F> allow a custom circuit with LED’s to be connected to (and derive its power from) the relay outputs (or digital outputs)?


R39 F relates to powered devices, which in the case of the DSC is the servo outputs with the jumper in place to feed 6 volts to those outputs. The digital I/O and relay outputs cannot sink or source enough current to feed LEDs without a driver of some sort. If you look at the schematic for the DSC, you will see that the relay outputs feed a small LED in parallel with the relay output. The output current on that LED and the relay drive about max out the chip capability of 16 ma per output. The LEDs on the board are low current devices speced at 5 ma each.


I did not want to do the engineering for the team. I was just pointing out that a custom circuit, powered in this manner, is possible within the rules.

On the engineering side, the outputs are derated and speced by FRC as a max 7.5 mA source and sink with a series 680 ohm resistor. IMHO, the team might be able to use low current LEDs. They would be hard to see but they don’t say what they wanted to use them for.



Sorry to jump the gun but I thought we were leading to the specifications eventually.

I was under the impression that the general purpose digital outputs on the DSC could source much more current than that, in fact, we’ve used it to successfully drive a 65mA blue LED with no observed side effects.

The TI device that drives the relay outputs is speced at 16ma max source/sink per output. The relay outputs then have a 680 resistor in series with the output and parallel feed another resistor and low current LED at about 5ma.

Sorry, badly phrased, I was referring to the DIO pins, not the relay outputs. I read your post as saying neither the DIO pins OR the Relay outputs were capable of driving an LED.

Sorry for any confusion,

The digital I/O is 2ma as I remember. That spec is hard to find on the NI site.

Not sure how that ended up working, I’m beginning to suspect that the 9403 module was actually just routing all of it’s current output to one pin, but my lunch break is just about over so I’ll have to dig through the spec sheets tonight to figure it out.

Apologies for any confusion I may have caused, thanks for the help,