Thread: Attaching a standard car relay to the Digital sidecar
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Unread 01-08-2009, 08:20
Russ Beavis Russ Beavis is offline
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Re: Attaching a standard car relay to the Digital sidecar
Please post a datasheet (or link to) for the relay. My first guess is that the sidecar can't directly provide sufficient current to drive the control coil since none of the outputs can drive more than about 10 milliamps.

You might, however, be able to use the Robot Signal Light output but I'm not sure whether the FRC-specific software allows you to directly control the RSL (since it's primary function is for safety indication and debugging). Even if you could control the RSL output, I'd wager that there's a fair amount of start-up code that would leave it blinking and twiddling while the cRIO is booting.

If you don't mind sacrificing a sidecar since mods are illegal (per 2009 FRC rules) and you'd lose the RSL functionality, you could check out the schematics for the sidecar and rewire it to drive the RSL FET using a GPIO.

Another couple of options that wouldn't break the sidecar would be to use a Spike to drive the control coil (a bit overkill since the Spike can almost drive your 40A load directly but it would work) or fabricate a little power switch that's controlled with a GPIO from the sidecar to provide the 100ma(?) or so for the control coil.

An even better option, I think, would be to use the NI 9472 module and the Solenoid Breakout adapter since each of its 8 channels can source 3/4 amps of current. I'd be surprised if that "relay output" module couldn't drive the relay's coil.

Russ