Arduino Ground Pins

[AroB]

I have a question about the ground pins on the Arduino Uno. Is it correct that the ground pins also have a max of 40mA each? I have a set of relays that I'm controlling through the Arduino. I'll be able to source up to 40mA since I can connect the relays to each individual digital output pin, but I'll need to connect the 10 relays back to ground. Would it be okay to divide the 10 up and connect them to the three grounds in sets of 3, 3, and 4?

If not, why does the arduino have so few ground pins? Even the boards that are much larger only have the same three ground pins.

Thanks!

[adafruit_support_bill]

The ground pins are not limited to 40mA. The pin contacts can probably handle 1A or more each. But when powering motors, relays and other high-current devices with an external supply, I typically ground the devices to the external supply and run a ground wire from there to the Arduino.

[AroB]

Ah, okay, that helps a lot. So if I am using an external power supply at 16V to power the load side (high current) of the relay, I can also connect the (-) side of the control side of the relay (low current) to the (-) terminal of the external power supply. In addition, I would connect the (-) terminal of the external power supply to one of the Arduino Grounds.

I put together a quick sketch to confirm that I'm thinking about this correctly. There are actually 10 relays for 10 solenoids but this just shows one.

Thanks!

[adafruit_support_bill]

Are you driving the relay coils directly from the Arduino pins? Most relay coils pull more than an Arduino pin can safely source. At the very least you should have a diode in parallel with the relay coil to protect your Arduino pin from the kickback

This is a typical relay driver circuit (actually, you could skip the relays and use it to drive the solenoid coils).
http://www.arduino.cc/playground/upload ... relays.pdf

[AroB]

I am using an off the shelf solid state relay board with a built in protection diode that is spec'd such that it can be powered directly from a microprocessor.

My question remains whether it is possible to connect the two (-) sides of the relay to the power supply GND and then to the Arduino GND. Is the sketch that I have feasible to use?

[adafruit_support_bill]

If your relays can be safely driven from an Arduino pin, then you can connect the grounds as you have drawn them or direct to the Arduino with no problems.

[AroB]

Great, thanks for your help!