PXI-6711 Analog Output Current Drive

[VI_Joe]

I have been using the PXI-6711 Analog Output to turn on relays with a rated coil current of 20ma. 

I was just looking at the spec sheet for the PXI-6711, and I noticed that the rated Voltage Output Current Drive is +/-5mA max.

Am I missing something or have I just been lucky so far?  Is there a better design technique I should be using?

Thanks in advance.

[Michael G]

Greetings VI Joe!

You are correct that the PXI-6711 is only rated to be able to drive an attached device with a maximum of 5mA.  Trying to draw more power form an analog output line than this could potentially lead to damaging the device.  Furthermore, as you are driving a relay, which is an inductive load, there is also the concern of "back EMF" that results from the discharge of energy in the relay coil when the voltage is no longer applied.  As such, I would recommend adding a layer of isolation between your outputs and relays. 

I would take a look at the following forum that discusses connecting a similarly rated relay to a USB 6009 device (which is also designed to only supply 5mA to its outputs). 

NI Discussion Forums - Drive relay coil with transistor powered by usb-6009?

http://forums.ni.com/t5/Digital-I-O/drive-relay-coil-with-transistor-powered-by-usb-6009/m-p/792925?...

Hope this information helps!

Michael G