Supply Greater Than 20 mA with NI Current Output Device

Issue Details

• I am using an analog output device like the NI 9265, NI 9266, or NI PCI-6704 which can output current between 0.1 and 20.2 mA according to the specifications. Is it possible to send more than 20 mA from these devices?
• Can I combine channels on a current output module, in order to output more than 20mA?

Solution

Yes, you can supply more than 20 mA from both the NI PCI-6704, NI 9266, and the NI 9265. 

These devices are limited to supplying 20 mA current outputs on a single channel but you can output greater than 20 mA of current by wiring two or more outputs in parallel. Refer to the respective User Manuals, for more information on outputting current. 

Although you can increase the current supplied from the analog output device, it is still limited to a maximum voltage generation. The NI PCI-6704 can supply a maximum voltage of 10V, and the NI 9265 or NI 9266 can supply up to 12 V. According to Ohm’s Law (V = I*R), as you increase the current supply to a constant resistive load, the amount of voltage that must be generated increases. Since the voltage generation is limited, the amount of load resistance that you are able to drive current through is limited to a maximum value. 

For example, if you supply 40 mA of current through your load, and the maximum voltage that the NI PCI-6704 can generate is 10V, then the load is limited to a maximum resistance of 250 Ω because 10V/40 mA=250Ω (V/I=R). If the load is greater than 250Ω the device must generate more than 10V to drive the set current level, which is not possible. 

When you wire current sources in parallel, the resulting current is the added values of the sources. 

Follow the directions below to wire current outputs in parallel. 

1. Wire two or more current output channels together. For example, when using channels 16 and 17 of the NI PCI-6704, wire pins 67 and 32 together. 
    
2. Wire the two or more respective analog ground pins together. When using channels 16 and 17 on the NI PCI-6704 this corresponds to pins 68 and 33.
    
3. If you are using LabVIEW, you can use the LabVIEW code below to send output current on multiple channels. Since the NI 9266 is a CompactRIO Module, this example program does not apply as it is using NI-DAQmx functions. However, the general setup for this task is similar and could be applied to the CompactRIO functions.
   • Select the correct device number of your current output device and appropriate channels for that device.
   • Assign between 0 and 20 mA to each current source that you are using.
   • Run the program. The current you measure between any of the current output channels and analog grounds is the sum of all the current sources that you have wired together.