Can the PXI-4110 be used as a Current Source? Hardware: Modular Instruments>>Power Supplies>>PXI-4110
Problem: I want to be able to use the PXI-4110 as a current source. How do I configure it to output a constant current? Solution: The PXI-4110 can act as a current source. In the application programming interface (API) for the PXI-4110, you can set a voltage level or a current limit. If you are using it as a voltage source, the voltage limit is the important number. If you are using it as a current source, the current limit is the important number. The reason we do not call it a current level, though, is because in order to set a current, you have to have a load connected - you can't just force current into air. For example, let's say you have a 10 ohm resistor connected to the PXI-4110. You can set a voltage of 5 volts across that 10 ohm resistor, and you will see 0.5 A flow through it. In that way, the 4110 is functioning as a voltage source. But, if you wanted to change it to a current source, you set a current limit less than 0.5 A. For instance, you could set a current limit of 0.1 A (with the voltage level set to 5 V) and you will see only 0.1 A come out of the 4110. You could then change the current limit to 0.2 A, and you would see 0.2 A out of the supply. You are limiting the current to a value less than Ohm's Law (V = IR) would have naturally allowed. But, in this same example, if you set a current limit of 0.6A, you will never see 0.6 A because you've only put 5 V across a 10 ohm resistor, and that will never draw more than 0.5 A. So, the current limit (or current source) functionality only works if your voltage setpoint and your load allow it to work. There is a critical equation: Rc = Vs / Is If the resistance attached to the 4110 is greater than Rc, the 4110 will be in voltage source mode. If the resistance attached is less than Rc, the 4110 will be in current source mode. This critical equation is depicted in the graph below.
For example, let's say you set the voltage setpoint to 3 V and you want to generate a steady 3 mA or 4 mA output. You can do this if you have a resistor connected to the 4110 that was less than 3 V / 3 mA = 1 kOhm. If you wanted to output 4 mA then you need 3 V / 4 mA = 750 ohms. So, if you were building a system where you wanted to output 3 V and also vary the current from 0-5 mA, you should have less than 600 ohms of resistance across the terminals. That way you could step through current values up to 5 mA before you needed to get higher than 3 V to maintain Ohm's law. To set the current limit in LabVIEW, use niDCPower Configure Current Limit.vi. After you set the limit appropriately and enable the output, the status indicator on the front of the 4110 should be amber instead of green signifying that the channel is functioning in the constant current mode. If the status indicator is green then the 4110 is acting as a voltage source and is not limiting the current. The different status indicator states are listed in the table below.
Related Links: Developer Zone Tutorial: Programmable DC Power Supply and Precision DC Source Tutorial Attachments:
Report Date: 03/13/2007 Last Updated: 01/21/2008 Document ID: 47CDOCE5 |
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