What is the Input Voltage Limitation of the 9425 Module?Hardware: CompactDAQ>>Digital Input Modules>>NI 9425
Problem: On page 12 of the 9425 module specifications (linked below), I can read that the maximum input voltage is 60 VDC if using 8 channels, and 30 VDC if using 32 channels. Why does the voltage limitation depend on the number of input channels? Solution: Each input pin of the 9425 has a 30 kOhm input impedance, made of several series resistors and connected to ground. The limitation is due to a power dissipation limit. Since the device only has resistive elements at each input (30 kOhms), the current flowing into the device is going to be Vin / 30 kOhm. When you have 32 channels at 30 V, you dissipate 0.96 W (32*30V*30V/30kOhms), and 8 channels at 60 V dissipates 0.96 W as well (8*60V*60V/30kOhms). Summarily, when you want to use 9425 input channels, you have to limit the total dissipated power to 0.96 W. Related Links: Product Manuals: NI 9425 Operating Instructions and Specifications (April 2008) Attachments:
Report Date: 08/07/2008 Last Updated: 11/03/2008 Document ID: 4O667DQ2 |
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