Understanding NI 6528 MOSFET-Based Current Limiting Circuitry

Updated Feb 1, 2019

Reported In

Hardware

  • PXI-6528
  • PCI-6528

Issue Details

In the NI 6528 user guide and specifications , I see that the card has MOSFET-Based Current-Limiting Circuitry. What is the purpose of this circuitry on the card and how does it affect the input current I can provide to the card?

Solution

The specifications for input current on the 6528 are as follows:
5 V input3 mA/channel max
60 V input3 mA/channel max

The MOSFET-Based Current-Limiting Circuitry will adjust the input current based on the input voltage. Think of the current-limiting circuit as a resistor that can change its resistance. It will automatically detect the amount of current that should be flowing through and change its resistance accordingly. If the user is inputting 60 V, then the resistance is 30 KOhms, allowing 2 mA of current. If the input is 5 V, the resistance is 3.3 KOhms, allowing 1.5 mA. If the user is inputting 28 V, then the 6528 will draw somewhere between 1.5 and 2 mA of current. 

Additional Information

It is possible for the current-limiting MOSFET to become damaged from voltage spikes above 60V. If this occurs then the device may still be able to measure a boolean true or false, but the amount of input current can be larger than specified. To troubleshoot a damaged MOSFET, first determine which channels operate outside of the specified current input limits. Then contact National Instruments technical support for further help.

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