Sensing DC voltages with NI 6528

[glingi]

Hi.

I have several questions about sensing DC voltages using NI 6258 Digital I/O module.

Below figure is a example of circuit configuration to measure the voltage with NI 6258 module.

Q1. "LOAD" should be located in the circuit to sense the voltage of supply?

Q2. If yes, what is the proper value of the load resistance?

Q3. When we assume that NI 6512 module is used as power supply, if there is no load in the circuit and switch is closed, NI 6512 can be broken?

p17, https://www.ni.com/pdf/manuals/372124d.pdf

Thanks,

Woohyun ahn

[Gevor]

Q1, Q2: Load is basically there to limit the current into module. Per datasheet input current should not exceed 3 mA per channel. So the load resistor must be chosen accordingly. 

Q3. Since you may anyway need a power supply for 6512 outputs, I expect the circuitry should not change for your particular case.

[Minions]

Woohyun Ahn,

The load resistor is acting as a current divider circuit.

The section Sensing DC Voltages on page 16 of the manual notes - Voltages:

• <= 1V will be a logic LO
• >= 3.2V will be a logic HI
• 1V < Vin < 3.2V may be unstable and transition between the two states

Seeing how the inputs are rated at -60V to 60V, You will need to size the resistor according to the following:

• What is the absolute max of the voltage ==> | Vin |
  • Example here will be the max of 60V
• Maximum Specified Current ==> 3mA
• Load Resistor that will cover all voltages ==> 60V/3mA = 20Kohms