USB 6008 current limits

[VeeJay]

one more sketch . (not so gud on paint) 🙂

V

[RavensFan]

VeeJay,

I think you have something wrong with some of the vowels on your keyboard.  The "a" and "e" were missing from "have".   "What" was missing the "a".  And "good" had a "u" instead of two "o"s.  I think some of those keys are registering when you press them.

[johnsold]

VeeJay,

The USB6008 only measures voltages.  To measure a current you need to convert it to a voltage, usually by passing the current through a resistor.

It seems apparent that you are not very familiar with electrical circuits.  Rather than you guessing what you need and trying to show it in Multisim, please tell us what you are trying to measure.

You mention a water level sensor with a 10:1 range of resistance.  Is that your transducer? What is its specification for excitation?

Lynn 

[VeeJay]

@Lynn

This makes things clear .

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The USB6008 only measures voltages.  To measure a current you need to convert it to a voltage, usually by passing the current through a resistor.

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I am familiar with electrical circuits. But the specifications of USB 6008 is confusing me. An NI applications engineer told me that Analog inputs can take in only upto 0.243mA as current. What is confusing me is with respect to Analog Inputs. Analog outputs and Digital I/O, I get. I am measuring voltages across the 10:1 sensor and passing them into the AI. But before doing that, I am connecting a resistance to limit the current to within 0.243mA so that I don't burn my DAQ card. At the lowest point of the sensor, The voltage is 0.5V and at the

highest point  it is 2.5V. 

My question is, Do I need to add the resistance before connecting the wires t0 AI. Because if I dont do, the current exceeds the aforementioned limit. 

@raven's fan 🙂 ah better??? 🙂

[johnsold]

VeeJay,

 "My question is, Do I need to add the resistance before connecting the wires t0 AI..."

As long as the voltage is within the limits for the device, which yours is, NO.  The input impedance of the AI device limits the current.  This is no different than connecting a voltmeter or oscilloscope. You do not connect a resistor between the circuit and the input of those instruments.

I suspect that the NI applications engineer answered the exact question you asked, but did not really discover what you wanted to know.  The way you worded the question may have resulted in some confusion between the two of you.

The origin of the 0.243 mA value is simple Ohm's law: The overvoltage limit of 35 volts divided by the input impedance of 144 kΩ = 0.243 mA.  So that is the amount of current which would be drawn by the input if the maximum allowable voltage is applied.  Similarly, if you drove the input with a current source of 0.243 mA the voltage would go to 35 V.  Any higher and the device might be destroyed.  The maximum limits apply regardless of what range or mode you may have selected.

For your voltage measurements of 0.5 to 2.5 V select the +/-4 V range and connect your sensor to the input.

Lynn