Breakdown Voltage on PCI-6225

[Paul567]

Hi All,

I am attempting to calculate current while changing the resistance of the circuit with my potentiometer.

I calculate current by measuring differential voltage (with my PCI-6225) across a 2 Ohm precision resistor and I verify my calculations by measuring current with an ammeter.

Below is a simple diagram of how my circuit is connected:

I find that once my potentiometer is set to about 512 Ohms, my calculated current know longer matches what my measured current (calculated current starts becoming greater than measured current). I can correctly calculate current again if I remove the wire connecting the analog ground of the card to the ground of my 12V supply. I don't want to do this because it adds a lot of noise.

My suspision is that at 512 Ohms the resistance becomes so great that current starts finding an alternate route through the DAQ card.

I welcome any ideas or suggestions.

[kekin]

Dear ohhhhhhh dear,

There is lot of shunting/loading effect.

The 200K connected to AI72 is in parallel with 1K pot.

& the 200K connected to AI64 is in parallel with series compbination of 2 ohm & (1K parallel with 200K).

So you are forming unbalanced bridge at 512 ohms & hence the current is taking another path.

Just form balanced bridge or use NRSE or Differential mode of measurement. You are mixing up these two modes. Plz refer M-series manual for more info. If you find lot of noise then use NRSE mode only.