voltages vary too much and calibration doesn't help

[wkamin]

Hi,

I'm new to the NI devices and most terminology.  I'm doing an accerlated life test experiment with small light bulbs.  I have my PCI 6229 connected to a SCB68.  I'm trying to read voltages across a 1ohm resistor (for each bulb - a bulb in series with a resistor.. and 10 bulb-resistors in parallel).  This means i'm indirectly measuring the current of each bulb (which is about .06 at 2v).  This small value is making it  hard for me to distinguish when the bulbs fail, since the 6229 shows voltages that bump around from negative values to +0.0x values.  I've tried using AISENSE and AIGND (and setting up the DAQ Assistant according to each type).  And I've tried the self-calibration in the test tools.  I've also checked to make sure that all ground connections are good.  Perhaps I'm missing something?

Sorry if this is all confusing... I appreciate any suggestions.

-Walter

[Jerry_X]

I am slightly confused by your description of the issue.  You are trying to read voltage across a 1 Ohm resistor.  How small is the voltage range do you expect the voltage difference would be in? in milli-volts? in micro-volts? What do you mean by ".06 at 2v"?  How are your signal connected to 6229?  In differential configuration or RSE?  When you self-calibrate, did you remove all external connections from the DAQ board?  Did you set the signal input range to a smaller range more suited for your signal?  I think the default one is  (-10V, 10V).

Looking at the specification manual for 6229

It has input ranges of  (-10V, 10V),  (-5V, 5V), (-1V, 1V), (-0.2V, 0.2V)

For the smallest range (-0.2V, 0.2V),  the range accuracy is 112 micro-volts and range sensitivity is 5.2 micro-volts.

Does this board's hardware specification meets the requirement to measure your signal?

[Brooks_C]

Hello Walter, 

It sounds like you're trying to determine when a bulb has failed.  If this is the case it sounds like you would need to have a constant current power source that you use to supply current to all of the bulbs.  Then when one burns out the voltage across the remaining bulbs would change to maintain the same current level.  If you set the supply voltage to 2 volts and adjust the current accordingly I would expect you to always read 2 volts regardless of how many bulbs were lit.

At this point I don't think that what I mentioned above is your problem--I think we need to make sure you're using the correct input configuration and connecting to the correct points in your circuit.  Initially I would recommend removing the bulbs from your setup and then confirm that you can measure the voltage output of your voltage source.  If you're just applying this voltage across the bulbs then the same configuration that works for the voltage source should work in the actual setup.  Table 1 of the Field Wiring and Noise Considerations document shows when to use which configuration and also whether you attach the signal to AI +/-, AI + and AI GND, or AI+ and AI Sense. 

If you're still having trouble reading the signals it would be helpful if you can post a picture of your circuit diagram so that we're all on the same page with your setup. 

Cheers,

Message Edited by Brooks_C on 08-19-2008 02:52 PM