Dear Colleagues,
I am attempting to use an NI 6008USB interface to make simultaneous voltage and current measurements from a working photovoltaic solar panel. I have searched available NI resources (tech notes, tutorials, this forum) for any information about how to do this. I have configured my 10 Watt solar panel with a 45 Ohm 20 Watt power resistor as the load resistor and have configured a 0.7 Ohm shunt resistor on the return current side of the panel (so-called "low-side" current measurement, even though the solar panel is not referenced to ground). The ground line to the 6008 is the node between the load resistor and the shunt resistor, the voltage pickoff comes from a 10 KilOhm voltage divider placed across the load resistor (the solar panel's open-circuit voltage under full illumination is 19.2 Volts) and the voltage pickoff for the "current" measurement is taken from the solar panel return-current side of the shunt resistor.
This simple circuit seems to work: the voltage I read is, correctly, one-half of the voltage across the load resistor, but the voltage measured across the shunt resistor is ~2X what it should be. I say "should be" because the mechanical connections of the 6008 for the analog inputs are tied to the internal multiplex through a series 127 KilOhm resistor that connects the middle of a voltage divider consisting of a 30.9 KilOhm resistor tied to a +2.5 V reference and a 39.2 KilOhm resistor to signal ground. Finally, this node of three resistors is connected to the input to the multiplexer for that data channel.
This is NOT the way I would configure the input to a multiplexer channel, but NI gives me no obvious means of turning off the voltage reference. I say "obvious" because it is possible to configure the 6008 in software for "current" measurements. But because NI does not explain how the 6008 is set up differently for "current" measurements from being configured for "voltage measurements" -- after all, the 6008 has no built-in shunt resistors --, I am not running the 6008 in "current" mode. So I assume that the fact the mux channel I have configured in hardware to measure the voltage drop across the shunt resistor is reading higher than it should is because of the interaction of the measured voltage drop across the shunt resistor with the complex voltage-reference-tied resistor network inside the 6008 that in software I have configured for voltage measurements.
NI does provide a hint at how to address this input problem in a single document dated March 2008 and titled "Make Accurate Power Measurements with NI Tools": use the equivalent of a Burr-Brown (now, sadly, TI) INA139 Current Shunt Monitor to buffer the voltage drop across the shunt resistor and output a low-impedance voltage to the mechanical input on the 6008. This is a nice idea, but the INA139 only comes in a surface-mount package. If it turns out I will need such a voltage buffer, I'll use the equivalent of an instrumentation amplifier as a substitute that I can easily prototype using DIP chip packaging.
So my question is, simply, in the absence of the voltage buffering proposed by NI in their March 2008 tutorial, is there a way to make both voltage and current measurements (as I outlined above) with a 6008USB interface?
Cordially,
Paul Rybski, Physics, University of Wisconsin-Whitewater
