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Probe circuit to read in 230 V(ac) with a 0-10V Daqmx card

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willnage

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I want to try design a probe circuit (similar to the probe's that come with oscilloscopes) that can measure an AC signal using a DAQmx card. The daqmx card can read in a signal between -10V and +10V. Any help or ideas would help.

Thanks
 
What is the input impedance of the card you have? I would imagine you could design a probe similar to a 10:1 or 100:1 scope probe. How high of a frequency do you plan to see on the AC signals you plan to measure?

Ron
 
The input impedance is 144k ohm for the card. The signal I want to measure is between 50 and 60hz. If I use the scope probe would it just be a voltage divider probe that differentially matches the 230VAC signal to the +-10V input?
 
The input impedance is 144k ohm for the card. The signal I want to measure is between 50 and 60hz. If I use the scope probe would it just be a voltage divider probe that differentially matches the 230VAC signal to the +-10V input?

Yes and No. The typical input impedance of a scope is 1 Meg Ohm. If you look at a 10:1 scope probe you will see a 9 Meg Ohm resistor placed in series with the scope input. This comprises a 10:1 divider when connected to the scope's vertical input channels. The 9 Meg is typically shunted with a capacitor in the 9 to 15 pF range.

This link affords a schematic of a typical 10:1 scope probe and explains the operation of it. To make a 100:1 probe with a 144K Ohm input impedance you would want to multiply your 144 K times 90 and build your probe based on that.

Ron
 
One obvious concern would be that if the Ground side of your probe ever comes in contact with the Line Voltage. Less of a concern if this is permanently wired in...
 
Another concern I'm having is how to isolate the DAQ card from the 230Vac signal. Is it possible to isolate the probe circuit from the 230Vac signal circuit through a opto coupler and in the same time proportionally "shrink" so that it can be input into the DAQ card. I don't want to blow the DAQ card its an expensive piece of hardware (NI 6008 USB).
 
It is ok to connect the probe as long as you can guarantee that the barrel of the BNC jack is connected to Neutral (never to Line). This is analogous to what I would do if I were using my Tektronics scope to measure line voltage at home; I'm fine as long as the probe tip goes to Line1 (Black wire), and the probe's ground clip goes to Neutral (White wire). God help me if the clip ever touches the Black wire! The scope frame is grounded through the Fat pin (Green wire) on it's own line cord. In the US distribution system, that effectively connects the scope frame to Neutral, meaning that the 10X probe ground clip is connected to Neutral, too.

The problem of using an opto isolator is that the transfer ratio is not that well defined. I would use a small plug-in AC transformer (12V?, without rectifier/capacitor). The turns-ratio is absolutely defined. At low currents, it will give you a good reduced representation of the input voltage. You also have complete isolation.
 
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Minor question, is the input to your data acquisition device a differential or single ended input? I haven't worked with the NI units where I am but we have some. I have worked with Dataq units which depending on the model and setup can be either differential or single ended inputs.

Short of getting into an expensive configuration it isn't easy to do an isolation. The probs I use for that are active probes and run about a grand each.

Ron
 
Ron: The DAQmx card I'm using allows for differential and single-ended input.
Mike: I'm a bit concerned about using a transformer for isolation as wouldn't this create extra inductance on the input signal which might distort the signal I want to read in. The signal I want to measure is a phase controlled sine wave (controlled by firing a triac at certain periods in the sine wave). I want to read this signal in to create a virtual oscilloscope in LabView. Is this possible with the transformer isolation method? Thanks a lot for your guys help thus far...
 
In lieu of what you want to do I would just fabricate some dividers or as I mentioned basically 100:1 scope probes. I do pretty much the same thing using the Dataq hardware I mentioned earlier and especially phase comparasons. I use differential inputs.

Ron
 
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