Signal Conditioning

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how to use NI 9205 to measure 120V AC waveform

Hello:

I am trying to use the NI 9205 to measure the 120V AC waveform of a wall outlet, with the purpose of measuring the use of power and get information from the AC waves of both voltage and current. I already have the form of the current by using an AC clamp that transforms AC current to a proportonal AC voltage of 2V range.

I am having problems is with the AC Voltage measurement. I want to adquire with the NI 9205 the AC Voltage waveform from the wall outlet, but I don't know how to condition the signal for the NI 9205. From what I can get from the datasheet its max. voltage range is 10V.

I wanted to ask you guys if you could recommend me a hardware solution for conditioning the signal so the NI 9205 can measure it. I was thinking of maybe using an AC/AC transformer to bring down the signal to a suitable voltage range while maintaining the waveform. I was also thinking of using a very small shunt resistor with a very low voltage drop and amplify that for entry to the NI 9205.

Also, any suggestion in electric protection would be greatly appreciated (ex: fuses).

Thank you for your time and patience.

Message 1 of 7 (4,099 Views)

Re: how to use NI 9205 to measure 120V AC waveform

There are signal transmitters that will convert mains voltage to a corresponding DC output like 0 - 10VDC which will interface directly with your DAQ.

For example, something like this

Richard

Message 2 of 7 (4,095 Views)

Re: how to use NI 9205 to measure 120V AC waveform

Broken Arrow escribió:

There are signal transmitters that will convert mains voltage to a corresponding DC output like 0 - 10VDC which will interface directly with your DAQ.

For example, something like this

Hello.

Thank you for your response. The signal transmitters you mention could be useful, but for the time being I need to use the NI 9205 (cant buy additional adquisition hardware). Also, I need to get the waveform into Labview to analize it, and take the DC measurement (RMS) from within the program, with other calculations as well. The signal transmitter would give me the RMS but I would still need a way to get the AC Voltage signal waveform.

Message 3 of 7 (4,091 Views)

Re: how to use NI 9205 to measure 120V AC waveform

The transformer is a far better choice than a resistor.  It allows isolation from the power system which you would not have with a resistor.  Think: Save your life. Save your instruments. Save your computer.  Save your job.

If you use a small power transformer such as a wall plug-in device with a low enough voltage rating, it will work for measuring the nominal voltage.  However, you mentioned needing the waveform.  That implies some non-sinusoidal signals.  The cheap power transformers (and some expensive ones) probably have very limited frequency response for harmonics.  The manufacturers generally do not publish such information so you would need to make your own measurements.

If can can specify your requirements (signal bandwidth, maximum input voltage - instantaneous, ...) it would help to define a suitable transducer for your application.

Lynn

Message 4 of 7 (4,084 Views)

Re: how to use NI 9205 to measure 120V AC waveform

Hello, thank you for your response.

Yes, as I see it frequency response is a problem with the transformer solution. Altough for the time being it is a simple, first-approach experiment, so I dont need to measure high-frequency armonics on the line. I need to get a decent- enough waveform to make simple calculations  like power factor and such, (an approximation). I don't need, for example, to make a spectral analizer. The transformer may give a distorsiones wave, but Ill have to see how distorsionate it gets.

I am sorry for not specifing my requirements, they are still on debate, but for the Experiment these would do:

At first we will measure power consumption on simple electronics (ex: Lamp). Bandwith I am not so sure but as I say, its a first approach so it does not have to be very restrictive, maybe 200Hz to get atleast the third armonic. Voltage should be in the 120V range, and spikes in voltage should be protected against but not taken into account in the measurement.

Im not an expert on transformers, but does someone know what frecuency or bandwith are they capable of? Also, if anyone has another suggestion for the implementation I would be very thankful.

Message 5 of 7 (4,079 Views)
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Re: how to use NI 9205 to measure 120V AC waveform

Here is a detail discussion and application guide for current and voltage isolation transformers.

http://www.lem.com/images/stories/files/Products/1-3_applications/CH24101.pdf

-AK2DM

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"It’s the questions that drive us.”
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Message 6 of 7 (4,068 Views)

Re: how to use NI 9205 to measure 120V AC waveform

DeweyMec wrote:
[...]

Im not an expert on transformers, but does someone know what frecuency or bandwith are they capable of? Also, if anyone has another suggestion for the implementation I would be very thankful.

I had good results with a small simple print transformer (1VA or less)  with a light load (resistive voltage divider to match the input range). A one point calibration was fine up to 2kHz ..for my proposes

(due to the winding capacties  the internal resonace of a standard line transformer is quite low)

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

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Message 7 of 7 (4,052 Views)