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AC current measurement with U3


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5 replies to this topic

#1 crmaris

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Posted 30 March 2010 - 11:15 AM

Hello. I want to measure the current that flows in a 230V AC line in order to calculate, with the most accuracy I can get, the energy (Watts) that a device is consuming (from this line). I have read a lot about this subject in this forum and I decided that the most straight-forward way is to buy a low current probe like the ESI-695 or ESI-687. Can you tell me which of the above is the best for my needs? Because I can't find it anywhere can you inform me what is the ration of mA/mVolts for the above current probes (e.g. for a 10mA current how much mV will I measure with the Labjack? I want to see if the resolution of the U3 is sufficient). Thanks!

#2 LabJack Support

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Posted 31 March 2010 - 04:52 PM

For the 695, the output is written right on the case, 100mV/A or 10mV/A depending on the range setting. The 687 is better for your range of interest, but looks to be an enclosed meter, not a DAQ device. Measuring power can be complicated. If you just measure True RMS of your current draw you have to assume voltage, waveform and power factor. If your load is capacitive or inductive there will be a difference in phase between the current and voltage. When the phases are not aligned you will be measuring apparent power, not real power consumed by the load. To get a good reading both current and voltage need to be read simultaneously then the products integrated. Streaming both at 25kHz would allow you to calculate a pretty good reading. The sample time between voltage and current would be 20us, so you'll need to determine if that will induce too much error.

#3 Gareth_SA

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Posted 11 June 2014 - 06:02 AM

Good Day,

 

This post was interesting to me as I am currently using the U3-HV to develop my 4th year Project which is a desktop solution that can be used by our University in some of their practicals and projects to understand and learn more about real-time data acquisition and signal processing. 

 

As I get further into my project I will be reading information from k-type thermocouples, 4-20mA industrial pressure sensors and strain gauges. However, first I am looking at measuring current of everyday items, such as an AC toaster (240V +-1000W, 50Hz)

 

I already have a  YHDC SCT-013-030 Split Core Current Transformer, Datasheet: http://garden.seeeds...SCT013-030V.pdf

 

However, I am having difficulty connecting this up. I am getting results that do not make sense. I understand that one of my problems is that I am using a 30A sensor and the toaster should only draw 4.2 Amps meaning that the range that the transformer would output 0-1V would now be 0 - 0.14 V.

 

I also have an LJTICK INAMP but am unsure if I should use this for my voltage range?

 

Regards,

 

Gareth



#4 LabJack Support

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Posted 11 June 2014 - 11:16 AM

I believe what they are saying is that a 30A RMS current would give you a 1V RMS output.  Assuming you have a nice sine, which I would expect for a toaster, a 30A RMS current is a sine from -42 to +42 amps, and the output of your C/T is a sine from -1.4 to +1.4 volts.  For your 4.2A current, you expect a sine varying from -0.19 to +0.19 volts.

 

You should be able to see this +/-190mV waveform using a high-voltage input on the U3-HV (e.g. AIN0/GND), but the resolution is just 5mV so it might be a little rough.  The frequency is 60Hz, so if you want 40 samples/cycle you need to collect data at 2400 scans/second, which means you need to use stream mode.  Try LJStreamUD.

 

 

Another readily available AC current sensor is the Mastech MS3302.  This convenient clamp sensor is currently just $20 from Amazon:

 

http://www.amazon.co...keywords=ms3302

 

It would provide a lower ouput so the LJTick-InAmp would probably be useful with that.

 

 

The Mastech MS3300 AC current sensor requires a battery but provides a DC voltage output proportional to True-RMS current, so it is easy to read (just grab a single reading whenever you want ... no streaming of an AC waveform).  It is currently $65 from Amazon:

 

http://www.amazon.co...keywords=ms3300



#5 Gareth_SA

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Posted 01 September 2014 - 04:49 AM

Good day, Thank you very much for your informed advice. I do however have one more question (an I hope I am not overstepping the limits of asking questions within another users post)

 

My project is specifically aimed as a ultra low cost solution. That is why I am looking at using a C/T rather than a more expensive device like a calibrated Current Clamp (no matter how wonderful those devices are). So I think I will look for a C/T that more closely matches the expected current I will be measuring to ensure the resolution is adequate. 

 

One further question. I am using a cheap, easily available k-type Thermocouple and building my own conditioning circuit to calibrate it using the "cold Junction Compensation" technique. (see the circuit here) using a linear temperature sensor and a small op amp (I could use the LJTick-InAmp, but would still need a second temperature sensor for calibration). Do you think the circuit (linked above) would be sufficient?

 

for 0 - 250 degrees Celsius it outputs 0.1 - 2.6v which should be ideal?

 

Regards,

 

Gareth



#6 LabJack Support

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Posted 02 September 2014 - 09:50 AM

You circuit should work fine, although the normal max input a low voltage analog input on the U3 is 2.44 volts, so don't plan on reading up to 2.6 unless you switch to the "special 0-3.6" range.

 

I really don't see a reason to build a circuit that does CJC.  Makes more sense to me to just use an op-amp to amplify the signal from the thermocouple, and use a LM34 sensor to measure the CJ temperature with the U3, so then you can do the CJC in software.




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