AC current measurement with U3
Posted 30 March 2010 - 11:15 AM
Posted 31 March 2010 - 04:52 PM
Posted 11 June 2014 - 06:02 AM
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?
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:
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:
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?
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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