How to wire Non-invasive AC Current Sensor (SCT-013-030) to TelosB Mote's ADC pin?

Discussion in 'The Projects Forum' started by liliandreapinero, Jul 27, 2015.

  1. liliandreapinero

    Thread Starter New Member

    Jul 27, 2015
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    Does anyone know here how to wire a non-invasive AC Current Sensor (current transducer - CT) to Telosb using ADC pin? or has any circuit diagram on wiring a CT to the TelosB? I attached the picture of the sensor I'm using.
    I am currently working on a project on collecting current consumption data using current transducer (SCT-013-030) and I don't know how to properly wire it on the Telosb.
    Could anyone help me, it would really mean a lot. Thanks :)
     
  2. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello liliandreapinero
    .
    It's really difficult to make a circuit to measure DC current with one Clamp-On.
    .
    Notice how, Hewlett Packard, developed the HP 428B meter.
    On page 16 onwards describes the method to measure DC current with one Clamp-On.
    (Attachment HP 428 manual.pdf).
    .
    Currently these current meters are made with a field effect device they are relatively easier.
    .
    However you can buy one already made around here:
    http://www.amazon.co.uk/HP-6203-Compact-Digital-Current-Pliers-Dunkelgrau-orange/dp/B00CBS1NZK
    .
    In this other link (From CR4) They discuss something related with Clamp-On:
    http://cr4.globalspec.com/thread/4522/How-is-DC-Current-Measured-by-a-Clamp-Meter
    .
    I hope this information will help you continue with your project.
     
  3. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    I don't see any indication that the TS plans to measure DC current with this device. I'd have assumed they were measuring AC current.

    I know nothing about the Telosb, so I can't offer much help with interfacing these devices. My guess would be something along these lines: burden resistor to convert current into voltage, rectifier to eliminate negative pulses, and maybe capacitors to smooth DC pulses into steady DC signal for ADC conversion.
     
  4. liliandreapinero

    Thread Starter New Member

    Jul 27, 2015
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    0

    Hello MrCarlos,

    Thanks for your kind reply. However, I'm not going to measure DC current but rather AC. I also have a clamp meter which I would also use to see, if and when my project works, the measurement is accurate as the actual.
     
  5. liliandreapinero

    Thread Starter New Member

    Jul 27, 2015
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    You are indeed correct ebeowulf17... I'm going to measure AC rather than DC current.
    TelosB is a small device used in R&D for wireless sensor networks. Anyway, I've made some readings and your guess might be correct. However, I still perhaps would just make the circuit on my own.

    Thanks,
    Lili
     
  6. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    Oops! Didn't see your attached data sheet before. Looks like the SCT-013-030 already has the burden resistor built in, so in theory you could just wire it directly to an ADC input.

    Depending on your sample frequency, bit depth, and required precision, you'll probably want to convert the AC sensor output to DC (otherwise you'd have to be sampling at well more than double your AC frequency to get any idea what's really going on) and you'll probably want to scale it up with an op-amp, at least scaling the 1V max output to whatever your ADC reference voltage is, and maybe scaling it higher if your measurement range doesn't extend all the way to 30 amps.
     
  7. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello liliandreapinero
    .
    Ok, sorry for my mistake.
    .
    Have You visited this link?
    http://openenergymonitor.org/emon/b...r-adaptor-installation-and-calibration-theory
    .
    However, there is much information on the Web. You can ask Google.com by: SCT-013-030
    .
    By the way: You already have data sheets for SCT-013-030 ??
    .
    I saw those sheets. They say that the current transformer is in the range of 0-30 Amp. and gives 0-1 VAC.
    .
    Now, to visualize, in some way, this voltage, is necessary to use an instrumentation amplifier with a gain of, say, 10. Then an Precision Rectifier and then such an ADC ICL7135.
    You can find circuits: instrumentation amplifier, Precision Rectifier asking Google.com
    Same for the ICL7135.
    .
    .

    I hope this information will help you continue with your project.
     
  8. liliandreapinero

    Thread Starter New Member

    Jul 27, 2015
    4
    0
    Hi ebeowulf17,

    I have stumbled upon this link http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface and it also uses the same CT sensor that I will be using. However, it is wired to Arduino.
    Another link here gives a circuit diagram of the same sensor to Arduino. http://myelectronichome.altervista....detector-with-sct-013-020-or-sct-013-030.html

    Thanks for your suggestion regarding op-amps. My measurement may extend upto maximum of 100A.

    BTW, do you know any software application where I could try to simulate my circuit?

    Thanks,
    Lili
     
  9. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    It looks like both links are the same. That circuit delivers AC to the ADC input, which would require high sample frequencies, and may be unnecessary, depending on your exact goals. I would expect a smoothed DC output to be easier to work with, unless you want to see the actual waveform like they did in their project, in which case that circuit design looks reasonable to me.

    The datasheet says that sensor is only good for up to 30A. I don't know what happens if you try to sense more, but I'm guessing it's some sort of saturation effect. I think you'll need a different sensor to accurately measure 100A currents.

    LTspice is free and pretty useful. It seems to be what the majority of this forum's membership uses. I'm still quite new at it myself, but I think it would help a lot with designing a circuit like you need. The interface is a little awkward to me, so there's a bit of a learning curve, but there are lots of great online tutorials and forums that can help you get started.
     
    liliandreapinero likes this.
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,150
    206
    Define your goals. I'm not sure you want to just look at current? Are you looking at energy consumption of inductive loads?
    Then it's a whole new ball game.
     
  11. dad98253

    New Member

    Aug 28, 2015
    2
    0
     
  12. dad98253

    New Member

    Aug 28, 2015
    2
    0
    Lili,

    The SCT-013 is a series of current probes. The SCT-013-000 that is mentioned in the openenergymonitor.org article is a "current transformer" version and requires an external burden resistor to interface it to your voltage measuring circuit. The SCT-013-030 model that you mentioned in your first message is a "voltage transformer" type (most current probes marketed are of this type... ). The SCT-013-030 has a built in 62 Ohm resistor. The resistor is sized to give accurate (low distortion) measurement up to the indicated 30 Amp maximum. Beyond that, the core will begin to saturate and produce a distorted output voltage. All of the SCT-013 models utilize the same core design; the different current ratings are because each of the sub-models have a different size resistor. If you try to measure a 100 Amp load using your SCT-013-030, the output voltage will start to limit ("flat-top") beyond 1V output (30 Amp input), and you may even blow out the burden resistor. If you truly need to measure 100 Amp, you should use the SCT-013-100.

    I'm not familiar with the Telosb. But, I have played with the Arduino and am currently working on a project using the SCT-013-030 to monitor the current draw of the pumps in my home septic system. I suspect that the Telosb is similar to the Arduino and may have similar issues when reading analog voltages. My Arduino has 6 analog inputs which are mux'ed into a single ADC. I found that noise (EMI) generated by the Arduino digital circuitry is easily detected by the analog inputs and ADC. There are some programming tricks that can be used on the Arduino to minimize this noise and improve the accuracy of the analog measurements. The Telosb may require similar care in its programming. Without using these tricks, I was seeing about 100 mV rms of noise in worst case (high impedance) situations. In my case, I minimize the effect of this noise by throwing out a number of measurements after initialization and each time I switch the mux. I determined the number to discard experimentally. However, once I hook up my SCT-013-030, the lowered load impedance of the circuit should hopefully eliminate most of this noise.

    I expect that you will learn a lot building your project.

    Good luck,
    John
     
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