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Help interfacing Current Transformers to ADC

Discussion in 'Electronic Design' started by beananimal, Sep 12, 2006.

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  1. beananimal

    beananimal Guest

    The Scenario:

    (20) 120VAC 5A and 10A devices. I would like to come up with a VERY
    CHEAP way to data log the power consumption of each device. I don't
    need exact power consumption with power factor correction or anything
    like that. I would gladly just deal with a reading of the amperage of
    each device and the assumed or measured voltage. I am more concerned
    about trends, not the real world kWH.

    I have found current transducers that provide a mV output that can be
    fed to an ADC, but certainly can not see buying 20 of them @ $200 or
    more each.

    I would like to use simple current transformers such as those shown in
    the following links: Both are Hall type sensors.
    or (the LTS 6 for example)

    The problem (as I understand it) is that the output of these devices
    can not be fed straight into an OP-AMP for scalling and then to the
    ADC. Is there a different approach I should be looking at?

    How exactly does one go about converting the AC signal from a current
    transformer into a DC signal usable by an ADC, and do it cheap enough
    to build 20 or more devices.

    The plan is to use the Dallas 1-wire 4 channel ADCs (DS2450 i think) so
    that I can toss this right onto my 1-wire network.

    I apologize if this is the wrong group.

    Thanks in advance!
  2. Joerg

    Joerg Guest

    beananimal wrote:

    A real name would be kind of nice...

    $200? Huh? Check again. Amveco, carried by Digikey, they call the
    category "current sense", $6.50 a pop for the 10A version. Then CR
    Magnetics and lots of others. If that's still too much you'd have to
    wind your own.

    Look up "ideal rectifiers" and stuff like that. Example:

    But you do need a power supply.

    It's the right group :)

    And never, ever, forget the burden resistor when doing this kind of
    stuff. I have seen lots of grief when that happened.
  3. John Larkin

    John Larkin Guest

    The low end here...

    is about $20 or so, as I recall. The output could be high enough to
    rectify, then go into an ADC to indicate average current.

    There are hall-effect ICs that are a couple of bucks, but need a pc
    board and such.

    There are probably also super-cheap, not very accurate CTs somewhere,
    or maybe you could use come stock torroidal power inductors at a
    couple of bucks each.

  4. Tim Wescott

    Tim Wescott Guest

    You can feed them into some ADCs and sample faster than 60Hz (1200Hz,
    say) and do some math to find the currents. This would require an
    all-up sampling rate of 24000Hz, which might require an expensive
    processor, unless you did them each in turn.

    You can feed them into ADCs and sample at a rational, but oddball
    fraction of 60Hz -- like 11/600ths, or 1.1Hz. Then when you've taken
    the right number of samples (11 in this case) you'll have a picture of
    the waveform and you can do some math to find the currents.

    If the idea of doing some math up front bores you, you can amplify the
    current, run it through a precision rectifier, low pass the result and
    sample that with an ADC -- you'll spend more on op amps and capacitors
    than you will on your microprocessor, but you can do it.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  5. Jim Thompson

    Jim Thompson Guest

    For AC why do you need a Hall device? Just use a current transformer
    into an OpAmp with gain and rectification.

    ...Jim Thompson
  6. I was in a computer store which has a plug in meter which reads and stores
    all of this data and more. I'm guessing $30 - $40 each.
  7. DJ Delorie

    DJ Delorie Guest

    If you have access to the AC voltage, say as part of your power
    supply, just use an opto to trigger on the zero crossing and use a
    timer off that to sample the ADC right at the peak.
  8. beananimal

    beananimal Guest

    I was refering to the prices I found for the DIN mount units with all
    the electronics built in and a mV signal ready for DAQ units.
    I will look into this.
    That will not be a problem, as I will have to supply power to the rest
    of the electronics on the project (some opamps, line drivers, and otehr
    Yes, I have read quite a bit about the dangers of an open secondary.
    Along with the rectifer and interfacing design help, I am sure I will
    need some burden resistor guidance!

    I will try and do some research before I ask any more questions with
    regards to the solution you have offered.
  9. beananimal

    beananimal Guest

    I am not opposed to doing some boards for this project (as the rest of
    the project needs a board anyway). My problem would be that I have no
    idea what particular IC to use or exactly how to use it (thus my delima
    in the first place).

    I am also not opposed to winding my own CTs if it is worth the trouble
    and not to involved. The fact that I have 20-30 of these to do would
    seemingly mean that I could save quite a bit of money winding my own
    CTs. However, I would again need some basic guidance and how-to info.
  10. beananimal

    beananimal Guest

    That is an interstesting solution, however I fear that 20 micros and
    the supporting electroinics would get somewhat pricey and take up a lot
    of space. A small Atmel or PIC for each CT maybe? Or a larger multi
    input unit for a bank fo CTs?
    The idea does not bore me, the problem is in the details and my lack of
    knowledge. I assume you are refering to an "ideal rectifier" followed
    by an op-amp and then an RC network as the low pass filter? I am game
    for either the micro or the second solution and would be willing to do
    either. The cheaper would be the best, as long as you guys can help
    with the details. Otherwise I may be as lost as when I started
  11. beananimal

    beananimal Guest

    If you have access to the AC voltage, say as part of your power
    Interesting, could you elaborate a little more on the details of how I
    would do this. I am familair with the zero crossing, and somewhat
    familair with using an opto to trigger on it. I am not sure how many
    supporting components each channel would take. Again, the key here is
    to get a basic power measurment for 20-30 devices in my DIY automation
    system and get those values to the user interface.

    Thanks again to everybody who has replied so far!!!
  12. Guest

    That is my thinking. I have some cheap doughnuts I got from Hosfelt or
    Allelectronics for a buck or so. I am not measuring but I do use them
    into my trusty old 324s for sensing if the motion lights are on or
    off. A couple amps through the doughnut will saturate the 324. I
    suppose you do some of that "engineering" stuff with feedback to get
    that down to a linear scale.
  13. For the size you need you could probably use a small, cheap audio
    transformer and use one piece of wire as a primary.
  14. John Perry

    John Perry Guest

    Since a CT is a pretty good current source for reasonable burden
    resistances, you really don't need precision rectifiers and all their
    baggage. A simple bridge will do fine -- especially since you say
    you're not doing measurement.
    Let's emphasize this one -- NEVER, EVER LEAVE THE SECONDARY OPEN!

    John Perry
  15. John  Larkin

    John Larkin Guest

    There's no need to sample fast or to alias the waveform. The object is
    to gather statistics on the waveform, not to reproduce it, so Nyquist
    doesn't apply; any old scattered bunch of samples will do. I sold many
    thousands of electric meters that did true RMS volts and amps, power,
    power factor, KWH, KVAH, all that stuff, 16 power channels with a 6803
    uP, sampling each channel at about 27 Hz.

    And it's easy to offset the sine waves roughly midscale on a unipolar
    ADC and software auto-zero the DC part out.

  16. beananimal

    beananimal Guest

    Since a CT is a pretty good current source for reasonable burden
    I am not sure what "baggage" comes with the precision rectifier but am
    doing this as much for learning purposes as I am to complete my
    automtion project (a learning experience in its own right). That
    op-amp circuit looked pretty simple, though 4 diodes is MUCH simpler
    and cheaper I guess. I suppose a rectifer package would be much better
    suited than 4 discrete diodes?
    I only hope to get a reasonably close picture of the power consumption
    of each device. I am looking to log more of a trend than the actual
    watts. I mean I would like the results to be resonably close... but I
    am nto going to lose sleep over a few percent.

    I kepe seeing these warnings and will heed them. I am just curious as
    to the damage that can be done without the burden. The measuring
    device will be a permantent part of each circuit. I don't suppose the
    rectifier is considered safe as the burden and a burden resistor is
    required for safety no matter what?
  17. You should carefully wire a shunt (burden) resistor across the secondary and
    use that to measure the current.

    An open secondary will try to achieve infinite voltage (approx). This is a
    bad thing. It won't achieve it but it'll be a very high voltage.
  18. I have used these with success as well. However, they are not very linear
    over a wide range with a bridge rectifier. It is actually fairly simple to
    shift the output AC from the burden resistor so it stays within the
    positive input range of an A/D on a PIC.

    Select the burden resistor so it produces 5 V P-P at maximum current, and
    tie one end to ground. Add a resistor about 20x that value to the A/D, and
    another of the same value to +5VDC, and the sine wave is centered on a 2.5
    VDC level. A simple software routine takes the absolute value, and you have
    a precision rectifier in PIC code, with only three resistors per input.
    Sample at about 300/sec and read average or true RMS.

    Another way to do this is with an active 2.5 VDC reference for one side of
    all the CTs. Use a burden resistor for 2.5 V P-P max, and a high value
    resistor (and cap) to protect the A/D inputs from spikes. You can get a PIC
    with at least 8 A/Ds for under $5.

    For a very sophisticated single IC solution, ST Micro's Electric Metering
    IC is very interesting. See:

  19. Tim Wescott

    Tim Wescott Guest

    Who said anything about 20 microprocessors? How about one, with an
    analog multiplexer going into one ADC? Or one ADC with a built-in
    "Cheaper" and "best" are always judgment calls, to be sorted out by
    free-market forces. If you're just starting out, "best" may well be
    what you can understand, duplicate and maintain.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  20. Joerg

    Joerg Guest

    beananimal wrote:

    The lower measurement range will be non-linear and somewhat temperature
    dependent with a plain old rectifier. Doesn't have to be a bridge BTW.
    As to rectifier packages I haven't used one in designs for at least ten
    years. Discretes are cheaper and with SMT machine stuffing being rather
    low cost there is typically not much of an advantage for a reduced parts
    Then a passive solution might suffice.
    A burden resistor is required for a CT. In the 50/60Hz world you might
    be somewhat safe if it fails because the core would saturate but it
    might fry the rectifier before that happens and then your electronics
    could think there is no current. When the core saturates it almost
    becomes a core-less inductor where its inductance drops to a very small

    In RF apps I have seen things like burnt up traces, transistors and
    diodes that blew off their plastic parts.
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