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4-20ma sensor — Parallax Forums

4-20ma sensor

SignalsSignals Posts: 3
edited 2007-10-11 02:03 in BASIC Stamp
Hi from newbie

I want to adapt stamp projects to existing 4-20ma equipment. At first I will not be
able to "connect" to the circuits. I will need to sense the current via a clamp on
type of device. Any ideas or products that can clamped or wrapped around a wire that
is conducting a 4-20ma signal for use with the Basic Stamp?

TIA

Comments

  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2007-08-30 05:35
    Signals

    This might be something to looking into


    http://www.fieldpiece.com/accessory-heads.html·· Almost at the bottom of the page

    Measures milliamps and microamps for flame diode test. Measures 4 to 20mA control circuits.

    Includes flame diode adapter kit (AQK3).

    Resolution: 0.1 for meters with 0.1mVDC.

    Accuracy:
    DC: ±(1.0%rdg + 1dgt)
    AC: ±(1.5% + 2dgts).

    Max current: 200mA
    AC or DC



    . I do not know of any heads that are for meters that will work well enough for what you want to maybe some one on the forum may know of one

    I hope this will help you

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    ··Thanks for any·idea.gif·that you may have and all of your time finding them

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    Sam
  • OzStampOzStamp Posts: 377
    edited 2007-08-30 06:33
    Hi

    Buy a precision 250 Ohm resistor .. 4-20MA thru this to GND converts it to 1-5V
    Measure this with a A/D converter..
    cheers
    Ronald Nollet Australia
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-08-30 13:52
    Signals are magnetically coupled (via a 'clamp-on device') only when the current changes. This works really well for 60 Hz AC circuits because the current is changing all the time, or for a "Timing-Light" on your car engine because the current to the spark plugs also jumps at a particular point in time.

    For monitoring continuous current, it's not too good. If you DID clamp on a device (or even wrap a coil of wire around another wire) that device would only get a signal when the 4 to 20 mA signal changed. I'm not sure that's good enough to reconstruct the signal -- but I could be mistaken about that.
  • cyplesmacyplesma Posts: 76
    edited 2007-08-30 18:08
    there are dc clamp on amp meters, but I don't know how they work.

    I typed the above before I checked out that web site listed above

    Post Edited (cyplesma) : 8/30/2007 6:30:28 PM GMT
  • SignalsSignals Posts: 3
    edited 2007-08-30 19:05
    I think Allenlane5 is right that a clamp or wire loop will not work on a DC wire.
    I have too many dead brain cells (almost 60).

    Have to figure out how to get the owner to let me add a pair of jumpers to the
    chart recorder (driven by 4-20ma) and use voltage at the wire nuts for the Basic Stamp.

    My next question

    Would using a set of jumpers across the 4-20ma lines at the chart recorder change the values on the chart recorder? I am sure I would not be allowed to hook up if the values change or if I had to re-calibrate things.

    I assume I will have to use a ADC chip and the current requirement will extremely low.


    TIA
  • cyplesmacyplesma Posts: 76
    edited 2007-08-31 00:43
    I'm almost 100% sure it'll need recalibrating if you do a direct connection.

    but I also think to help any further need to know what type of device your trying to tap signals from
  • FerretMK4FerretMK4 Posts: 11
    edited 2007-08-31 00:57
    You can't connect in parallel with the current loop and get a reading of the signal: you'll get the system voltage. You have to put a resistor in series and read the voltage drop. If you use a small resistor it won't drive the system voltage up too high, but your voltage measurement will of course be small and you'll need to use an instrumentation amplifier (full differential) to feed your data acquisition.
    Current loop systems maintain a given current for a given input (that's their calibration) but there is a limit on the driving voltage that forces the current (it will be maximum at 20 ma). This is one reason why current loops are used instead of voltage levels... you don't worry about line loss if the system can jack the voltage up to maintain the current through whatever the resistance of the lead wires is. The other reason is lower noise. However if you put in a large enough resistor to generate 5 volts across it at 20 ma, you're asking the system to boost its voltage by 5 volts to keep the current up and it's highly unlikely that the system will be able to do that.
    Adding a small resistor shouldn't affect the calibration at all.. it's just like adding some lead wire. Be sure to use full differential amps and be careful not to introduce noise.
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2007-08-31 16:56
    ·cyplesma

    There are dc clamp on amp meters, but I don't know how they work


    There are dc clamp but for a good one you hve spend a lot of money
    and they are hard to find


    Here you find one

    ·http://us.fluke.com/usen/products/category.htm?category=CLMP(FlukeProducts)&parent=CLMP(FlukeProducts)&PK=CLM&gclid=COmfroWcoI4CFSMOgQodkjwMTw

    Here one more

    http://www.extech.com/instrument/categories/clampon/subcategory/acdcClamp.html

    http://www.multimeterwarehouse.com/clamp.htm

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    ··Thanks for any·idea.gif·that you may have and all of your time finding them

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    Sam

    Post Edited (sam_sam_sam) : 8/31/2007 5:13:11 PM GMT
  • TechnoRobboTechnoRobbo Posts: 323
    edited 2007-08-31 22:26
    why don't you split the signal and read it?
    http://www.api-usa.com/pdf/api/api4393dinl1.pdf

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    Have Fun


    TR
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2007-09-02 22:34
    TechnoRobbo

    · Thanks for sharing this i did not know that there was such a thing
    split the signal and read it

    That is something to keep in mind when you need·to do more than one·thing with a input signal

    Signals

    I do not see any reason·why the chart recorder would work with this·

    This a Single 4-20 mA Input & Dual 4-20 mA Outputs unit· Send One Process Signal to Two Locations

    S
    Single 4-20 mA Input & Dual 4-20 mA Outputs
    ingle 4-20 mA Input & Dual 4-20 mA Outputs
    S
    Single 4-20 mA Input & Dual 4-20 mA Outputs
    ingle 4-20 mA Input & Dual 4-20 mA Outputs
    DC to DC Signal Splitter, Isolated
    DC to DC Signal Splitter, Isolated


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    ··Thanks for any·idea.gif·that you may have and all of your time finding them

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    Sam

    Post Edited (sam_sam_sam) : 9/2/2007 10:57:00 PM GMT
  • Philip GamblinPhilip Gamblin Posts: 202
    edited 2007-09-08 02:26
    Fluke recently introduced a task specific 4-20mA clamp on meter. Very accurate ...$300-$400... very pricey.I'd go with the 250 ohm loop resistor, myself.
  • SignalsSignals Posts: 3
    edited 2007-09-25 05:06
    Thanks FerritMK4

    Your answer was the closest I wanted. I want to build something to read a 4-20ma signal and interface it to a Basic Stamp (via ADC)
    and for a couple of dollars, not $400 like some replies.

    I want to use a Basic Stamp to read the output of a chlorine analyzer - Hach CL17. I agree with

    FerritMK4 -->

    --"you're asking the system to boost its voltage by 5 volts to keep the current up and it's highly unlikely that the system will be able to do that.

    I believe the max the chart recorder goes is 10 mg/L but the high alarm is 4.0 mg/L and that is the max I would have to read.
    (the chlorine pumps could not dose higher than this)
    The low alarm is 1.0 mg/L
    and that is more important (this is for drinking water treatment plant- the water has to have some chlorine in it).
    So the chart goes from 0-10 but all I should need to read is 0-4 mg/L.

    SO lets say I read 0-5 mg/L that should be 4-10ma.

    --"Adding a small resistor shouldn't affect the calibration at all.. it's just like adding some lead wire. "

    What voltage range for 4-10ma would work for a ..

    --"use full differential amps and be careful not to introduce noise."

    Do you mean differential op amp ? (I do not have experience with these) and can you recommend a good one?

    Would .4 - 1.0 v drop be ok across the series resistor for the differential amp?

    Going to stop - getting myself confused.

    (FYI - the plant is in the southwest desert and no AC in an uninsulated building - ie very hot environment so I need to use good quality parts)

    TIA
  • Bryan K.Bryan K. Posts: 47
    edited 2007-10-11 02:03
    are u using the HART protocol with that? or just the 4-20 mA?
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