Emerson Exchange 365

Delta V charms

Good morning, I have an S series AI charm that was just installed to read analog from a table top scale. it is terminated on 2 and 4. when it is landed on the charm, the mA drop from 4.2 to 0. I can remove the device and simulate 4-20 with my fluke and it is fine. I can also install an interposing device like a digital scale, then output from the digital scale and it reads fine, diagnostics shows open loop with just the scale, but with the interposing device it is closed loop and reads correct? any assistance would be greatly appreciated.

Tom Light

• 1 Sep 2023 11:57 AM
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7 Replies

• Ralph Kitts

  

  • 1 Sep 2023 1:03 PM

  Tom, is it possible your table top scale has an analog output that requires power? In other words, it is a 3-wire device as opposed to a 2-wire device?
  You can get power from a Fused Injected Power Terminal Block (pin 3). Or wire a +Vdc to your scale where it needs it, and connect this supply ground to your CHARM (pin 4).

  Ralph Kitts  

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• Tom Light

  

  • 1 Sep 2023 1:27 PM

  In reply to Ralph Kitts:

  thank you Ralph, I am looking into that right now.

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• Andre Dicaire

  

  • 1 Sep 2023 1:59 PM

  In reply to Tom Light:

  Is your scale a voltage out? AI uses load resistor and that would be in parallel with the scale output. Has to be a current out or a voltage driver, not a voltage drop from an internal resistance in the scale.

  Andre Dicaire

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• Tom Light

  

  • 1 Sep 2023 2:51 PM

  In reply to Andre Dicaire:

  the scale provides its own 18VDC source for the analog output circuit. the manufacturer recommends the output be used with a 100-150Ω resistor. but I would assume the internal resistance of the Charm would be suffice for this purpose. I spoke company rep and this has been an issue at other locations. he suggests an isolated ground. I don't see that helping but I could be wrong.

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• Ralph Kitts

  

  • 1 Sep 2023 2:58 PM

  In reply to Tom Light:

  Have you tried measuring the mA output of your scale with your Fluke meter?

  Ralph Kitts  

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• Andre Dicaire

  

  • 1 Sep 2023 3:00 PM

  AI has 250 ohm load to prove 1-5 v reference to instrument tech as a nonintrusive measurement. Note this is not a calibrated load R.

  Andre Dicaire

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• Gary Law

  

  • 5 Sep 2023 1:04 PM

  In reply to Andre Dicaire:

  It is possible the scale is not capable of driving 250 ohm load.
  An alternative would be to use a Voltage input CHARM and a 100 ohm resistor (since this is what manufacturer reccomends)

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