Emerson Exchange 365

Analog output load resistance and DVC valve positioners

The max load resistance for an AO CHARM is 750 ohm.  If I measure resistance across the terminals of a DVC positioner, I will get a meg or tens of megs of resistance.  The resistance will slowly decrease, perhaps to hundreds of kilo ohms as I keep the meter leads on the terminals.  My multimeter has a 9V battery, whereas a CHARM has a 24V power supply.  For that reason, perhaps the resistance drops below 750 ohm when a CHARM is connected to the valve.  I suppose it must be the case; otherwise, we wouldn't be able to drive any of our valves.  This behavior is making it difficult for me to troubleshoot bad signals.  Can anyone explain the circuitry at the DVC terminals, why I'm seeing this behavior and how a CHARM could possibly drive a valve with such high resistance?  Are there methods I can employ to troubleshoot the AO connection and determine whether or not a DVC positioner must be replaced?

Mark Bendele

• 9 Oct 2020 7:19 PM
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3 Replies

• Jeffrey Mach

  

  • 10 Oct 2020 3:25 AM

  I have seen similar on a different device. The device would provide a 4-20 mA signal out with a 24 VDC input, but the device needed a minimum voltage in to activate and if you tried to investigate the signal with anything other than an ammeter in line with the output, you would get very odd readings. It turned out that the device consumed a certain amount of power internally, and when the input voltage was too low, it would shut down including the output regulator circuit, so it would appear to have a high impedance. Other devices need a minimum receiver impedance on the line for them to function (this is really fun if the minimum is undocumented), and if you are not careful with your test setup, you will find a strangely malfunctioning device that appeared to work normally in the field.
  
  It never hurts to invest in a good process meter / calibrator like the Fluke 715, which can supply 24V power and be put in line in place of a 4-20 mA instrument to check out your control system input (don't get those wiring configurations confused). However, I would also recommend portable power supply and a good precision 250 Ohm resistor for rigging up a test in the proper configuration for the instrument to work. When in doubt, read the manual and find out how the instrument wants to be wired.

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• Jonas Berge

  

  • 10 Oct 2020 4:34 AM

  Generally speaking, smart positioners like DVC have semiconductor components like diodes and transistors in series and zeners in parallel with the loop so cannot be measured as a resistance with a multimeter. The way to engineer the current loop is to make sure that you have a budget of 11 V for the DVC. An AO CHARM has a 15 V output voltage swing ("compliance voltage") so you have a balance of 4 V for series loop resistance like the cable. Remember you don't need a 250 ohm resistor for AO. Some of this is explained in the essay found here:

  

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• Riyaz Ali

  

  • 10 Oct 2020 6:49 PM

  In reply to Jonas Berge:

  Thanks Jonas for your help. Would like to provide supplemental details for Mark on compliance voltage, sample calculation, shown on DVC6200 Instruction Manual D103605X012, Page 10. If you need further help, please contact your local Emerson office or Impact Partner. You can get IM copy through Emerson Website. If no luck, please send me email riyaz.ali@emerson.com and will be glad to share the copy.

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