Limiting 4-20ma transmitters output to 20.5ma max.
Limiting 4-20ma transmitters output to 20.5ma max.
(OP)
Is there a way to limit the output of a pressure transmitter to not exceed 20.5ma output? Thanks Frank.
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Limiting 4-20ma transmitters output to 20.5ma max.
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RE: Limiting 4-20ma transmitters output to 20.5ma max.
RE: Limiting 4-20ma transmitters output to 20.5ma max.
RE: Limiting 4-20ma transmitters output to 20.5ma max.
a couple of starting points :
http://www.centralsemi.com/cld/index.html
http://www.vishay.com/docs/70195/70195.pdf
These only go to about 5 ma, so it might take a few in parallel. There used to be higher current parts.
Another approach is to create your own current limiting diode with a depletion mode fet and a single resistor.
DspDad
RE: Limiting 4-20ma transmitters output to 20.5ma max.
RE: Limiting 4-20ma transmitters output to 20.5ma max.
When the voltage reaches 5.1 V the shunt device will start to conduct. It will shunt pressure transmitter current in excess of 20.4 mA around the resistor. The spec for the "zener" should be 5.1 volt at a worst case current of (Pressure transmitter maximum current - 20.4mA) with "ideally" a zero ohm dynamic resistance.
Of course such a strategy will kill off your transmitter error warning!!!
RE: Limiting 4-20ma transmitters output to 20.5ma max.
It sounds like your DCS is displaying a reasonable response to an error - the PT signal going out of range. What you are proposing sounds, no offense, like a bodge rather than a proper solution.
RE: Limiting 4-20ma transmitters output to 20.5ma max.
RE: Limiting 4-20ma transmitters output to 20.5ma max.
This transmitter is used to control vacuum on a kettle. When the batch is complete the kettle gets pressured up 10 15psi which is well above its range of 1000 mmg. So thats why this discussion was started. If there are to many nusance "Bad transmitter errors" no one will pay attention to a real transmitter problem. On this process there are about a dozen of these issues. Thanks for all the ideas...
RE: Limiting 4-20ma transmitters output to 20.5ma max.
One solution we have used successfully on our DCS (Foxboro I/A Series) is two have two transmitters, one low range, and one high range. The transmitters have a degree of overlap.
In our application we can have a low flow condition which must be accurately monitored, but the process can also have flow levels an order of magnitude greater. The FT capable of handling the full-flow does not have the resolution for accurate control at the bottom end. We use a transfer block within the DCS logic which selects either the low range or high range FT depending on the flow. A suitable deadband on the transfer prevents excessive toggling between the two.
Basically, if the flow exceeds 95% of range on the low range FT, the measurement transfers to the high range FT. If the measurement falls below 5% of range of the high range FT, the measurement transfers to the low range FT. 95% on the low range FT equates to about 10% on the high range FT.
Hope this helps.