Added resistor should be the remaining 'load' that the transmitter can put out. In other words, if your device that needs to see this is equivalent to a 250 ohm load and your transmitter can handle a 500 ohm load then put a 250 ohm resistor in series. Use a metal film resistor for a 0.5%-1% resistance tolerance. Also, this may require a little tweaking of the resistance value. Not sure how critical your circuit is but make sure if it is exposed (the resistor) to temp fluctuations that when the temp is high that the resistor does not drop to much voltage to make obtaining the 20mA level impossible. This is likely not a concern but should be considered.
Thanks for all the replies! This is a pressure transmitter sending a 4-20ma representing 0-1000mmHg. When the system get pressured up to 5 psig the full scale value exceedes the 22ma limit in the DCS system and reports a transmitter error. There is a 250 ohm resistor in parrellel with the 4-20ma making the input to the DCS 1 to 5V. How about a 5.1 zener in series with a 1000 ohm resistor to keep the input from going over 5 V?
Surely all you need is a 5.1 volt zener or an electronic precision shunt regulator across your 250R resistor.
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!!!
This is going to sound really obvious.... but why not re-range the PT so it remains in its linear range for all operating pressures?
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.
Just a zener will give you a non linear error near 20 ma. A amplified zener with an open collector op amp would be better. If you have voltage to spare, you could use a low voltage 3 term voltage regulator operating in current mode (2 wire connection) that was set at the upper current limit. 78L05, 2 bypass caps, 2 resistors is a pretty low parts count.
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...
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.
