120426-2012 EDT
bpk:
I do not know your parameters but suppose the following:
1. Internal to the automation system is a 50 ohm resistor for current sensing. At 20 MA this has a drop of 1 V. I will assume one input terminal is connected to the system chassis. So call it ground. The second input terminal will be at + 1 V relative to ground at 20 MA flowing into the second terminal..
2. Connect a 1200 ohm resistor to the second terminal. The power rating should be maybe 2 times I
max2 * 1200 or 1 W. The voltage to ground from the input end of the 1200 ohm resistor is 0.02 * 1250 = 25 V.
3. Connect your sensor from the input end of the 1200 resistor to ground. Then your sensor shunts current to ground instead of thru the system input.
4. Provide a constant current source of 24 MA to the input end of the 1200 ohm resistor.
5. When your sensor provides a 20 MA load 4 MA flows to the system input and the voltage across your sensor is 0.004 * 1250 = 5 V. When your sensor is a 4 MA load, then the system sees 20 MA, and the voltage across the sensor is 0.02 * 1250 = 25 V.
6. You can probably build a good constant current source with an LM317T regulator. See p 19 of
http://www.ti.com/lit/ds/symlink/lm117.pdf for a current regulator schematic. With 49.9 ohms for the current sensor I got about 23 MA. Using the National 1.2 ohms for 1 A I calculate 50 ohms for 24 MA, but from 49.9 and 23 MA in my experiment I would calculate about 47 ohms. This show you some of the variation from one sample part to a nominal part. For the string I am suggesting above I would use a 30 V DC source and heat sink the LM317. From about 3 V to 30 V across the regulator the current holds very constant.
i do not know if the 317 is still in production, but there should be other comparable parts.
Check my calculations.
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