Extending 4 to 20 ma Signal
Extending 4 to 20 ma Signal
(OP)
I have a device that has a 4 to 20ma output. I need to have two remote meters that will receive this signal, One is 1/2 mile away, the other is 1 mile away. Does anyone have any suggestions on how this can be done?
Thanks.
Thanks.





RE: Extending 4 to 20 ma Signal
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RE: Extending 4 to 20 ma Signal
David Baird
mrbaird@hotmail.com
Sr Controls Designer
EET degree.
Journeyman Electrician.
RE: Extending 4 to 20 ma Signal
RE: Extending 4 to 20 ma Signal
If you use an AWG 18 conductor (1 sqmm), one mile will have somewhere between 50 and 100 ohms total resistance. Most 4-20 mA instruments are designed to handle at least 500 ohms.
So you should be home free. Use some overvoltage protection if your cable is between buildings and/or there is much lightning in the area.
Gunnar Englund
www.gke.org
RE: Extending 4 to 20 ma Signal
Thanks.
RE: Extending 4 to 20 ma Signal
It is a matter on how many volts are available at the source, if you add all the loads in the path at the max current. With Gunnar's exemple, if the total resistive load is at 500 ohms, and the source has 12 volts, there will be only two volts of play for all active devices and linearity problems will occur, as some devices may start saturating at their extremes. Some devices also use zeners to in the current loop in order to feed the sensor dirently from the loop. There is an additional voltage requirement for such devices.
Find what the requirements are for your input devices and what is the voltage available at the 4-20mA source.
As a corollary, once you have these numbers, you can choose to increase the available voltage instead of using a thicker gage wire. One mile of wire isn't cheap. It depends on what devices you have.
RE: Extending 4 to 20 ma Signal
1) A powersupply. (a voltage source)
2) The transducer. (with a voltage limit and a voltage drop burden )
3) The wire run. (with a voltage drop burden)
4) A meter. (with a voltage drop burden)
5) A meter. (with a voltage drop burden)
Your power supply must be able to supply enough voltage to exceed all the burdens and NOT exceed the transducer's maximum allowed voltage.
RE: Extending 4 to 20 ma Signal
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