Extending 4 to 20 ma Signal

[Stanfi]

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.

 

[analogkid2digitalman]

I think that is within range of standard 4-20mA spec. If not, could use a 4-20ma to fiberoptic converter.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[dbaird]

Per ohms law I = E/R the obvious thing that will affect your 4-20mA is the extra resistance of the long run of wire. You may have to increase the conductor size to compensate for voltage drop/losses.

David Baird
mrbaird@hotmail.com
Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[dpc]

You can run 4-20 mA signals one mile and beyond. Since it is basically a dc signal, all you need to consider is the resistance of the wire used and the power requirements of the receiving device. As David mentioned, you may want to increase your wire size a bit. You can also increase the power supply voltage a little if you have control over that. Main concern is to keep the dc voltage at the transmitter or whatever active electronics you have within required voltage range.

 

[Skogsgurra]

No problems.

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

http://www.gke.org

 

[Stanfi]

Do you think 2 Input devices could be conneceted in the loop at this distance?

Thanks.

 

[felixc]

I understand it as two devices in series where you can read the current. Not as two sensors in series on the same twisted pair, right?

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.

 

[itsmoked]

Stanfi; You would have in your system.

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.

 

[ScottyUK]

Convert the current loop signal to a frequency signal, and reconvert at remote end. There are commercial devices which use voice-frequency tones which are intended for remote telemetry over either a leased line or the PSTN; you have very long range capability over low-cost light-gauge wiring. This technology is probably less common than it once was in these days of digital data transmission, but you could build your own (if you keep off the PSTN - there are all sorts of regulations about what you can and can't connect to it).


----------------------------------

If we learn from our mistakes,
I'm getting a great education!