4-20mA Signal from Level Transmiter

Sir.

Thanks for the information and support. One more point! If I use multi core (3 sets twisted pairs shielded and armured) cable can I use the one of the pairs for energising a 24VDC coil Relay to ON/OFF solenoid valve and one pair for getting the feedback. Will it effect the signals?

Warm regards,

A K Saseendran

Good practice for instrumentation is to never mix analog with discrete (on/off) signals. Sometimes, on/off devices with coils (like relays and solenoid valves) can produce voltage spikes that can affect your analog signals. The shield will offer some protection but don't push it. Use a different cable for your on/off signals.

Quite; good answer.

Another possibility might be to increase the power supply voltage a little to get over the cable resistance, should length prove to be a marginal problem at higher currents.

Lower down, the Original Poster reveals that there are additional pairs available in this cable. A further option available to the Poster would be to wire these pairs in parallel with the original circuit, to further reduce the loop resistance.

Yet another option would be to run the cable using larger diameter conductors.

Yet another option would be to add a 'current mirror' unit at intervals along the loop, which provide a new voltage source and a 1:1 relationship between input and output.

'All roads lead to Rome'.

No problem if your loop resistance doesn't exceed the manufacturer's recommended specs. On a 20mA current, your voltage drop will be negligable, calculated at typically a 24vdc source. Figure your voltage drop at 20volts to give yourself some headroom. My experience with analog signals is that they go nuts below 19vdc, depending on the system you are using.

Have you thought about using a wireless option? Check out www.remotecontroltech.com and look at the Wireless Switch System 151 Mhz Digital and Analog system.

Another option would be to use optical fiber.

Hi A K S,

If you are worried about the distance, besides the above suggestions, you may check first if it affects your controller performance due to the resistance of your wire. Have a 4-20 ma current source.This instruments is standard when calibrating instruments and controllers.

Feed your 4-20 ma at the end of the line (at the level controller side) then check your controller it should read the following, 4 ma at 0 level or 0%, 8 at 25%, 12 at 50%, 16 at 75% and 20 at 100% respectively, if this result is fine, you have no reason to worry.

If this result is not good, you may opt to use transmitters, repeaters or converters. this are DC to DC, this are available in the market.

I forget to mention, please also calibrate your controller and level transmitter, to have an optimum response of your system. And if still in doubt, ask for professional help in your area.

good luck!