switch for analog signals


Thread Starter


I am looking for a good low cost way to select one 4-20mA signal from three to send to a electronic controller. I have a heat cool loop taking care of three jacketed vessels, each with their own RTD transmitters. Is a simple switch OK? Is there concern with surges damaging equipment? Is there a DIN type commercial product available?

Audun Alvestad

Hi there! What you need is a multiplexer unit. I have developed DIN mounted unit but it's for 8 or 16 channels input to one output, and with signal converter (if you need). It fully digital with no mechanical relays that disturbes the signal (and uses extra power). These will be commercially available soon. But for now they are "specially handmade". To choose which channel to "lead through" I just use 4 DO 24DC from the PLC. It is also important to think about if you need galvanic isolation or not. They are a bit overkill for your application maybe but they will same your system for analogue inputs.

Harry Wilson

Yes there is a din rail mounted device that will select one of four analog signals to send to a controller. Please see attached or contact M-System in Dallas (800/544-3181). Regards, H. Wilson [email protected]

Al Pawlowski

A switch should be OK, but there are a few things to consider. There will be a "bump" when the signal is switched. Its magnitude will depend upon controller adjustments (tuning, input filtering) and the difference between the new transmitter reading and the old. You may want/need to provide separate power for the un-selected transmitters if they are 2-wire types. A transmitter that can be damaged by open circuit to loaded switching is not unheard of.