Switching 2*4-20mA to one AI

[Lancie1]

NW, he is going to install another analog input, with no relays needed.

I told the customer that I will not go to the former Soviet republic...and then it was no problem..........

 

[nwboson]

Thanks for your input, Lancie1. I did get that the OP has successfully sold the optimal solution we would all usually prefer. However, since these threads do last forever, I thought a reply about increasing multiplexed IO reliability using bifurcated relays in low current/low voltage applications might be helpful.

If, however, you are suggesting we should quit posting as a matter of forum protocol once an OP has settled on a solution, I'm OK with that. I was not aware of such forum protocol - as an engineer, I can be socially dense

I was just hoping to be helpful to future folks that do a search before they post.

 

[Lancie1]

No, that is perfectly okay with me. Keep on posting!

 

[shooter]

However i do use switching relays (no fast logging system, i switch relays over the 250 ohms resistor, this way the current signal is not disturbed, and yes i have to wait until the module is ready with conversion. Just after the input is done, i switch, then i go out to do other things and on polling a timer i go to next channel etc.
It runs every hour one full cycle. or on demand.

 

[Saffa]

I've got two of these at a plant: http://www.intech.co.nz/products/remotestations/2100-m.html

Bit over-kill for only 2 analogs into one though.

They are very reliable units.

 

[Aabeck]

Just a quick question:

If you were to switch between 2 signals with a DPDT relay, wouldn't that cause an open circuit in the unused signal?

I would think a 4 pole relay would be needed to close a contact between the 2 wires of the unused signal, so it still could be tested & calibrated when not used.

 

[shooter]

@aabeck yes it would if you need to switch the mA, however i switch the voltage signal and that is no problem.
I do the same with Pt100 (however tyhen i first close a relay and then open the previous one)

 

[Lancie1]

I would think a 4 pole relay would be needed...

If the signals have a commom (-) wire, then a DPDT relay will work also, with 1 pole relay common for the PLC signal, the other pole common for a 250 Ohm dummy load. Each time the relays switches, one (+) signal (out of 2) is switched to the dummy load.