suggestion for HW redundancy in current loop

mikas_m

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Join Date
Feb 2007
Location
ck
Posts
154
Hello to all,

I have one application where I created a simple control loop which works very well. Now, I want to increase the availability in the following way. I already have two transmitters that measure the same process variable. One transmitter is used for contrl loop other is used for some signalization purpose. I have only one available analog input for control loop in one microcontroller.

Now I wonder if there is some kind of device that behaves like redundant source of mA so I can use signals from both transmitters to increase availability of the control loop in case one transmitter fails.

What would you suggest? I have already two mA signals but one analog input. It would be ideal if this device takes two current signals and provide only one on its output. In case one signals goes out of the range, it continues to forward the other correct mA signal to the controller. It's something like 1 out of 2 logic in HW.

Thank you!
 
Thank you Mickey!

I'm not sure about this Chinese solution. I wonder if phoenix makes something like this.
 
simplicity: use two diodes in the voltage line that is convert the mA to Voltage with a resistor. if one of them fails the voltage will drop however the diode will keep the voltage up from other input.
 
Shooter,

Is this what you propose?

2_xmtr_1_load_diode_isolation.jpg
 
Ooops, I got my diodes reverse biased in the diagram. They'd have to the other way for current to flow at all.
 
Ooops, I got my diodes reverse biased in the diagram. They'd have to the other way for current to flow at all.

I think that it can't work. Current is now sum of both at resistor point so also voltage is wrong. You need change both currents first to volts and then connect with diodes to voltage input?
 
I would use a relay with NC contact to 1 transmitter and NO contact to 2 transmitter, the C contact to analog IN and have the controller switch the relay
 

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