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4-20 mA Analog Output Wiring
How to drive one flow control valve from two different 4-20 mA signals
0 out of 1 members thought this post was helpful...

Hi All

I have two 4-20 mA signals coming from different controller. This 4-20 mA signal will drive flow control valve.

Please let me know how to drive one flow control valve from two different 4-20 mA signal.

I assume you need to switch between the two analog output signals, not combine them in some fashion.

Way back in the early 80's I had to do something like that using a couple DPDT relay (I seem to remember sourcing gold plated contacts which are supposed be best for low current 4-20mA signals) to switch one of two analog signals to a common load "output". There were issues with the opening of the current circuit during the time it took to switch. The analog sources didn't like momentary or sustained open circuits.

I found an old sketch with one of the DPDT relays used to switch the 4-20mA to a common load (the valve positioner in your case), the other DPDT relay to switch a shunt resistor into the 4-20mA circuit that is not connected to the load so that the outputs didn't drive operations crazy with output error codes, messages and faults.

If I had to do it today, any Process Automation Controller (PAC) has analog switch function blocks that can be used to toggle an analog signal to owherever it needs to be without the mechanics of electromechanical relays.

By Steve Myres on 14 October, 2017 - 11:09 pm

There are two ways around the open-circuit problem. You might be able to configure the 4-20ma load for voltage mode. Then leave a shunt resistor in each 4-20ma signal, so that whichever one is switched out still has its shunt. Or, you can put a zener and a resistor across the 4-20's, again on the source side of the isolation contacts, selected so as not to conduct when the load is in circuit. Then when the load becomes open circuit, the zener takes over and gives the 4-20 a path.

I like this answer the best, I have also used something similar when I ran out of analog inputs for a shoebox PLC I needed to monitor 6 wells on a dewatering project, each well transmitter was wired in series with a diode to ground. An 8 output relay module was connected to sample the Current across each diode in sequence. I could easily measure all 6 in a second but it was set much slower. This PLC had a 0.1 to 0.5 input range so there was no need for zeners, regular diodes were ok.

>There are two ways around the open-circuit problem. You
>might be able to configure the 4-20ma load for voltage mode.
>Then leave a shunt resistor in each 4-20ma signal, so that
>whichever one is switched out still has its shunt. Or, you
>can put a zener and a resistor across the 4-20's, again on
>the source side of the isolation contacts, selected so as
>not to conduct when the load is in circuit. Then when the
>load becomes open circuit, the zener takes over and gives
>the 4-20 a path.

Thanks!!!!

By Ikhtiander - on 14 October, 2017 - 9:37 pm
1 out of 1 members thought this post was helpful...

Hi Prakash,

if you need to combine two or more separate 2 wire (positive/sourcing leg and common leg) 4-20 mA analog output channels to drive one 4-20 mA actuator (control valve I/P), just simply combine all of common legs (also common leg of I/P) to one reference to make all of channels on same reference/potential. then combine all of positive legs; then connect to control valve I/P positive leg. So the amount of current flowing into the load (control valve I/P) will be total of mA current delivered by each analog output channel.

Another method by using SPST relay that the coil is controlled by Digital output channel so PLC should decide and select which mA output channel to be used. but In this case there may be a small bump to the I/P because of relay energisation time (10 msec).

Another idea is to use Acromag 892M DC Output Math Module, and set it up for "High Select" so this module will decide and select the channel with highest mA current.

By Bob Peterson on 14 October, 2017 - 10:34 pm

Perhaps you can have a relay that would switch to the signal that you want to control the valve with at a particular time.

Can you describe more as to why you have two control outputs going to the same valve? Are they redundant controllers?

thanks, i have two redundant controller.

If you use that Acromag 894 four channel math module, then you could probably use two channels for your control analog outputs and use the 3rd channel as an analog/fake discrete channel to switch between the control outputs.

That module uses a sort free-form math so something like:

A = channel 1 (control output 1)

B = Channel 2 (control output 2)

C = for a 0-10Vdc analog/fakediscrete input (put a DC power supply and a load resistor across the contacts of a relay and drive the coil with whatever switching logic is)

A statement on the order of IF(C<5.0) THEN(A) ELSE(B) (I'm guessing at both the syntax and the logic expressions).

>thanks, i have two redundant controller.

Prakash, it seems to me that you might need some sort of intelligence/logic/math to select which of your two redundant controller outputs to send to the valve. In that case, you might try to use a micro PLC with analog I/O capabilities and maybe a couple of discrete inputs in case you have malfunction outputs from your redundant controllers. Automation Direct has some reasonably priced micro PLC's. Just my opinion...

I am not sure totally of your set up, but maybe one of these 2 products may help.

Multi-port to Ethernet Converters
https://www.icpdas-usa.com/documents/I-7188E-ConvertersCutSheet.pdf

Or tSH series. For when you have 2 masters on 1 slave port.

https://www.icpdas-usa.com/search-result.php?searchproduct=tsh-


Hope these help you.

Ever heard of a High Select Relay?

They used to make these for the old pneumatic systems to select the 3-15 psi with the highest value. You just need the electronic equivalent.

Arrange it so the PLC that is off line sends it's signal to zero. If the power supplies are referenced to a common ground you might be able to use just a pair of diodes.

Here's an example where they don't have two control systems but do have two controllers - same idea.

http://www.contek-systems.co.uk/Documents/high_low_selects.htm