Adjustabel resistance

Hi,

I have a analog output signal 4-20mA for controlling a propertional valve. I want to limit the current, so if the I/O card get broken og software error etc... the mA can never goes under X mA...
We have eralier used a constant resistance in parallel, but I want to look for adjustable resistance?

Hello,

in my opinion you have some solutions, set point on HMI or if you have some analog input you can set it via Voltage/Current value. In my opinion pay attention to some filtering that analog input control value in condition, simply your value can oscilate around your set point, put proper condition when that value has really importance for you.

Hi,

I want these signal to be failsafe, it shall not lay on the software. Is I got software error or I/O card get broken det signal shall never goes under xmA....

My suggestion was to add a resistor in pralelel, these will keep the signal above xmA, but the customer don`t want a static resistor.

So I was thinking about a reistor who can be adusted by a srewdriver or something?

If you have other option are you welcome, but not any software solution :)

Hello gordonene,

if you want to have a "safety" system then you should use a signal conditioner like this 

https://www.phoenixcontact.com/online/portal/us?uri=pxc-oc-itemdetail:pid=2902028&library=usen&pcck=P-13-15-05-01&tab=1

At this part can you define the reaction of the output signal at wire breaking, overflow or underflow of the input signal.

Then have you at a PLC failure 4 mA at your propotional valve, so long the signal conditioner works fine.

I hope this is for you safety solution.

regards
CaOd

Hi thank you for advise reagardering these module.

I cannot find the documentation you can adjust the resistance? Lets say I want to force the loop to be 6mA minimum, so if the raw value in the software goes lower then 6mA the current to valve will not go under 6mA? But if the raw value in software goes over 6mA the current to valve shall increase. So you create low level outside the software, was my idea and you will have a failsafe system.

Hello,

please look at the attachment, there is a manual.

You connect this signal conditioner between your PLC and the proportional valve.
Then you can configure the signal conditioner.

You can configure your output signal if the input signal is lower than x.x mA (e.g. IN <4 mA then OUT = 4 mA). This is eplained in the manual.

If you have more questions, let me know (part list etc.)

regards
CaOd

Thank you for the information.

Just for double check, so  I don`t misunderstand something, the module can protect against under and overflow that is ok, but  can it be configurated so output not go under 8mA example?
So if the analog output from the PLC goes under 8mA the module vill not go under 8mA out to to prop valve?

These has to do be done with software and cable, cannot set the mA level by dip switches?

Or will the module only protect against under and over flow?

Hello gordonene,

you must configure the signal conditioner with cable or cell phone app, because you don't need a "standard" configuration.

You understand it right, if you configure the module if IN <8 mA then is the OUT = 8 mA.

The configuration e.eg. must look like this
INPUT: 
start 8.0 mA
end: 20.0 mA

OUTPUT:
start 8.0 mA
end: 20.0 mA

wire break or short circuit: 8.0 mA
underflow: 8.0 mA
overflow: 20.0 mA

Then you have never a current under 8.0 mA at your proportional valve.
Your signals are the follow
PLC - valve

0.0 mA - 8.0 mA (wire break)
4.0 mA - 8.0 mA (underflow)
12.0 mA - 12.0 mA
20 mA - 20 mA
24 mA - 20 mA (overflow)

I hope this help you for understanding the signal conditioner.

regards
CaOd