4-20mA control loop fault - Siemens PLC

Thread Starter

Dan Tucker

Joined Jul 1, 2017
I am installing a 4-20mA control loop using a Siemens Logo PLC driving a Kinetrol I/P actuator. I have used many of these pneumatic actuators over the years and I am familiar with its operation. This is the first time we are using the PLC setup to drive such an actuator. The actuator has been proven to work correctly and multiple have been tried with the Siemens PLC system.

When the PLC system was bench tested with only a fluke multimeter as the load we are capable of achieving 4-20mA.
When connected to the Kinetrol actuator the output will not drop below 5mA. We can achieve 20mA without issue. What can cause current to be "pulled up" in the way?

The Kinetrol actuator impedance is specified as 300 ohms. I have tested the PLC output with a resistor(s) making 570 ohms and I can achieve 4-20mA with no issues. On Monday I intend to increase this load until I can potentially replicate the issue.
So with a fluke with very low impedance and with a resistance greater than that specified for the actuator I can achieve the desired 4-20mA. This has a few of us confused and I am hoping an expert here may be able to point me in the right direction.

Thanks in advance.
Last edited:


Joined Sep 22, 2013
I'll give it a try. Not sure I'm following your observations. Never heard of using a meter that way with current loops.

I would suggest disconnecting the loop leads at the actuator location. Connect a 300 ohm resistor across the leads. I don't know your setup.....but have a buddy adjust PLC input......for a zero output on the loop. With a zero output on the loop......you should have 4 ma flowing thru resistor. 4 ma X 300 = 1.2 volts DC across resistor. Have your buddy adjust for 100% output. Now you should have 20 ma X 300 = 6 volts DC.

If you don't get those results........it could be the cabling or the PLC. But this will tell you if proper signal is getting to actuator. If not proper........

Disconnect loop leads at PLC. Connect 300 ohm resistor to PLC output and retest.

If proper...it's the cabling....if not....it's the PLC. If PLC and cabling ok......then bad actuator.

My procedure is just general and base on on info given. But you should get the idea.


Joined Feb 20, 2016
It does sound odd. If the PLC will run 4-20mA into 570R, just add a 270R in series with the actuator and see if that will work. There is no problem with leaving the resistor in line.
It maybe the electrical characteristics of the actuator is making the PLC current driver unstable, or the cabling is injecting noise and mucking up the driver....
Put the resistor at the current output of the PLC as it will help isolate the load a bit.
I'll be interested to see what you discover.

Thread Starter

Dan Tucker

Joined Jul 1, 2017
Thanks for the replies. Apologies for my initial statement, amazing how something can read so badly when you return, its been a long week.

BR-549 - Your first suggestion of removing the actuator and installing resistor(s) at the end of the cabling. This has been done using resistors from 250R up to 570R to cover any possible variation in the actuator impedance. The POT control of 0-10V into the PLC was varied across its full range which in turn gives me 4-20mA output. A good result. Add the actuator back in to the loop, 5-20mA.

Second suggestion, the resistors have been added directly to the PLC output and we have the same result, 4-20mA output. I too considered poor cabling and tried these variations in setup.
The actuator has been proven independently, I have however contacted the manufacturer for assistance in case there is something I have missed. A second actuator was also installed with the same 5-20mA characteristic.

dendad - I will try your suggestion of adding 270R to the loop and report back. However I have proven the circuit to work with resistors at 250R up to 570R, so I have limited hope, I will let you know how this goes.

If the actuator impedance was much higher than specified you would expect to see issues trying to achieve 20mA due to the voltage required to get there, it is being unable to achieve 4mA which has me puzzled! Any further help is welcomed and will be trialed on Monday.



Joined Sep 17, 2013
Is the actuator impedance purely resistive?
Is there perhaps a diode or other component in parallel for spike/back-emf protection?