PWM 4 to 20 mA Signal To Go To PLC


Thread Starter


I would like to connect a sensor outputting 4-20mA PWM signal to an existing PLC that can only accept analogue 0-10V or 4-20mA signals as well as digital inputs.

The sensor (dissolved oxygen) outputs a PWM 4-20mA signal. Recommended settings for sensor connections a sampling rate below 3.5 kHz, an averaging over more than 1 s, and the use of galvanically separated inputs to avoid oscillations.

I believe I will need to convert the PWM signal to an analogue 0-10V or 4-20mA signal for the PLC. Can this be achieved simply or what converters can be recommended.
It was not clear on what you meant on PWM 4-20 mA , I guess there are 2 possibilities :

1. You are talking about simplex current loop communication which has MARK or "1" represented by 20 mA and SPACE or "0" represented by 0 mA.


2. You are talking about superimposing digital communication on 4-20 mA physical layer (example HART protocol)

Please clarify on this first before we can continue our discussion.
It's been a while but I recall working with Endress-Hauser Flow Meters that used a pulse output (one pulse for x amount or weight of product) and the pulse time or width could be adjusted i.e. how long a pulse would stay on. With a relatively low flow and so fewer pulses you could set the pulse width longer or adjust the one pulse for x amount. As I recall we used a PLC with a counter programmed in to process the data into the needed information. You might need an input card capable of handling quickly cycling inputs if you want very accurate data counting (the card has to be able to handle the pulse width of the on signal. It does take some coordination between the meter configuration and the input device.

Dave Butler