Good morning all. I've searched for this topic and have found some pretty in-depth answers, but I can't seem to wrap my head around them and I'm still searching for some clarification. The actuator I'm hoping to control isn't actually in my possession yet, I'm just trying to be as prepared as possible when it finally does arrive.
I've ordered a Siemens SKD62UA proportional valve actuator that I'm hoping to control with a 4-20 mA output from the Arduino (I'm using a Mega Board). I have a separate power source for the unit (24 VAC). I've been successful with reading a 4-20 mA reading from a pressure sensor by applying a 250 ohm resistance to the signal headed for the analog pin. My main question is whether or not I can apply a 250 ohm resistance to a signal from an analogWrite command to control the actuator.
I've seen people mention using an op-amp, but I'm wondering if that is necessary since the unit will be powered separately and only looking for the signal.
The whole system is actually a test stand for a water pump we're developing. We have a manual control system, but we need automatic controls for a continuous 2+ week test. I've had success with all other aspects, this is just the last piece of the puzzle. I'd be happy to share more once I'm in the office.
Thanks in advance, and I apologize for bringing up such a basic question again. I just couldn't apply what I was reading to the project I'm working on.
A voltage is required before current can flow. Check the idle voltage of your valve controller input, and if there is none you have to provide your own voltage. Then add a switchable constant current circuit that supports 20mA in ON state (PWM 100%), or use a ready made module. Insert a RC low pass filter between the Arduino PWM output and the current source input.
That depends on which Arduino you have (not provided in the OP).
analogWrite on most Arduinos provides a pulse width modulated signal not a 0-5V proportionial signal that you could use to drive a 4-20mA transceiver. (even if the arduino had a true analog output, it would not supply 20mA).
So analogWrite will not work. You need a DAC to provide the 0-5V signal, which you can then use to drive an opamp to supply the 4-20mA signal.
Hi,
To generate a 4 - 20mA signal you cannot just use a 0 to 5V source.
The load can be any resistance, so there will have to be current feedback to the Arduino to just adjust for leads and load characteristics.
Most industrial current loop systems use 24Vdc potential as standard loop voltage.
In my opinion you should have 0 to 5V from a DAC on the Arduino and purchase a level converter, 0 to 5V to 4 to 20mA.
https://www.ebay.com/itm/Converter-Voltage-to-Current-0-5V-to-0-20mA-Voltage-Current-Convert-module-/182644785707
google voltage to current converter module
Tom... 
How hard is it to filter PWM?
I remember enough from turning full-wave rectified AC into almost flat DC that it should be possible. You define limits and color inside the lines.
Thanks for the input, everyone. Seems like there's more than one way to tackle this. I'll mess around and see what sticks.
Thanks again!