DavidH (Customer) asked a question.
So I'm currently working with the C0-12DRE-D Click PLC and I'm trying to simplify things for our customers when it comes to the 4-20mA outputs. Given the "strange" setup of the 4-20mA outputs on the Click series (sinking 4-20mA outputs), I'm trying to devise a way where the customer can easily connect the 4-20mA to their PLC or check the signal on their multimeter using only two wires.
I've designed a simple PCB and I was wondering if you guys could have a quick look and see if there are any potential problems(I have attached a couple of simplified diagrams). Since the I'm using the +24V power supply in my enclosure(the same one that is powering the Click PLC and everything else) and a 250Ω resistor that is soldered to the custom PCB, does this mean that the maximum load impedance on the customers PLC will now be reduced to around 650Ω?
I don't understand why you have the 250 ohm resistor. While the specs say 250 Ohm typical, there is no minimum impedance for a current loop, since the current is bein controlled (however for a voltage output there certainly is a minimum) Anyway, the "customer PLC" input card no doubt has a "typical" impedance anyway. You will need to be sure the "customer PLC" input is isolated or at least docent' mind having both pins above 0V.
So the reason for the 250 ohm resistor is simple. We have had several scenarios where the customer complains that the 4-20mA output is "not working". We know that it does work, they just haven't connected it properly to their PLC or data logger.
The simple solution here would be to connect an ammeter to the 4-20mA output and see if you are getting a signal(as shown in the second example). Without the 250Ω resistor this solution would not work. There needs to be some sort load in series with the ammeter in order to get a signal.
That being said, I was wondering if I should/could lower the resistor value? I just picked 250Ω since it translates to 1-5V across it with the whole 4-20mA range It is an easy range to work with in case the fuse in your multimeter is gone and you can only take a voltage measurement to test for a signal output.
You could add a C0-04DA-1. It is a sourcing 4-20mA output module.
I have thought about this, but given the fact that cabinet space is an issue and we are only monitoring a couple of variables (and only one output is required), I can't add an additional module.
The C0-12DD2E-1-D, C0-12DD1E-1-D, C0-12DRE-1-D, C0-12ARE-1-D have sourcing 4-20mA analog outputs. I believe C0-12DRE-1-D would drop in to replace your C0-12DRE-D
This is an option, however I like the C0-12DRE-D due to it allowing you to have both voltage or current analog inputs and outputs. I'm looking to standardize a design here so we don't have to order so many different parts(which the C0-12DRE-D gives us).
if the customer is having trouble shooting they do make clamp on amp meters that are designed for 4-20 ma loops
https://www.amazon.com/Fluke-Milliamp-Process-Clamp-Meter/dp/B000R81ARM/ref=sr_1_3?crid=836FD6Y0M5EJ&dchild=1&keywords=ma+clamp+meter&qid=1617728712&sprefix=clamp+on+ma+%2Caps%2C161&sr=8-3
This still doesn't solve the problem. As an example, we had a customer where everything was wired properly by our technician to the customers PLC. Customer's calls us a couple of weeks later(when they actually power up their equipment and ready to commission and complains that they are not getting any readings on their PLC).
It turns out that it was a programming issue on the customers PLC. Now unfortunately I didn't have the above proposed PCB where I could have asked the customer to disconnect the wires going to the PLC and measure the current directly(since there was no 250Ω resistor acting as a load).
Also asking the customer to get a special current meter like the one you proposed would be a nice option, but most technicians have a multimeter(and the moment they aren't measuring any current that is supposed to be coming out your device there must be something wrong with the equipment in their mind).