Do PLCs alter the amperage at all?

BigJon1023

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I have a Meter putting out 4.2XmA but as soon as I connect it to the PLC it drops down to 3.7x. However the PLC still shows a number when I would assume it should be a 0 if it is below 4mA.
 
No, current inputs do not 'load' current outputs.

If you were attempting to drive current through a higher resistance than the analog output of the field device is capable of (insufficient drive voltage), then current output might decrease. But that's not very likely if the field device is really a current output and the input is really a current input.

What is more likely is that the potential (voltage) difference between the field device ground and the PLC AI input ground is probably creating a ground loop. Some of the current goes through the AI, other current goes through the ground path.

It usually takes a loop isolator (4-20mA in/4-20mA out) to solve the latter issue.
 
BigJohn1023 said:
However the PLC still shows a number when I would assume it should be a 0 if it is below 4mA.
Generally, no. To give you a more specific answer, we'd need to know part numbers of your analog input card, but most analog input cards that handle 4-20mA actually read somewhere in the range of 0-21mA. That's deliberate - it's to take advantage of the "raised zero" inherent in a 4-20mA signal.

Imagine you have a 0-10V analog input. Your input is zero. Is the process just at minimum scale, or has your transmitter failed, or had a wire break? You can't tell. If it's the latter, nobody will know until they deduce it from secondary or tertiary issues that arise out of the initial problem. Likewise, what if your analog input indicates full scale (10V)? Is the process variable actually at 100%, or has someone shorted 24V to your input? Or has the sensor failed to a high-scale position? Same thing again - you have no way of knowing.

On the other hand, consider a 4-20mA sensor, wired into an input that measures 0-21mA. Your input is 4mA. You know the sensor is healthy, and the process variable is at 0%. If your input is 3.5mA, you know that your sensor is connected, but has probably gone to a fault condition (as many sensors do when they fail). So you have a prompt to go and look in the right place to start with. If your input is zero, chances are you have a wiring problem, or your sensor is completely kaput. Likewise, at the other end of the scale, if you're getting 20mA back you know that the sensor is healthy and your process variable is at 100%. If you have 21mA, again, your sensor has probably failed, and is set to fail to 21mA.

tldr; if your analog input can measure from 0-21mA you can to a large degree distinguish between normal operation, abnormal operation, and a wiring problem within the PLC. You just need to scale the input from 4mA to 20mA as 0-100%, and then any readings outside that range are cause for concern.
 
Is the PLC programmed in RS Logix 5000? If so, look at the default engineer units in the module Properties > Configuration tab. I had a similar issue the other day with an 1734-IE2C, when I was outputting a measured 4mA from the transducer, but the controller tag was ~37xx. The defaults for Low and High engineering units are 3277 and 16383. Changing engineering limits to 4000 to 20000 fixed the issue. My understanding is the default limits are based off the resolution of the Analog to Digital Converter in the analog module.
 
I have a Meter putting out 4.2XmA but as soon as I connect it to the PLC it drops down to 3.7x. However the PLC still shows a number when I would assume it should be a 0 if it is below 4mA.

What is the meter connected to when it is not connected to the PLC, how is the current loop completed?

We can give you a better answer if we have more information about what you are connected to, including which PLC card.
 
I had a customer that was having a problem troubleshooting a 4-20mA signal, no matter what he did his meter inline only showed 6.5mA when the analog output should have been giving full output.

I disconnected his meter & put mine inline & got 20mA, then put his meter inline with mine & the signal on both meters dropped to 6.5mA. His meter (actually a $3000 mA/mV calibrator) was restricting the current flow internally.
 

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