Hello, I am installing some load cells to a system. 8 load cells and 8 amplifiers, I am trying to calculate the dB drop to determine how long of runs I can have without too much issue. We will be using 4-20mA signals to monitor the weights using a RIO. I will add the data sheets for the devices I am using and hopefully, someone can give me a walk-through. Basically, I need to know what my max length of a run could be. Amplifiers can support up to 6ft from the load cell. Plan on using 22AWG Twisted Pair wiring for signal and powering.

24v Power supply:
Mean Well NDR-120-24 AC/DC 24V 5A 120W Industrial

RIO:
Stride IO terminates at 500R


Amplifers


Load Cell

 

With a 4-20mA system, voltage drops are basically ignored, since it's the current that carries the information.
So the length of wire you can run depends upon the maximum voltage the 4-20mA load-cell driver can generate and the resistance of the wire.

 

With a 4-20mA system, voltage drops are basically ignored, since it's the current that carries the information.
So the length of wire you can run depends upon the maximum voltage the 4-20mA load-cell driver can generate and the resistance of the wire.

Thank for the reply. Well, what would be the correct formula to use to calculate this? Or what is typically listed as "maximum voltage outputted'.

Do need to worry about voltage drop to some degree correct? I must power the amp.

 

Do need to worry about voltage drop to some degree correct? I must power the amp.

Yes.
What voltage can the sensor output that generates the 4-20mA signal, and what is the receiver signal voltage at 20mA?

 

1. 24±10%

2. Would be .02A x 500R = 10V ?

 

1. 24±10%

2. Would be .02A x 500R = 10V ?

Okay, if the sensor driver can generate 24V to drive 20mA and the receiver is a 500Ω resistor, then you can tolerate a 12V drop in the line when carrying 20mA.
Using Ohm's law, you can calculate the wire drop by multiplying twice the wire length (round trip) times its resistance per unit length times 20mA.

 

So check my math here I have the resistance of a 100ft of TPS 22awg wire being 3.2R assuming a 100ft (round trip) I will have my dB loss be around .0232 dB. Does this seem correct to you? Either way, I will be fine at some pretty long lengths. Maybe I am forgetting the resistance from the load cell itself in this situation.

 

Forget dB, that is not a useful unit here. We are looking at voltage drop, which is just resistance x current.

3.2 x 0.020 = 0.064V

Which is not even close to the 12V that would be a problem.

Do you understand that the 4-20 mA interface is automatically adjusting for voltage drop on the wiring? And that there is a limit to how much to can tolerate?

Up to that limit, there is no reduction in signal strength. You are nowhere close to that limit.