Voltage drop calculation to connect a 3 wire type transmitter to aPLC analog input.

azetou

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Mar 2010
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Algeria
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Hi,


Calculation of voltage drop of a 2 wire transmitter connected to plc AI is pretty simple and straight forward.


My Question is could you please share the method for voltage drop calculation for 3 wire transmitter with the power coming from the PLC. the purpose is to determine the maximum allowable cable length. And it would even better if you guys share a reference for the method, i.e ; A standard , guide , book, article ... etc.


Thanks in advance:site:
 
if you are talking about a current output transmitter ( for example 4-20 mA) then the the resistance of the cables does not really matter, the only condition is that the resistance of the loop wires plus the resistance in the AI (for example 250 ohm) is not greater than the maximum resistance of the load specified in the specs of the transmitter.

And in the case of a 3-wire transmitter, the voltage of the power supply minus the voltage drop across the supply wires (+ & - wires) should be greater than the minimum supply voltage of the transmitter, with the maximum current consumption also specified in the transmitter characteristics.
 
Last edited:
Thank you for your reply and yes I was talking about 4-20 mA transmitter.

Is there any reference for these kind of calculation like a standard or somthing ?
 
Its Ohms Law. You should be able to get resistance per km of wire from your cable supplier. Unless you're pulling more than a km or two of wire I wouldn't even worry. Or if your 3 wire device pulls an amp or more excluding the 4-20mA current.

Get the specs on your cable, the length of the run and the power consumption of your device and I'll show you my working. If you draw it out with the wires depicted as resistors I'm sure you'll intuitively be able to solve for max cable length
 
I recently had a 3 wire 0-10V device with about 1A. (0V, 24V, 0-10V). Allowed a 0.5V drop on the wire, but that threw my 0-10V out by 0.25V. If it was 2 or 3 wire current it would have been fine, or 4 wire voltage should have been fine also.

It didn't contribute much to your question, but just a rambling anecdote for you :)
 

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