PhaseShift
Senior Member
Lets say I have a 4-20mA loop where my PLC is looking at an input from a field transmitter. My PLC is supplying the 24V for the loop.
So as far as I can tell the 24V dropped across this loop will be dropped in 3places, the wire, the transmitter and the input. Assuming 20mA current flow then the drop in the wire will be small lets just say 2V and the drop at the input (usually 250 ohm resistor) will be 5V. Does this mean that the remaining 17V is dropped across the transmitter? If lets say there was only 4mA flowing, does this mean that 21V would drop across transmitter? Does the transmitter care how much voltage is dropped across it?
What about if my PLC was supplying an output to a device in the field? Now the output transmitter is located in my PLC with the field device most likely having the 250 ohm resistor? So for the same case above, with 20mA flowing, will the 17V drop across the output transmitter in my PLC card? So if I measure between the two analog wires in the field I would see less than 24V (maybe 5V or so) if this current is flowing? Obviously without current flowing I'd seel all 24V between wires in field?
So as far as I can tell the 24V dropped across this loop will be dropped in 3places, the wire, the transmitter and the input. Assuming 20mA current flow then the drop in the wire will be small lets just say 2V and the drop at the input (usually 250 ohm resistor) will be 5V. Does this mean that the remaining 17V is dropped across the transmitter? If lets say there was only 4mA flowing, does this mean that 21V would drop across transmitter? Does the transmitter care how much voltage is dropped across it?
What about if my PLC was supplying an output to a device in the field? Now the output transmitter is located in my PLC with the field device most likely having the 250 ohm resistor? So for the same case above, with 20mA flowing, will the 17V drop across the output transmitter in my PLC card? So if I measure between the two analog wires in the field I would see less than 24V (maybe 5V or so) if this current is flowing? Obviously without current flowing I'd seel all 24V between wires in field?