How much Voltage Across two Wires carrying 4-20mA current

[umery2k75]

Hello,
I was wondering, what is the voltage across the wire that is carring this much current from say differential pressure transmitter. In 4-20mA, what's the voltage. There should be some voltage, otherwise current can carry along, even PN junction needs few tenths of a volt to overcome the barrier and let the way for majority charge carrier in a semiconductor. Are there any standards that governs on this.

 

[FvM]

I guess, you are refering to the transducer rather than the wire voltage drop? The latter could be kept effectively
near to zero, although it's not reasonable of course. Two wire transducers are e.g. specifying a supply voltage range of 8..35 V.
this means, you have to calculate a transducer voltage drop of 8V in the supply budget.

There's an IEC standard (IEC 381-1) for current loop analog signals, but curiously no automation equipment manufacturer refers to it.

 

[umery2k75]

Re: How much Voltage Across two Wires carrying 4-20mA curren

Yes, it's about the transducer.You mean to say the voltage outputing from the transducer could have any voltage across it, provided the current remains in the range of 4-20mA and in practice, it's usually put close to zero? If voltage is put close to zero, then what about lengths in hundreds.Screen cable would definitely have a voltage drop. I have large length about 100meter Screened Cable length, just 2 Corex 0.75Sq.mm, see the schedule and the output signal was HART.



I'm attaching a ABB 600T Temperature transmitter manual just for a look, how it looks like, actually it's of pressure transmitter, but I can't find 600T instead I found 600T EN which is for pressure.The appearance is the same.

 

[FvM]

Re: How much Voltage Across two Wires carrying 4-20mA curren

You mean to say the voltage outputing from the transducer could have any voltage across it

The transducer is a current source, it can have "any" voltage acroos it, but it needs at least 8 V to operate. The cable resistance can be almost neglected, it's only about 5 ohm in your case.

 

[IanP]

Re: How much Voltage Across two Wires carrying 4-20mA curren

Voltage across transducer will never be zero, it needs some voltage to operate and that minimum voltage (in two-wire system) is ~8V, and converted to current 4mA ..
If the total voltage in the loop is below 8V (4-20mA) transducers won’t work ..
That's what it needs to "supply" itself ..

Put the wire resistance into a perspective ..
If, for argument's sake, the wire resistance is 100ohm (in your case it looks like below 10ohm) the voltage drop over the wires (in the worst case) is 20mA * 100ohm ~ 2Vdc ..
It's like deducting this 2V from the power supply voltage, as everything is in one loop ..

Everything becomes clear once the idea of a current source is well understood ..

See attached picture .. it’s self-explanatory ..
If you have more doubts goto:
**broken link removed**

Rgds,
IanP