Voltage Drop Concern
- Thread starter jaykool
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- Illinois
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- retired electrician
This appears to be some type of instrument circuit. The manual for the device will specify that it can deliver the 4-20mA signal into a specific resistance. If you exceed that resistance then the device will not be able to supply the full 20mA. With many instruments the maximum resistance is 750 ohms, but that included the "receiving" device, often 250 ohms.
As Don stated this appears to be a telemetry circuit of some type. The limit is a function of the supply voltage and current. In this case the maximum loop resistance including the receiver is 1200 ohms (24/.02).
So the gauge of the wire will have a huge influence on the distance that 20 ma can be transmitted with a 24 VDC source supply.
OK with that said I will make you feel a little better. Let?s assume your receiver resistance is 250 ohms. So 1200-250 = 950 ohms maximum loop resistance. We want to know the one-way distance in ohms so we take 950/2 = 425 ohms.
So if you were to use 22 AWG wire has a resistance of .019 ohms per linear foot. So the maximum distance using 22 AWG wire @ 425 ohms = 425/.019 = 25,000 feet, or about 5 miles.
So the gauge of the wire will have a huge influence on the distance that 20 ma can be transmitted with a 24 VDC source supply.
OK with that said I will make you feel a little better. Let?s assume your receiver resistance is 250 ohms. So 1200-250 = 950 ohms maximum loop resistance. We want to know the one-way distance in ohms so we take 950/2 = 425 ohms.
So if you were to use 22 AWG wire has a resistance of .019 ohms per linear foot. So the maximum distance using 22 AWG wire @ 425 ohms = 425/.019 = 25,000 feet, or about 5 miles.
Sounds good...in addition, Ive had quite a bit of experience with pressure and temperature transmitters that utilized a 24vdc 4-20loop. These installations were very long runs in cable tray,mostly 14awg some 16awg, probably in that same 1500ft range. In these cases we found very few cases where the voltage would drop was a issue because the transmitter impedance was such a small small load. So with a quick calibration telling the intstrument where
"0" (4ma) and "Span" 100% or 20ma the loop worked fine
"0" (4ma) and "Span" 100% or 20ma the loop worked fine
cadpoint
Senior Member
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- Durham, NC
I know I don't know telephone terms or much of DC Work.
I'll never catch up...
Telemetric, loops, Why the use of these terms?
Its DC, your firing a signal down a line, and one wants the same signal to be the same at the end of the line.
I frankly got stuck on dereckbc example with these terms getting thrown in.
It is a great example just the same.
Theres usually a circuit for both transmit and a circuit for receive... thats a telephone, or is this slang, and in fact a loop, in the DC sense ?
:-?
I'll never catch up...
Telemetric, loops, Why the use of these terms?
Its DC, your firing a signal down a line, and one wants the same signal to be the same at the end of the line.
I frankly got stuck on dereckbc example with these terms getting thrown in.
It is a great example just the same.
Theres usually a circuit for both transmit and a circuit for receive... thats a telephone, or is this slang, and in fact a loop, in the DC sense ?
:-?
You could be. Not enough information.jaykool said:
Distance is not insurmountable. You're not stuck with 24V - no matter what they told you. If 24V is not enough 36V and 42V power supplies are available, and bigger wire is available
These circuits are a series loop. the voltage at the end of the loop really doesn't matter much - but there are some limits.
The receiver end is generally a 250 ohm resistor - develops 1V-5V.
Then there is the Vd of the interconnecting wire.
Next issue is the compliance of the transmitter. Consider the transmiter a current source. It is designed such that when the input parameter is 0% the output is 4ma. And when the input parameter is 100% the output is 20ma. The transmitter will drop whatever voltage across it that is needs to to limit the current to the value it is susposed to have. Compliance is a measure of how much voltage the transmiter can drop to hold the current. All can handle 24V, some quite a bit more.
Another issue is the minimum drop across the transmitter. Two wire transmitters use power from the current loop to run their electronics. These will have a minumum Vd - usually around 3V. Four wire transmitters have a separate pair to supply 24V to operate the transmitter electronics. On these the minimum drop is around 0.7V.
Get the specs for the receiver (probably 1V to 5V). Get the transmitter specs (compliance and minimum drop). Figure the wire Vd at 4ma and 20ma. See if the numbers add up the 24V. You are adding voltage drops around a loop and then seeing if the tramsmitter compliance is sufficient for the supply.
This is different than typical electrical work which is voltage source - not current source.
I read this over and it sounds overly complicated. Probably can slim it down with some more specific information on the application.
carl
carl
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