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Old February 9th, 2019, 06:41 AM   #1
bodoo23
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Several Questions about Analog Input

I am new in automation industry and nowadays, I focus on learning analog input wiring with detailed since last time I damaged input module because of wrong wiring.
Here I have several questions about analog input wiring. They seems pointless to you but I need these answers to fully understand analog input concept. I would be appreciated if you answer these questions.

1- When using 2 or 3 wire 4-20ma current transmitter. Does analog output wire(4-20ma) always produce current with referenced to power supply common ? (I mean we can think this transmitter as a fixed voltage source and a variable resistor so the minus side of the voltage source is always linked to power supply common ?)

2- When using 2 wire 0-10v voltage transmitter, if the input module and transmitter is fed by same power supply. just wiring + side of transmitter to module will be enough (by wiring " - " of module and " - " of transmitter to common) ? I'd like to learn if the voltage produced by transmitter is always referenced to ground or can be any floating value.

3- The second question is valid for 4-20ma 4 wire transmitter. if both input module and transmitter is fed by same power supply. just wiring + side of transmitter to module will be enough (by wiring " - " of module and " - " of transmitter to common) ? It seems to me 4 wire transmitter output can be any floating value but what happens if we use same power supply for module and transmitter ?
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Old February 9th, 2019, 07:26 AM   #2
the_msp
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I'll add more detail later but I'll start by recommending these two tools:

https://uk.rs-online.com/web/p/curre...ators/5372580/

https://uk.rs-online.com/web/p/clamp-meters/0287642/
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Old February 9th, 2019, 01:12 PM   #3
lfe
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Quote:
Originally Posted by bodoo23 View Post
1- When using 2 or 3 wire 4-20ma current transmitter. Does analog output wire(4-20ma) always produce current with referenced to power supply common ? (I mean we can think this transmitter as a fixed voltage source and a variable resistor so the minus side of the voltage source is always linked to power supply common ?)
I do not understand the second part. In the case of 3 wire you simply supply the correct voltage to the power terminal and the transmitter will generate the milliamperes corresponding to the measured value and of course the common for power and output is the same.
In the case of 2 wire, the output current is also the current that makes the transmitter internal electronics work.

Quote:
2- When using 2 wire 0-10v voltage transmitter, if the input module and transmitter is fed by same power supply. just wiring + side of transmitter to module will be enough (by wiring " - " of module and " - " of transmitter to common) ? I'd like to learn if the voltage produced by transmitter is always referenced to ground or can be any floating value.

3- The second question is valid for 4-20ma 4 wire transmitter. if both input module and transmitter is fed by same power supply. just wiring + side of transmitter to module will be enough (by wiring " - " of module and " - " of transmitter to common) ? It seems to me 4 wire transmitter output can be any floating value but what happens if we use same power supply for module and transmitter ?
In the case of a 0-10V transmitter the answer is no, you have to use a separate cable for the 0V output different from that of the 0V power supply, since if you used the same there would be an error due to the voltage drop in that cable.

Instead with a current output transmitter you can use the same common wire for power and output as it will not be affected by the voltage drop in the common cable.
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Last edited by lfe; February 9th, 2019 at 01:15 PM.
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Old February 9th, 2019, 01:36 PM   #4
danw
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Quote:
Originally Posted by bodoo23 View Post
1- When using 2 or 3 wire 4-20ma current transmitter. Does analog output wire(4-20ma) always produce current with referenced to power supply common ? (I mean we can think this transmitter as a fixed voltage source and a variable resistor so the minus side of the voltage source is always linked to power supply common ?)
I would substitute the term "loop" for 'transmitter', and use the word "power supply" for 'voltage source'; because the transmitter is a component in the loop that is the variable resistor and the power supply is the voltage source and then add in the analog input.

we can think of this "loop" as a fixed voltage source [the power supply] and a variable resistor [the transmitter] and an analog input.

The minus [negative] side of the "last component in the loop" is always linked to power supply common. That is usually the negative side of the analog input.

Quote:
Originally Posted by bodoo23 View Post
2- When using 2 wire 0-10v voltage transmitter, if the input module and transmitter is fed by same power supply. just wiring + side of transmitter to module will be enough (by wiring " - " of module and " - " of transmitter to common) ? I'd like to learn if the voltage produced by transmitter is always referenced to ground or can be any floating value.
There is no such thing as a 2 wire 0-10V transmitter. A 0-10V output is from either a 3 or 4 wire device.

The concept of a 2 wire, loop powered transmitter is that the transmitter uses 3.5mA-3.6mA of the loop current to drive its own electronics to do the measurement and to control the loop current regulation needed. The remaining loop current, 4-20mA, is used to represent the measured variable.

A loop powered transmitter can never be 0-20mA because there's no current available to run the transmitter electronics. A 0-20mA output is from a 4 wire device, like the analog output of a PLC card, which is powered by the PLC power supply.

The same thing applies to a voltage output: there needs to be a power source for the transmitter and a 0-10V output does not allow for that.

Regarding different power supplies, there can be problems when analog outputs and analog inputs are powered from separate power supplies, because the 0V common can be at different potential, creating a ground loop between the devices.

Quote:
Originally Posted by bodoo23 View Post
3- The second question is valid for 4-20ma 4 wire transmitter. if both input module and transmitter is fed by same power supply. just wiring + side of transmitter to module will be enough (by wiring " - " of module and " - " of transmitter to common) ? It seems to me 4 wire transmitter output can be any floating value but what happens if we use same power supply for module and transmitter ?
No, a current loop is a series circuit.

For a 2 wire transmitter, starting at the power supply (+):

power supply (+) to
Transmitter (+)

Transmitter (-) to
AI module (+)

AI module (-) to
power supply (-)

the 4-20mA output of a 4 wire transmitter omits the power supply:

Transmitter (+) to
AI module (+)

AI module (-) to
Transmitter (-)
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Old February 11th, 2019, 04:14 AM   #5
bodoo23
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Thank you for your detailed answers.
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