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#1 |
Member
![]() ![]() Join Date: Jan 2018
Location: Athens
Posts: 16
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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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#2 |
Lifetime Supporting Member
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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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#3 | ||
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![]() ![]() Join Date: Jun 2007
Location: Barcelona
Posts: 346
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Quote:
In the case of 2 wire, the output current is also the current that makes the transmitter internal electronics work. Quote:
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. Last edited by lfe; February 9th, 2019 at 02:15 PM. |
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#4 | |||
Member
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Quote:
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:
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:
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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#5 |
Member
![]() ![]() Join Date: Jan 2018
Location: Athens
Posts: 16
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Thank you for your detailed answers.
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