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Define 2
- Thread starter mrvee
- Start date
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1
- #2
Skogsgurra
Electrical
Using only two wires. These wires carry power and signals. Bi-directional communication may be possible, but not very common.
The better known loops are the 4-20 mA loop and the Dallas "single wire" protocol (plus ground). There are certainly others, but these are what I have worked with.
Gunnar Englund
The better known loops are the 4-20 mA loop and the Dallas "single wire" protocol (plus ground). There are certainly others, but these are what I have worked with.
Gunnar Englund
Loop powered- An instrument or series of instrument that accepts power for the signal loop from a seperate power source.
Those power sources generally are the control system, Usually an input card will provide 24v power and also carry the 4-20ma signal back to the card or a power supply used to power one or several instruments with the signal going back to the input card (skogsgurra's dallas).
So your generic defintion would be- a control system which supplies power to a loop (instrument, display's) that carries the 4-20ma signal back to the control system.
Good luck!
Quote: "Its not what you know, its who you know" - anybody trying to find a decent job
Everyhtings a learning experience-Everything
Those power sources generally are the control system, Usually an input card will provide 24v power and also carry the 4-20ma signal back to the card or a power supply used to power one or several instruments with the signal going back to the input card (skogsgurra's dallas).
So your generic defintion would be- a control system which supplies power to a loop (instrument, display's) that carries the 4-20ma signal back to the control system.
Good luck!
Quote: "Its not what you know, its who you know" - anybody trying to find a decent job
Everyhtings a learning experience-Everything
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- #4
Loop powered circuit consists of 3 components:
A power supply provides power to the loop typically 24 vdc (sometimes is incorporated into one of the other devices)
An amperage controlling device such as a pressure transmitter. It uses the power from the loop to read the value and increases or decrease its resistance in the loop to make the current equal to the value it is calibrated to transmitt. Such devices will be rated for a certain amount of loop load, 800 ohms is common
An amperage controlled device such as a chart recorder or digital display, there may be more than one amperage controlled device in the loop. It has a fixed resistance in the loop and measures the voltage accross the resistance to provide the output desired. typically, 250 ohm with the result that a 4 mA signal produces 1 Volt and a 20 mA produces 5 volts as measured across the device's terminals.
4 mA is used as a low value so that if the current is less than 4 the loop is considered in a 'fault' state, such as if the wires are cut, the current will go to zero, and the control scheme can be programed to alarm and ignore that signal.
Hydrae
A power supply provides power to the loop typically 24 vdc (sometimes is incorporated into one of the other devices)
An amperage controlling device such as a pressure transmitter. It uses the power from the loop to read the value and increases or decrease its resistance in the loop to make the current equal to the value it is calibrated to transmitt. Such devices will be rated for a certain amount of loop load, 800 ohms is common
An amperage controlled device such as a chart recorder or digital display, there may be more than one amperage controlled device in the loop. It has a fixed resistance in the loop and measures the voltage accross the resistance to provide the output desired. typically, 250 ohm with the result that a 4 mA signal produces 1 Volt and a 20 mA produces 5 volts as measured across the device's terminals.
4 mA is used as a low value so that if the current is less than 4 the loop is considered in a 'fault' state, such as if the wires are cut, the current will go to zero, and the control scheme can be programed to alarm and ignore that signal.
Hydrae
Skogsgurra
Electrical
I think that a generic definition shall avoid implementation specific details. That is why my definition is short: "Using only two wires. These wires carry power and signals. Bi-directional communication may be possible, but not very common".
My added added examples could even have been left out. I do not think that mrvee asked about how "loop powered" works.
Gunnar Englund
My added added examples could even have been left out. I do not think that mrvee asked about how "loop powered" works.
Gunnar Englund
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