Electronics > Projects, Designs, and Technical Stuff
4-20mA current loop
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OM222O:
on the schematic that they show, the 5v output is directly connected to one of the output wires  :-// how can you back feed that with a higher voltage on the sensor side without causing issues?
Edit: I'm not an expert on this topic and have not used that chip before, so it's purely based on my speculation but it seems like common sense.
rs20:
Holy crap OM222O. You need to look up a primer on what 4-20mA is. The whole reason that it's 4-20mA and not 0-20mA is because that means at least 4mA is always flowing, and that 4mA can be used to power the circuitry. No separate power supplies needed. The 4-20mA sensor can be in a remote place with just a loop (pair) of wires running to it, and those wires carry both the power to the sensor and the signal coming back.

This is the whole point of 4-20mA, the whole reason it was created in the first place. If you can't understand that, please feel free to do some research and/or create your own thread (in the Beginners forum). But you're clearly at a level of understanding way below the OP's, and you're contributing absolutely nothing to this thread by sending him facepalm emojis while he's trying to explain the above to you.


--- Quote from: OM222O on May 06, 2019, 09:19:22 pm ---You cannot power op amps from a current source   :-// or any other IC for that matter   :palm:

--- End quote ---

Yes you can, and yes you can. Isn't powering an opamp from a current source the basis of 4-20mA? Aren't voltage regulators ICs? Just because you haven't seen something yet doesn't mean it's impossible and stupid.
soldar:
You can have two wire sensors which get the power supply from the loop current and three wire sensors which have a separate wire for power supply.

Typical implementation of a 4 - 20 mA two wire loop:
IanB:

--- Quote from: dagobah700 on May 06, 2019, 09:01:03 pm ---What you are saying with the simplied version is true, but if I connect 100 meters of cable to it's output, the feedback measurement will be corrupted with the cable's resistance.
--- End quote ---

I don't think this is how it should be. With a 4-20 mA system the remote sensor determines how much current is in the loop. If the remote sensor sets the current to 10 mA, then the receiver will see 10 mA also since the current in a loop is everywhere equal. The loop resistance only comes into play when considering the source (compliance) voltage driving the current around the loop. There must be enough voltage to overcome the loop resistance and provide additional headroom for the sensor to regulate the current.
OM222O:

--- Quote from: rs20 on May 06, 2019, 11:20:19 pm ---Holy crap OM222O. You need to look up a primer on what 4-20mA is. The whole reason that it's 4-20mA and not 0-20mA is because that means at least 4mA is always flowing, and that 4mA can be used to power the circuitry. No separate power supplies needed. The 4-20mA sensor can be in a remote place with just a loop (pair) of wires running to it, and those wires carry both the power to the sensor and the signal coming back.

This is the whole point of 4-20mA, the whole reason it was created in the first place. If you can't understand that, please feel free to do some research and/or create your own thread (in the Beginners forum). But you're clearly at a level of understanding way below the OP's, and you're contributing absolutely nothing to this thread by sending him facepalm emojis while he's trying to explain the above to you.


--- Quote from: OM222O on May 06, 2019, 09:19:22 pm ---You cannot power op amps from a current source   :-// or any other IC for that matter   :palm:

--- End quote ---

Yes you can, and yes you can. Isn't powering an opamp from a current source the basis of 4-20mA? Aren't voltage regulators ICs? Just because you haven't seen something yet doesn't mean it's impossible and stupid.

--- End quote ---

regardless of how much you search, there is nothing about "Powering ICs (or op amps for that matter) from a current source" ... they require voltage sources! simple as that! op amps specifically have voltage rails!


--- Quote from: soldar on May 06, 2019, 11:36:41 pm ---You can have two wire sensors which get the power supply from the loop current and three wire sensors which have a separate wire for power supply.

Typical implementation of a 4 - 20 mA two wire loop:

--- End quote ---
also this is exactly what I said ... the 250 ohm creates a 1 to 5v voltage drop
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