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4-20mA current loop

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Zero999:
Yes, it is confusing. I thought the OP was asking how to build a circuit which is powered by 24V and varies its current, as the input voltage is swept from 0 to 5V or 0 to 4V, which wasn't clear.

The schematics I posted just take 24V in and vary the current, depending on the input voltage: V2. Here's it showing a voltage regulator, which just used as a reference, but could to power external circuitry.

soldar:
OK, I think I get it now. I cannot imagine a situation where this could be useful though. I guess with some sensor with separate power supply and the output is voltage.

I am curious about the behavior if the circuits I posted in #19 which convert resistance to 4-20 ma. The original circuit is for a three wire sensor but it could be converted to two wire as in the third circuit. I wonder how linear they are.

Zero999:

--- Quote from: soldar on May 08, 2019, 10:19:52 pm ---OK, I think I get it now. I cannot imagine a situation where this could be useful though. I guess with some sensor with separate power supply and the output is voltage.
--- End quote ---
I'm not sure you get it, otherwise you'd immediately see the potential applications of this circuit. The whole point is no separate power supply is required. All the power is taken from the current loop. U2 provides the 5V to the circuit (including the sensor and any additional signal processing) from the 24V powering the current loop.

Suppose you have a sonar proximity sensor, which you want to output a 4mA to 20mA signal, depending on the distance.  This is a bit more complex than a simple resistive sensor. A chirp needs to be generated and the distance calculated from the return time. The circuitry for the sonar (transducer, pre-amplifier, MCU etc.) is all powered by U2. The MCU outputs a 0 to 5V signal using a DAC or filtered PWM signal, which is converted to 4mA to 20mA, using the op-amp circuit.


The challenge is power is scarce: the whole circuit needs to run from 4mA minus the power required by the op-amp and U2. The solution would be to use a low duty cycle for the chirp, so even if the traducer is driven with way more than 4mA, the average current consumption could be kept below that.

There are ICs available which are specifically designed to the purpose. Look up the XTR115 and XTR116.
https://www.ti.com/lit/ds/symlink/xtr115.pdf

soldar:
Yeah, no, I understand that. I have worked with many two and three wire sensors and understand how they work.

I was referring to the circuit in the OP which seems to have its own voltage supply.

Although I guess that can be a sort of "general idea" and the voltage could be derived from the loop as you point out.

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