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I have a 3 wire 4-20mA output probe simulated here, where during normal operation the 20v and 250 ohm resistor are variable chosen by the user.

One of my users has a 2 wire 4-20mA meter, and I am unsure what the normal approach to 2 wire 4-20 is.

Can i plug my probe into the user's controller? If so what wires go where?

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enter image description here

Figure 1. (a) 2-wire and (b) 3-wire. Source: copied from my answer to Several Questions About Analog Input.

... but if I can have non isolated power provided externally can I tie my 3 wire signal line to the + terminal on the 2 wire controller and tie ground to the - terminal?

Yes. Consider Figure 1b. The 24 V supply is feeding the 3-wire transmitter while the grounds are connected. The 24 V could be supplied from the receiver panel or could be supplied locally at the transmitter.

As an aside, the 250 Ω resistor shown in the receiver is the typical way to convert the mA signal to a 1 to 5 V signal for the internal ADC.


enter image description here

Figure 2. OP's schematic.

You have placed the 250 Ω current sensing resistor in the high side of your device. That means they can't share a common ground.

One of my users has a 2-wire 4 - 20 mA meter, and I am unsure what the normal approach to 2-wire 4 - 20 is. Can i plug my probe into the user's controller? If so what wires go where?

You can provided the user's meter is not grounded. If it is you will have a short across the transistor and lower resistor - always give designations to components on schematics - resulting in 20 V DC applied across the 250 Ω resistor which will pass 80 mA.

Your configuration is unusual and this could give rise to problems on installation.

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  • \$\begingroup\$ Is the transmitter in your figure (b) a current source? i edited my post to have my output circuit and want to make sure its the same type. \$\endgroup\$ – Hatman Jul 30 at 16:23
  • \$\begingroup\$ I thought I understood what you were asking but with the addition of the simulation link I now have no idea. Can you edit your question to make it clearer? \$\endgroup\$ – Transistor Jul 30 at 18:00
  • \$\begingroup\$ i rewrote the question \$\endgroup\$ – Hatman Jul 30 at 18:04
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    \$\begingroup\$ Be careful with rewriting when folk have answered already as it makes their posts look irrelevant. Please see the update. \$\endgroup\$ – Transistor Jul 30 at 18:22
  • \$\begingroup\$ Thanks, i will just redo the design to be a current source similar to figure 1 \$\endgroup\$ – Hatman Jul 30 at 18:53
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This TI chip is how a 2 wire input to 2 wire 4~20mA current loop is used so that ground noise does not degrade the result , yet is not galvanically isolated.

A 5V regulator inside the XTR117 is like the LM117/317 and needs a Vcc = 7.5 to 24V to drive the IC.

The chip is basically a current amplifier and the series input R converts the input voltage to a small input current then the chip amplifies this current with an output offset of 4mA and a gain of Io=100 x In to 20mA full scale. The input may also be offset with a bias resistor to +5Vreg or some other source. THere is also examples of how to protect it from reverse and over-voltage and RF interference.

This means you can choose Rin to have just about any scale of Vin {min,max} to equate to {4:20}mA output. e.g. Vin = 0:1V, or 0 to 3.3V or 0 to 5V or 2.8V to 4.2V ...etc.

enter image description here

This IC can work in most applications unless there is a real need for high voltage or galvanic CMRR optical isolation. But don't reinvent the wheel, THis IC is 0.3% accurate over temp and supply range.

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Can I plug a 3 wire 4-20mA current sink probe into a 2 wire 4-20mA loop powered controller?

Short answer: Yes

Longer answer, the configuration will change depending on both the sender and receiver.

A three wire sender includes a + line, 0V line and sender output (source from + or sink to 0V).

You should read this.

If your Rx is loop powered then the connection would typically look like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The first diagram shows a 4-20mA Sink, the second a 4-20mA source.

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