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?


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.

  • \$\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
  • 1
    \$\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

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.


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:


simulate this circuit – Schematic created using CircuitLab

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


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.