How do you use an Arduino to control a 4 - 20 mA transmitter?

Question

I am using an Arduino Mega to control the speed of an AC pump. This can be done by connecting to the variable frequency drive. Here is a link of the relevant datasheet, pages 66 and 67 relate to what I am doing.

http://www.parker.com/Literature/SSD%20Drives/AC%20Drives/AC650/HA464828U003.pdf

From what I have read and been told already, I should be able to control this using a 4 - 20 mA loop, which is fed into AIN2, as long as I connect the 0V terminal as well?

My question is what is the best way to make a current loop transmitter using an Arduino Mega.

So far the options I have are:

• Create a circuit out of discrete components, this would then be controlled using the Megas PWM
• Use an IC circuit, I was looking at the AD694JNZ
• Use an add on board, I was looking at this one: http://uk.rs-online.com/web/p/products/8828929/?grossPrice=Y&cm_mmc=UK-PLA--google--PLA_UK_EN_Semiconductors-_-Semiconductor_Development_Kits&mkwid=sm9dTYvCc_dc|pcrid|88057061283|pkw||pmt||prd|8828929&gclid=CjwKEAjwk6K8BRDM3aCSkdCtzSQSJAA3Vf38mNs9lZrg2A1AjrVpS3SCubT0AbAr7L8OImUZbfCPzRoCIXjw_wcB

I have a budget for my project so cost is not an issue, however I am running out of time. What would be the easiest method to implement and how?

I have been thinking of using options 2 or 3, however my issues are that this is my first Arduino project. I understand what PWM is but I don't know how I would go about implementing it. I also can't find any tutorials online that demonstrate how to program the MIKROE board using an Arduino.

Thank you in advance for any help :)

Answer 1

I can recommend the Click boards from MikroElektronica (I am the chief engineer porting support for them, and the PIC32-based Clicker development board series, to the chipKITTM system).

The 4-20mA-T Click board is controlled by the Microchip MCP4921 DAC. This is a common DAC family used in many Arduino projects and there are plenty of libraries around for them (mine is here though it's aimed at the 4822 not the 4921).

Basically you set the DAC output voltage which sets the current output from an XTR116 current loop transmitter.

The XTR116 takes an input of 0 - 4.095v, and provides a convenient 4.095v reference voltage to the MCP4921.

• Datasheet for the XTR116
• Datasheet for the MCP4921
• Manual for the 4-20mA T Click

The whole thing is isolated from your main board through an ADu M1411 to keep everything safe from industrial process noise.

Answer 2

I don't know how I would go about implementing it.

Both option 2/3 require coding - generally simple and doable, even for newbies.

To go down the DAC route, current-feedback DACs are quite common, especially in the good old days. they are ideal here -> make sure your device matches up with the dac in terms of current source vs. current sink.

option 1 is good if you don't like coding. turning a voltage into a current is fairly simple.