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Topic: 4-20mA output options (Read 3 times) previous topic - next topic

Constantin

Hi there,

I have very little experience with the Arduino and would appreciate a couple of pointers, if possible. I am considering using an Arduino to control a variable-speed PSC controller from varidigm, specifically VSM-CA2, which can be controlled with a remote potentiometer, 4-20mA signal, or 0-10VDC signal.

For simplicities' sake, I'd like to control the varidigm board using the 4-20mA option. Based on reading I have done, I gather I should use a capacitor to smooth out any ripple (0.1uF?) from the PWM signal. Other than that, is it as simple as putting a 220Ohm resistor in series with the output pin from the arduino (I don't want to fry anything)?

The other option is to go the 0-10VDC route, either with something like the XTR110 mentioned elsewhere, or the much less expensive solution of using a transistor, a 12VDC power supply and a similar smoothing cap to create a 0-10VDC signal.

So, is 4-20mA straight from the analog pin kosher? If so, what external smoothign capacitor should I use? Many thanks in advance for any advice, pointers. Cheers and happy Easter. And since I don't know if posting links here is allowed or not, I won't post a link straight to the Varidigm data sheet. But if someome would like a link and it is allowed, please let me know. Many thanks, Constantin

retrolefty

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So, is 4-20mA straight from the analog pin kosher?


No. Even if you filter a Arduino PWM analog output you will have obtained a programmable analog output voltage, not a programmable constant current output. 4-20ma current outputs require external components. You are probably better off with the 0-10vdc option for your controller. That will still require external components like an op-amp to amplify the 0-5vdc of the filtered PWM output to a 0-10vdc analog output.

Lefty

Constantin

Hey Lefty, thanks!

The varidigm board also has the option of being controlled with an external potentiometer.  Do you suppose that a digital pot might work? The only reason I'd go this way is to keep the number of power supplies to a minimum.

IIRC, it was Grumpy Mike that posted a similar solution to the one you mentioned, i.e. using the arduino to pulse a transistor attached to a 12VDC power supply and smoothing the output.

Seeing that I spent $66 on the variable-speed drive, I want to use something as intrinsically safe as possible so I don't blow it out... So, while I could even feed the output back into the Arduino to ensure that it never exceeds 10VDC on the output side, the digital potentiometer approach seems to have merit also. Have you ever tried this?

retrolefty

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Do you suppose that a digital pot might work? The only reason I'd go this way is to keep the number of power supplies to a minimum.


I haven't played with digital pots yet. My impression is that they probably won't work for your application. I think most are designed for lower voltage signals, like line level adudio which is just a AC voltage of less then 0-2vac.

Lefty




Senso

Well, in what I know a "digital pot" is an rotary encoder, with or without limited rotation, so it won't work without reading it and doing some processing of its output to then control something.

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