PWM Converted Into 4-20ma (Vs) 1-5V Converted into 4-20ma

[ArduinoWannaBe]

Hello Everyone,

Background:
I recently purchased a cheap relay board off of amazon, and the spec which I should have checked prior says it works off of 15ma-20ma signals. (Because a relay is ON-OFF) This doesn’t seem to make a lot of sense to me. Wouldn't be much easier to just make the board 0-5V or PWM much less work to switch a relay on or off that way I would think.

Anyways back to my real question. According to Ohms Law V=IR so a 250 Ohm resister minus the resistance in the circuit (Could be hard to measure the resistance in the circuit). That should give me my 4-20MA out, because this is just on/off I think this should work with a couple of resister sizes.

Secondly, I have seen people turning PWM into 4-20ma with a small R-C- Filter and then connecting to a Voltage-to-Current Converter like XTR110 which should work.

What is the correct way to do this?

This is just a demo setup so I think a crappy relay to save the wait time on component shipping is the way to go. When I build my final product I will not use 4-20 Output to a relay board.

Am I missing an easier solution?

Cheers
ArduinoWannaBe

[ivaylo]

A schematic of what you are trying to do would help, but I can't even tell if you understand what "4-20mA" means (and if your final product is not going to use why even mention it here)...

[Rerouter]

4-20mA is an industry standard signal, the purpose of that relay would be something like a high alarm (a sensor reading over ~75%)

as for driving it, if you have access to both ends, or even just the bottom end, then a single op amp current sink (look at daves dummy load) will fit the bill, even using that 250 ohm resistor as the measuring resistor if you want to keep the 1-5V signal logic

Just please search this site a bit, there are 20 odd threads showing the small tweaks that had to be done to make them stable (Dave oversimplified the schematic) which also include working component combinations,

[ArduinoWannaBe]

Thanks Rerouter!

I will read the artical. Just looking for the best practice to make my prototype work well for testing.

Cheers
ArduinoWannaBe