4-20ma converted to 1-5v

I want to use a few rtd's on a project that use converters that can be calibrated to a set range the trouble is they are 4-20ma output. I work with these every day so I understand wiring in loop etc.

I also have all the equipment to calibrate the sensors which is why they are my first choose.

Now the last posts I found were from 2012 and involved using a 220 ohm resistor and reading the voltage potential across the resistor. A extra 10k was added to protect the arduino pin just in case the loop was broken. Most industrial sensors that are rated 4-20ma fail high which is closer to 23-24ma so 220ohm makes sense instead of 250ohms and the signal can be corrected in the code. The real question is has anything changed since 2012 or is this still the best option. I will also probably add a 5.1 zenor diode (haven't used one since high school so I can not remember if they open at 5.1 or close but I will read up on that)

no idea why the premade boards are so expensive for a 4-20 to 1-5v unless there digital?

no idea why the premade boards are so expensive for a 4-20 to 1-5v unless there digital?

No demand therefore high price.

20ma wiht 250R = 5V

~10K good to use. Maybe cathode of a Schottky diode to Arduino +5V, anode to analog input. .1uF capacitor across the 250R

5.1V zener is a good idea. (> 5.1V acts as short)

Careful with your grounding.


Isnt 4-20 ma an old protocol?

It is used in industry very much.

I believe a 5.1 zener would impede the accuracy of your sensor as the signal approaches 5V as its hard to know where the actual "knee" voltage lies. The leakage could be around 10µA at 2V and increase to ?? at 5V. Here, an incredible 38mA was measured at 5V.

I'm sure its possible to have high accuracy, no loss in precision and over-voltage protection with something like this:

The Maxim 14626 is a good device to protect an Arduino from industrial-strength spikes coming down the 4-20mA wire. You still need a resistor to convert current to voltage but this chip handles most any fault condition.