alt Hacker News> What’s the deal with 5v, 3.3v and 24v “standards” for sensors?
Historical garbage and different manufacturing technologies. Be happy if you can get away with only 5V and 3V3 rails in your project. 24V is usually to interface with industrial sensors. And sometimes you see 12V as well, for stuff that's RS232 based.
And on top of that you got a fifth standard, 4..20 mA current loops. That one is used for long range transmission of analog values of a single sensor per wire pair, with 4-20 mA being seen as the value (4 mA = 0%, 20 mA = 100%), and anything less being seen as a cable break, anything higher as a short circuit somewhere.
Replies
4 to 20mA signaling is only the start of a very specific rabbit hole. Someone had the brilliant idea to encode digital signals on top of the analog current loop. The result is the HART communication protocol, which is old, bloated, confusing, quirky - and it is really popular in industrial automation.
Ah yes, I was wondering why my ClearCore supported that, seemed oddly specific!
Those voltage standards are kind of meant for very local things. If you really get into industrial things, one should look to industrial standards that work over longer distances. That is things like RS422, RS485, and increasingly industrial versions of ethernet that use differential signals. One should also learn what a PLC is and understand that in an industrial context, implementing controls in an Arduino or rpi is probably reinventing the wheel to achieve less reliability than industry standards.
4 to 20mA sensors are great. Invented in the 50s (!) to replace pneumatic controls and to this day work great. iirc they are usually 24V these days. You missed an important detail; the first 4mA (96mW) powers the sensor/local microcontroller (no local power supply required), and the remaining 4-20mA gives a calibrated current output for voltage/pressure/whatever you are measuring. If the output is less than 4mA or more than 20mA you know something is wrong (and many devices will output 20.1, 20.2 etc currents as a kind of fault code).