3.3V input protection when measuring 4-20mA signal

Thread Starter


Joined Sep 4, 2010
Hi folks...

I have an esp32 project on the bench which needs to read a 4-20ma signal from a couple of sensors.
at 12 bits resolution isn't a problem and I can afford to ignore the 4mA.

The instruments typically drop about 8v, minimum, so I need to keep other series resistance low to run from a 12V battery which is the plan.

A 250 Ohm resistor, in parallel with a 1K trim pot, gives a little adjustment and adding circa 30 Ohms to the supply will limit fault currents to something 'potentially' manageable if the instrument were to get shorted in the field.

However my original thought to add a PPTC to the supply and a 3.3v Zenner from the input to the ground is looking like it will not work well, the more spec sheets I look at the worse the plan looks, at least it dose if I am reading the sheets correctly.

Specifically I was looking at a MICROSMD005F-2 and 22 Ohn resistor in thre supply and a BZX85C3V3 between the input and ground.

I do not think this is going to do what I need, quickly enough to protect all its parts. I have also read that a 1K series resistor on the input will give the internal clamping diode a fighting chance without affecting the measurement too much.

Is there a 'standard' way to do this?
Any and all comments / suggestions welcome.