NanoIoT33 new rev. 2023-01-15

New updated version with current inputs för the industry standard 4-20mA. Have never worked with this simple signal interface before and of course things went wrong. The idea is to drive a current thru a known resistor and then let a ADC read the value thru an OP configured as a voltage follower. I choose 150ohm which will give around 3V at 20mA. The conversion itself works as designed.

Noticed on two boards that something was wrong and that input seems to b shortcircuited. After some troubleshooting it was clear that the +input on the OP was burnt and was around 1,5ohm. But why?

Well the reason was found quite fast. I used a testinstmrument that can simulate the input current. If this testinstument is unloaded on the oututput it supplies 24Vdc out.

So when connecting the probe from the instrument to the input there will shortly be an overvoltage transient of around 20Vdc into the OP which has a max tolerance of 6V. This will damage the input. I don't know if real sensors work the same and not having this high out voltage initially bet it's better to protect the input. The test setup below shows the otherwise great test instrument which can be purchased for around 25USD

Issue is solved by putting a 3.3V zener diode in parallel with the input and clamping the initial high input voltage. Scope pictures below show that input is around 20V before and around 6V when diode was added. The zener is a bit slow when clamping and therefore it goes higher then 3.3V but it works fine in this application.