HVAC/PLC Universal Input Design

[StuntMonkeh]

I have been working on a universal input design to be used with an MCU from some existing schematics I found.

I was lucky enough to find the resistor network to setup the input in a datasheet from one controls manufacturer but I have been trying to figure out what other components were required to make it resistant to the expected voltages without killing the MCU ADC input.

Is what I have done so far correct, can anyone suggest any improvements upon this?

[SeanB]

Other than the LM358 being the wrong choice with the supply voltage you have all else will work and survive pretty much anything up to mains being applied ( except in current mode where the resistor will die a fast death) to the input.

[StuntMonkeh]

Yes input to expect up to 24vAC no higher.

I wasn't sure what type of opamp, I need to really investigate that part further although the datasheets I can find indicate most LM358's are 3.0v to 30v.

Can anyone point me in the right direction of how to choose correct opamp selection.

[orion242]

That diagram looks identical to the inputs on the controllers I use.

What brand did you hock that from?

[SeanB]

LM358 may be rated from 3V but at 3V it can barely do 100mv in the middle. You need a rail to rail input and output opamp to work off 3V properly.

[StuntMonkeh]

Am I on the right track if I read this?http://www.ti.com/lit/an/sloa098/sloa098.pdf