Have a project where I need to sample the mains voltage into my MUC (an Arduino)

A silly question but am little confused as to which one to go with. I would really appreciate if somebody could advise/comment on the schematic. With the best of my knowledge and google i guess my schematic would work. I don’t have the parts to test this out but before i do i just need some advise. I have a project where i need to sample the mains voltage into my MUC (an Arduino) so i came up with this schematic of a precision rectifier, attached as CS.

Explanation Idea here is to detect when the secondary of the Transformer (MOT) is shorted. The LIVE LOAD here is a transformer (MOT) used for Spot welding, when shorted i can a voltage drop from 1.0vac to 0.5vac i just need to send this voltage to the ADC of an Arduino. So here for isolation i used a MOC3052 optocoupler from fairchild. The LT1078 is a dual opamp precision rectifier from LT. C24 is a decoupling cap and C25 is to smooth the ac signal from LT. Then the second opamp is a buffer.

The optocoupler would be triggered by the MUC via 220Ω resistor there by shoring the contact from the LIVE_LOAD to the voltage divider and that goes on to send a dc voltage to the MUC. This happens when the MOT is not powered. (I’ve googled how optocoupler works and I found this article,which helps me a lot: http://www.apogeeweb.net/article/69.html )

When the secondary is shorted, there would be a voltage drop at the primary contacts of the MOT and that would also be seen by the LT1078 and using code, the voltage drop would be sensed by the MUC then the the optocoupler would be disabled and the MOT would be powered for spot welding. Basically an Auto Sense, instead of having to press a button to switch ON the MOT.

Then i noticed this schematic from TI, attached (http://www.ti.com/lit/an/sboa097b/sboa097b.pdf) page 4 Fig3 where a similar approach with less components. My basic requirement is to sense the voltage drop at the primary of the MOT and switch ON the MOT.

Would really appreciate if anybody could advise.

Well I would advise you against this project.
First of all that opto isolator has a triac output, the text is a bit small to see but it apperes to be isolating the wrong thing. It is not clear to me what is lighting the LED in it and if the op-amps are on the live side of the circuit. A triac will only act as an on/off detector. If you want to see and analogue value you have to use a transistor output opto isolator. Even then you need a dual isolater to make the transfer anything like linear.

But perhaps most importantly is the application. We have had several people over the years here post about welding projects and they all suffer from the electromagnetic pulse produced by the welding kit disrupting the running of the Arduino, causing it to reset. It would seem that no amount of normal decoupling measures can make the Arduino operate correctly in the vicinity. I don’t speak from any personal experance here, just what I have read here over the years.

Yuszho: to detect when the secondary of the Transformer (MOT) is shorted.

The best isolation would be to use a hall sensor to measure current in the transformer secondary circuit.

Measuring "voltage drop on the primary" would be prone to variations in mains voltage and the state of the supply wiring.

I'm not sure what you are intending to do with the "Secondary Is Shorted" information... but I do agree with Grumpy_Mike, it will not be easy to make it reliable.

Yours, TonyWilk

Expanding on what TonyWilk suggested, perhaps it would be better to measure the current in the primary (using an isolated Hall sensor). You might find the secondary current is so high the current selection of hall devices not capable.