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How to set reference for a sine wave with dc supply

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anishkgt:
Hey all,

I am trying use a comparator to read an ac voltage and get HIGH when the voltage goes below 30vac and a LOW it goes above 30vac. Can somebody please help me here. I can understand setting a reference voltage for a dc when sensing the same but sensing the AC is something new.

pelule:
Do you want to controle/regulate VAC peak to peak 30VAC (+/-15V referred to zero/gnd)?
So you could use a AC-to-DC circuit - a bridge rectifier (diodes) and a buffer capacitor to convert the AC voltage to a DC level and compare with a comparator.
/PeLuLe

anishkgt:
Well i just need to monitor the 30vac. I did think of using a bridge but had a few concerns,

* would the dc voltage change as the voltage ac changes
* the same line would have a mains supply as well
The LOAD here is a MOT (microwave oven transformer). The 30vac is the voltage when the MOT is not energized for spot welding but will have this voltage just to sense the voltage drop when the secondary are shorted, which simulates a weld.

exe:
I'm not sure I understand the task because there are so many things on the schematic that look to redundant.

Please see "my" way. It's not ideal, but should be good enough. As a precation, I picked random opamp, no clue if it's suitable for this application. Also, many opamps have diode clamp between input, those are not suitable for this. Also, opamp works quite slow because output has to swing a lot. I'm pretty sure there are better comparators.

anishkgt:
Thank you exe. Yes the schematic is not the best just on the paths of learning and following google images.

I've got one as well which i've done with the best i've learned. The truth table for a comparator is like this

If V+ > V-  then, Vcc
If V+ < V- then, Vee

but i don't see any change in the output, wonder why.

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