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How to set reference for a sine wave with dc supply
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Zero999:

--- Quote from: exe on June 29, 2018, 12:51:42 pm ---**But this also makes this circuit unsafe.**
--- End quote ---
Why? I think you've forgotten this circuit is ELV.
anishkgt:

--- Quote ---What's the purpose of the project?

If you want to control a load, with zero crossing, then an opto-coupler can be used with built-in zero crossing, such as the MOC3041.

If it's an undervoltage protection circuit, then use a low pass filter, before the comparator circuit, to get the average voltage of the rectified waveform.
--- End quote ---

This is part of a spot welder using a MOT. This part of the design is used to avoid using a pedal to trigger the MOT for a weld. The difference in voltage is seen when the secondary are shorted. hence using that as a way to detect when the electrodes touch the nikel plates for a weld.

The idea of a optocoupler with built-in zero crossing is good. But in a inductive load it should not start at zero but has to be at the peak of ta sine wave as to overcome the inrush current from the core of the transformer. Supporting notes (http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3206_AppNote&DocType=CS&DocLang=EN) and (https://www.st.com/content/ccc/resource/technical/document/application_note/2f/f7/75/f5/9d/3d/44/04/CD00003867.pdf/files/CD00003867.pdf/jcr:content/translations/en.CD00003867.pdf)

This is not a protection circuit but yes it does require a low pass filter but i was concentrating on this and latter add it.

--- Quote from: exe on June 29, 2018, 12:51:42 pm ---To me the resistor divider and diode work: I see truncated sine wave. Not sure where 4V offset comes from. But!

Do you have opamp's ground connected to neutral? They must be connected for this to work. **But this also makes this circuit unsafe.**

--- End quote ---

I was thinking of connecting the Neutral to the common GND as the opamp does. Should i do that ?

I've attached the schematic as well with some changes. My PCB is made as per this.
anishkgt:

--- Quote ---I was thinking of connecting the Neutral to the common GND as the opamp does. Should i do that ?
--- End quote ---
Nope that did not work either. no voltage at all.
C:
   anishkgt

There are two ways to do what you want.

You can sense the voltage between the spot welding contacts.

You can sense the current when the spot welding contacts are shorted.

So think of it this way, When you short the spot welding contacts it turns on a light, The light turning on causes trigger to fire your high current source.

As Hero999 stated you can use a opto-coupler in place of the above light and get isolation from your micro-controller,
 
So you have an opto-coupler led in series between one connection from transformer and a spot welding contact.
Your high current output switch(SCR's) is connected in parallel to this.

When on the SCR's will cause
1. opto-coupler above to go out, the scr's are placing a short across the opto-coupler input.
2. Enable the high current path to the spot welding contact.

Now the input led of opto-coupler does not turn on at 0 Volts but requires some voltage & current. Here you will need a series resistor to protect the LED from full voltage of transformer. By using a combination of voltage divider and series resistor you can make the turn-on of led higher.

To say the above a different way
You are using a opto-coupler to detect the state of the SCR's when your spot welding contacts are shorted.

You have
spot welding contacts open, opto-coupler output off, SCR's off
spot welding contacts shorted, opto-coupler output on, SCR's off
spot welding contacts shorted, opto-coupler output off, SCR's on

C

anishkgt:
Can you please share a rough diagram. I feel you have a valid point but i am unable to follow it completely.
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