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

In the PDF of yours you have

A circuit containing a VOM160NT controlling two SCR's

Below that you have a box labeled Contact Sense.

Delete this box and replace with spot welding contacts.

If you were to connect a lamp to "Live" & "Line_Out" the lamp would light when you short the spot welding contacts.

When you trigger the VOM160NT, the lamp would go out and you would do a spot weld.

The lamp is just an AC Voltage sensor.
It can be replaced with an opto-coupler led & a series resistor.
If the opto-coupler  is only one LED then you need to add an external diode in parallel but reversed across LED to work with AC.
You can find the circuit in data sheet for opto-coupler.


Just guessing, but think circuit connected to VOM160NT pins 1 & 2  is wrong.

C
anishkgt:

--- Quote ---If you were to connect a lamp to "Live" & "Line_Out" the lamp would light when you short the spot welding contacts.
--- End quote ---
its labeled "LIVE" and "LIVE_OUT" basically they are live line and this part of the circuit is in series. That is, one terminal at the primary of the transformer is connected to the LIVE_OUT and the other terminal is connected to neutral. The mains live goes into "LIVE".

hence connecting a lamp at "LIVE" and "LIVE_OUT" would not work but connecting at "LIVE_OUT" and neutral would glow.


--- Quote ---Just guessing, but think circuit connected to VOM160NT pins 1 & 2  is wrong.
--- End quote ---
The pulse from MUC is TRDRIVER and not PULSE. They work as expected.

The way you suggested, are they at the mains side or the secondary ?
anishkgt:
Was this something you had in mind ?
C:

Your something like this drawing

 needs a series resistor for LED.
You select resistor value to prevent over current of LED at peak AC voltage.
This is a AC voltage sensor.

With out the series resistor you would have a AC current sensor for very low current. But you need to remember that the forward voltage drop of an LED is much greater then a Power Diode and would not handle the spot welding current.

What I described was secondary side control with SCR's connected to secondary. These let the led current of the AC Voltage sensor supply the contact sense current.


Now lets look at

--- Quote ---its labeled "LIVE" and "LIVE_OUT" basically they are live line and this part of the circuit is in series. That is, one terminal at the primary of the transformer is connected to the LIVE_OUT and the other terminal is connected to neutral. The mains live goes into "LIVE".

hence connecting a lamp at "LIVE" and "LIVE_OUT" would not work but connecting at "LIVE_OUT" and neutral would glow.

--- End quote ---

To sense contact closure you need to sense voltage or current.
Simple would be to use your transformer to supply the sense current, and you do not need much current for this.

If you connect your meter(#1) to "LIVE" and "LIVE_OUT" you would be measuring the state of your SCR's.

Today's meters are very high input resistance so very little current will flow through the meter. If the measure the secondary voltage with a second meter you would measure close to 0 Volts AC.

Start with a high value resistance between "LIVE" and "LIVE_OUT".
As you decrease the resistance
Meter #1 will start decreasing.
Meter #2 will start increasing.
This is creating a sense current for contact sense.

Now if you short out the secondary of a transformer, The primary will change to supply more current. The effect is that the primary of transformer will act more like a short.
Here the primary current is limited by that resistor connected between  "LIVE" and "LIVE_OUT".
This will cause the meter #1 voltage to increase due to increased voltage drop across the resistor & SCR's.

Now think the same in a simple way.
You have a transformer rated for X volts on the primary. If you connect a lamp rated for X volts in series with one wire of primary of transformer you have a voltage divider.
Going from open to short on secondary changes the voltage divider. The lamp gets brighter as it now has more voltage.
The lamp is acting just like the above resistor with meter sensing the voltage.

So summery
The resistor between "LIVE" and "LIVE_OUT" is supplying the low current to secondary so you can do secondary contact sensing. The Voltage change across the resistor lets you sense secondary contact change on the primary side.
You replace the meter #1 with your AC Voltage sense opto-coupler.

You have three things in parallel.
1. the SCR Switch
2. the resistor
3. the AC Voltage sense opto-coupler.

You just need to adjust the resistance value such that you get a good contact sense with your  spot welding contacts.

When you fire the SCR's you have your high current to spot weld.

When SCR's are off you have a low current limited lower voltage on the spot welding contacts that is used to sense contact.

C








anishkgt:
i get what you mean, well partially. A schematic would have helped.


--- Quote ---You have three things in parallel.
1. the SCR Switch
2. the resistor
3. the AC Voltage sense opto-coupler.
--- End quote ---
The SCR that you are referring, is it a third one excluding those in the schematic ? How would it be connected ? like it does require a gate voltage to trigger. Would it be like a crowbar between the optocoupler ? The resistor, is it just one resistor or a series of them before the opto-coupler ?
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