How to set reference for a sine wave with dc supply

[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]

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

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.

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

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

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.

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.

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

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 ?

[exe]

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

Nope that did not work either. no voltage at all.

Yes, GND and neutral must be connected. That's because for electric current to flow there must be a return path.

Concerning your circuit. If it still doesn't work after all suggestions, then there is a mistake in wiring or something. I suggest start from scratch and add "building blocks" one by one: diode, resistor divider, zener clamp, opamp, and output circuitry. Strictly in this order checking with the oscilloscope the circuit works the way it should. Otherwise troubleshooting may take forever.

[anishkgt]

Thank you for your time.

I think i would go with the optocoupler idea and see how that goes.

[anishkgt]

Would it be like a crowbar between the optocoupler ?

Something like this ?

[anishkgt]

if i understand it correctly

Note: Replaced attachment.

[anishkgt]

Scope view at the secondary.

[C]

here

Nice an simple

[anishkgt]

Thank you.

That looks like it is driving the AC side of the transformer Which was not my initial question or did i misunderstand it. I was looking at the secondary side of the transformer.

Secondly those SCR's would need a gate current in order for them to conduct.

[C]

The SCR's shown and VOM are your existing ones, use your circuit.

Think about what happens to transformer and secondary when you spot weld
All kinds of nasty happens.

R2 just reduces the line voltage to transformer to a lower value and restricts current. This gives the secondary side a source for contact sense.
Feed back through transformer on contact closure will cause a voltage change on R2

you just have to adjust R2 value to your transformer.

When you know how voltage across R2 changes you can adjust the voltage sense of opto-coupler voltage sense.

[C]

As I understand it,
You are wanting to replace a foot switch that triggers the spot weld.

You are trying to

detect when the electrodes touch the Nickel plates for a weld.

But to create a good spot weld you also need to force the Nickel against something, a second Nickel plate or a battery terminal.

So in-place of detecting contact, I think you want a force sensor or displacement sensor.
The better spot welders have spring loaded contacts, so you could just detect the spring deflection to trigger the spot weld.
This would guarantee that you had enough pressure for the spot weld and be much simpler also
.

[anishkgt]

he SCR's shown and VOM are your existing ones, use your circuit.

The existing schematic has one of the SCRs inverted but yours has them parallel, was that an error or intentionally drawn. So based on my circuit shouldn't R2 be connected to the anodes of the SCRs as you have drawn it ?

i checked the voltage at the LIVE and LIVE_OUT and it reads mains voltage of 240V but i read 33V at the primary of the transformer and zero volts at the secondary. The primary voltage of 33V was intentionally induced for the whole idea for sensing.

adjust the voltage sense of opto-coupler voltage sense

Sorry did not get what meant here.

[anishkgt]

As I understand it,
You are wanting to replace a foot switch that triggers the spot weld.

You are trying to

detect when the electrodes touch the Nickel plates for a weld.

But to create a good spot weld you also need to force the Nickel against something, a second Nickel plate or a battery terminal.

So in-place of detecting contact, I think you want a force sensor or displacement sensor.
The better spot welders have spring loaded contacts, so you could just detect the spring deflection to trigger the spot weld.
This would guarantee that you had enough pressure for the spot weld and be much simpler also
.

That is correct, as a replacement for a foot pedal. Initially i did think of such a design but then that would take some time to get the hardware fabricated with a spring loaded electrode and a switch, takes valuable time. Sensing when the electrodes makes a contact is easier and pushing down on the electrodes was ok.

For another reason i feel sensing the contact at the secondary would help in the future as well, reason being  measuring current passed on to the spot during the weld.

Edited: primary replaced with secondary.

[C]

https://www.eevblog.com/forum/beginners/how-to-set-reference-for-a-sine-wave-with-dc-supply/?action=dlattach;attach=459484
In the above you is your AC Switch where you use SCR's, This is my #1.  The doted box part is removed.
This is the power switch to Spot Weld Power. ( state #3 in below list)

You have a microwave oven transformer.
guess
Rated for a Primary voltage of 240V
Secondary of  30VAC peak to peak with nothing connected.
A large transformer, could be rated for over 1000 Watts.
Would be nice to know the transformer primary resistance(impedance) with nothing connected to secondary( transformer's idle current).

You want to sense a contact change.
This is a resistance change to V= I x R.
This requires a power source to supply V & I.
A sensor that detects a change in V or I,
To sense this you need a second point of change that prevents short circuit current.


Now to be clear, here are the states I see for the secondary of transformer.

1. OFF  no power.
2. With Contact sense Power.
   2a. Open contacts
   2b. contacts shorted
3. With Spot weld Power.
   3a. Open contacts
   3b. contacts shorted.

Note that a & b are limits of a large resistance change range.

You should note that with #3 you have very high currents & No current states. At these current levels just wire will act like an inductor. You could have large voltage changes from inductive kick.

So #3b above is a nasty state that wants to destroy things. Here the max current is only limited by transformer. The transformer is not operating as designed.

Now think about #3 above Spot weld power to secondary.
You have #3a where you have transformer idle current.
You have #3b where you nave more current then transformer is rated to handle.
There is a huge current change between these two states.

In the above list, State #2 needs protection from state #3

There are many ways to create power for #2 above.
My R2 is part of a AC based Ohm meter with line power source.

You have a voltage divider composed of R2 & transformer primary winding. It is using the bidirectional nature of a transformer.
Line voltage is the #2 power source with R2 supplying the Current limit.
This voltage divider lets you sense a spot weld contact resistance on secondary. The transformer is actually providing some protection to this voltage divider from the nasty secondary side.

With R2 being part of voltage divider, it is also setting the spot weld contacts open circuit voltage.

so #2a is open circuit secondary voltage R2 creates by supplying power to transformer primary.
#2b is short circuit secondary current limited by R2.


Now run some values for R2

If R2 = idle current of transformer primary you have.
1/2 line voltage across R2 & primary.
Secondary voltage = 1/2 of when powered by line. If Line gives a 30 volt P-P then secondary is 15 volt P-P when open circuit.
The ratio of R2 & primary changes as resistance across spot weld contacts decreases to a point where you have min voltage across primary and max voltage across R2.

You are working with ratios.
The ratio of R2 resistance and Transformer resistance.
#2a is R2 and idle current(secondary open)
#2b is R2 and primary resistance with secondary shorted.

You have a voltage sense that detects change from open to desired contact resistance.

In


You have a AC voltage sense circuit composed of R1 & SFH6206 parts.
This will detect very low levels of AC. The R1 resistor is protecting the SFH6206 from over current, but also setting the turn-on voltage of SFH6206. Increasing value of R1 will increase turn-on voltage.
Output of SFH6206 will be in form of pulse as AC input voltage changes above or below the turn-on voltage for each half cycle of line..

In addition to adjusting R1, you could connect one input to a line power voltage divider to adjust turn-on. The AC voltage sense would then be between voltage divider and junction of R2 & primary winding.

Need to keep in mind that when secondary is shorted, you could have full line voltage across R2 so the wattage rating of R2 needs to handle this.

In the past I worked with a circuit that had a power button shorting the secondary of a transformer. The primary of transformer was in series with a relay. The power button would cause the relay to change to energized state. With the power button open, the relay would not energize. This change allowed a Line voltage relay to be used instead of a low voltage relay connected to secondary.




 

 

[anishkgt]

Thank you, appreciate the time you took to write this.

you could connect one input to a line power voltage divider to adjust turn-on. The AC voltage sense would then be between voltage divider and junction of R2 & primary winding.

Input as in pin 2 ? but what is the poin in connect mains power to the input ? Just to make sure we are in the same page, the schematic is meant to be put in series with the secondary.

Need to keep in mind that when secondary is shorted, you could have full line voltage across R2 so the wattage rating of R2 needs to handle this.

What about the zener diode D1 does wattage matter ? The max amps the transformer can put out is 800A but will not be driving to that much. The secondary open voltage is 3.2vac. So a 3.6v @500mW zener and 3W resistor as R1 would be ok ? attached updated schematic with a triac

[C]

Start with this


Now break it down
It takes two connections to measure voltage.
It takes opening a connection and  putting something is series to measure current.

You have a series circuit composed of R1 & DK1. This gives two connections that need voltage & current to function.
At some voltage enough current will flow to cause output of DK1 to change. with AC connected you would get two pulses per cycle as long as AC voltage is high enough.

Now look at the remaining part.
A trac is a device that when triggered starts conducting until current drops below holding current.
As connected the trac can only turn off DK1 until 0 current( <holding current).
So the result would be just shorter ON time for DK1.

The second change caused by trac would be the very low voltage drop across trac between AC1 & AC2.

The when the two happen is controlled by trac trigger.

Now with NO AC connected to AC1 & AC2 nothing happens.

Now look at zener diode D1. One direction it conducts a a low voltage, other direction at rated zener diode D1 voltage.
The result is a almost full half cycle with other half cycle starting at higher voltage after 0 cross.

Now if AC1 & AC2 is in secondary  and in series with spot weld contacts, trac will need to survive the 800A+ amps.

The secondary open voltage is 3.2vac.

So you could have 1/2 of secondary voltage across trac in on state.

I see a mess

===========

Now think about your contacts.
3.2 volts is not much to work with poor contact.

========
You need to look at some resistance values.
Just the resistance of the wires connecting the transformer to spot weld contacts will limit current.
You need some force to reduce the contact resistance between two Nickel strips.

======
The advantage of circuits in primary is the higher voltages and lower currents.

[anishkgt]

A more refined schematic. The TRIAC is triggered by the same pulse to trigger the SCRs when a weld is initiated but i see a problem with the TRIAC, would it be able to withstand the current passing through. Would it be ideal to have 2 in parallel ? i guess i would have to test it out and see how it goes.

And to be honest i still haven't got your circuit completely.

[C]

 
-Voltage drop across TRIAC = ____?

[anishkgt]

Haven't found a TRAIC of suitable rating. Getting a feeling that this idea would be close to impossible.

[C]

Haven't found a TRAIC of suitable rating. Getting a feeling that this idea would be close to impossible.

You are starting to get the picture.

3.2 vac is not much to start a high current needed to spot weld.
As current increases the connecting wire is actually limiting max current.

The lower the starting voltage the cleaner the parts need to be to get the current started.

[anishkgt]

You are starting to get the picture.

3.2 vac ? what measurement is that ?

The lower the starting voltage the cleaner the parts need to be to get the current started.

which voltage and to start what current ?

[C]

Thank you, appreciate the time you took to write this.

you could connect one input to a line power voltage divider to adjust turn-on. The AC voltage sense would then be between voltage divider and junction of R2 & primary winding.

Input as in pin 2 ? but what is the poin in connect mains power to the input ? Just to make sure we are in the same page, the schematic is meant to be put in series with the secondary.

Need to keep in mind that when secondary is shorted, you could have full line voltage across R2 so the wattage rating of R2 needs to handle this.

What about the zener diode D1 does wattage matter ? The max amps the transformer can put out is 800A but will not be driving to that much. The secondary open voltage is 3.2vac. So a 3.6v @500mW zener and 3W resistor as R1 would be ok ? attached updated schematic with a triac

A spot weld is process of producing a lot of local heat.
To produce heat you need power. To produce a good spot weld you need a lot of power concentrated in a small area.
You need to heat the small area faster then you heat the larger area.