Electronics > Beginners
How to set reference for a sine wave with dc supply
anishkgt:
--- Quote from: C on June 30, 2018, 11:52:05 pm ---
As I understand it,
You are wanting to replace a foot switch that triggers the spot weld.
You are trying to
--- Quote ---detect when the electrodes touch the Nickel plates for a weld.
--- End quote ---
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
.
--- End quote ---
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.
--- Quote ---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.
--- End quote ---
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.
--- Quote ---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.
--- End quote ---
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.
--- Quote ---The secondary open voltage is 3.2vac.
--- End quote ---
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.
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