Electronics > Beginners
How to set reference for a sine wave with dc supply
C:
If your meter gives these values you will need to look at signal with OSope.
The inductance of transformer can cause current lag from a pure resistance circuit.
anishkgt:
I am not sure if it would ok to check the ac voltage with a scope. The 34v does not come from isolated source but comes from the same line as the PCB is powered. The 34v is just a phase shifted voltage.
I could check secondary side but not sure how to do it at the primary without blowing the scope.
C:
OK
might not be a great idea with scope.
Starting over.
To sense a contact between the two spot weld contacts you need to create a current flow or voltage change.
You stated you are using a microcontroller.
I see you have created a Zero-voltage cross circuit.
This way is not nice to power line
Fpr creating spot weld you turn on TRIAC after zero volt cross but near 0. Due to inductance causing current lag, TRIAC will actually drop below holding current after zero volts.
For contact sense you turn on TRIAC a short time before Zero cross. Again off is after Zero cross.
This will create a short voltage spike on secondary side and can show with a primary side current sensor. Open will be one low current value. Contact will be higher current value.
Just so you know, how hot a spot weld gets is heat vs time or here number of 1/2 cycles of AC in this case.
Contact would be too to little heat to do anything but sense contact.
The advantage is no series resistor in primary as contact will use current sensor.
To keep it simple, really should use ADC to measure Current but this will take better software then just contact & no contact.
Creating digital input is not that hard so software could be just a digital input pin.
So micro will sense Zero cross of line
Will sense primary current with ADC or digital pin
Will control TRAC with driver that does NOT have zero cross switching.
anishkgt:
I am trying to make sense of what your trying to say but half way through you seem to jumping between ideas and the first part leaves me puzzled.
Just to make it clear the 33vac is actually produced by the microcontroller by switching on the SCRs at 9000ms after the zero cross. I could increase the voltage by switching on the SCRs few seconds after the zero cross, like maybe 7000ms or 8000ms. So you say i put a resistor in series with the primary and determine when electrodes short by measuring the voltage difference ? well what happens when at full voltage ? I may have misunderstood but without a clearer schematic am just guessing.
Mean time I've been thinking at the secondary side. The TRIAC here is the BTA440Z-800BT (http://www.ween-semi.com/documents/BTA440Z-800BT.pdf) The non-repetitive peak on-state current (I-tsm) at 20ms is 400A. So based on that comment in the datasheet i guess this should work but not sure about the heat dissipated and how long the TRIAC would hold up.
anishkgt:
--- Quote from: C on June 30, 2018, 10:05:58 pm ---here
Nice an simple
--- End quote ---
In your schematic, as i understand it, the resistor in series with the opto-coupler should be high enough drop the voltage to zero when the secondary are shorted and low enough such there is some voltage for the LED in the opto-coupler to work, am i correct ?
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