My Miller 330 A/BP has a start circuit to provide a boost to initiate the arc, then the weld current goes to the primary amperage setting. Despite studying the schematic (I am a layperson relative to electrical circuits), I have been unable to figure out how the start circuit "knows" to disconnect after the arc initiates. How does the start circuit know the arc has initiated and that it should disconnect itself? Thanks. John
Not having the schematic in front of me, my best guess is that it senses current flow through the leads, probably by measuring or otherwise reacting to the voltage drop across some component in the main circuit. Or, it could be marketing b.s. that is calling the higher open circuit voltage a "boost" which naturally drops when the arc is initiated...I'm not familiar with your machine, so this could be totally off-base, especially if it's an inverter machinge...
I looked at my miller 180SD - the HV is an LC tank circuit - oscillator that is controlled from going to wild by the arc points. The main control board senses the lead current through a current transformer. It determines current flow and turns off the AC/DC supply to the LC Tank. (LC means inductor capacitor).
The nice thing - it will re-start when current halts or limits down. No idea at to what current level it does what....
There appears to be, what I think is a timer circuit across the output circuit. This circuit. consists of a tapped resistor, full wave rectifier, two capacitors and a relay coil. The relay contacts are in the control circuit, and since these contacts are in the control circuit they probably alter the output of the machine. I don't see anything that would detect when the arc is started. I think the time starts when the foot pedal is pressed and closes the main contactor.
When the main contactor is closed, causing voltage to appear in the output circuit, This timer circuit will begin to charge the capacitor that is connected in parallel with the relay coil. The resistor and capacitor are probably sized to allow the capacitor to charge slowly, allowing an acceptable time to pass before the relay coil has enough voltage to activate the contacts in the control circuit. The full wave rectifier makes the circuit work about the same regardless of the polarity setting, and I think the second capacitor helps to protect components from any high frequency that may get back into the machine.
I'm not an expert on this machine, but this how I think it works.