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
The nice thing - it will re-start when current halts or limits down. No idea at
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.
Been a while since I've been into one of those.
If memory is any good, and it has been getting worse!!-there is a current
sensing device and time delay relay for this function.
Email me directly if this doesn't answer your question.