I'm hoping someone can help me understand a couple aspects of the attached circuit. It's a simple, low end MIG welder (Miller Cricket) rated at about 120 amps, 20vdc at 20% duty cycle. Even though it's a small machine, the basic design features are similar to arger transformer-rectifier MIG's, but without all the obfuscating bells and whistles.
BTW: VR1 is a 10 joule 68vdc varistor, R2 is a 20w 50 ohm wirewound and C1 is 46,000 uf 35vdc.
1) First of all, what's the purpose of (what I'm sure are) ceramic capacitors C2, C3 across the diodes of the center-tapped FWB? I understand these might have some role in supressing noise coming off the diodes--but what difference would that make in this simple machine? There are no IC's anywhere to be victims of noise. Anyway, some manufacturers use them, and some don't. I've seen them in 4-diode FWB's too. What would be a good size for these? .01uf, .1uf, 1uf, ... all of the above? I haven't been able to find a welder circuit diagram that calls out a value.
2) Some manufacturers also include ceramic capacitors from the output terminals (labeled work and torch in this diagram) to ground. What would be the purpose of such capacitors? What would be reasonable values?
3) What's the purpose of varistor VR1? Varistors are similar in some ways to diacs--at a threshold voltage they switch full on. So is this varistor to protect the big capacitor C1 against over-voltage spikes, or to protect the fellow with his hand on the work? NB: A varistor in this location doesn't seem to be a common feature in MIG circuits I've seen.
4) Finally, what's the purpose of the big resistor R2 across the output? Maybe to limit current when the electrode wire shorts to the work? Not every manufacturer uses them, but they are not uncommon on MIGs. A big 250 amp machine might have a 300w 50 ohm resistor in this location.
Thanks in advance; I really appreciate the help.
BTW: VR1 is a 10 joule 68vdc varistor, R2 is a 20w 50 ohm wirewound and C1 is 46,000 uf 35vdc.
1) First of all, what's the purpose of (what I'm sure are) ceramic capacitors C2, C3 across the diodes of the center-tapped FWB? I understand these might have some role in supressing noise coming off the diodes--but what difference would that make in this simple machine? There are no IC's anywhere to be victims of noise. Anyway, some manufacturers use them, and some don't. I've seen them in 4-diode FWB's too. What would be a good size for these? .01uf, .1uf, 1uf, ... all of the above? I haven't been able to find a welder circuit diagram that calls out a value.
2) Some manufacturers also include ceramic capacitors from the output terminals (labeled work and torch in this diagram) to ground. What would be the purpose of such capacitors? What would be reasonable values?
3) What's the purpose of varistor VR1? Varistors are similar in some ways to diacs--at a threshold voltage they switch full on. So is this varistor to protect the big capacitor C1 against over-voltage spikes, or to protect the fellow with his hand on the work? NB: A varistor in this location doesn't seem to be a common feature in MIG circuits I've seen.
4) Finally, what's the purpose of the big resistor R2 across the output? Maybe to limit current when the electrode wire shorts to the work? Not every manufacturer uses them, but they are not uncommon on MIGs. A big 250 amp machine might have a 300w 50 ohm resistor in this location.
Thanks in advance; I really appreciate the help.