Blown Capacitors

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Hello,I am not an electrical engineer but I have built many electrical projects & constantly rebuild electrical equipment. I recently purchased a very large MIG welder that had an issue. I found a six-pack of capacitors connected to the hot electrode. There are six 10k microfarad in parallel hooked to the wire going down the hose to become the wire. They are slightly smaller than a soda can. I saw the first capacitor was bad. In my pile of stuff I found two 30k microfarad large enough to work. After I swapped out the six-pack for the two replacements, I test welded for 20 minutes or more. Yeah! I saved a welder to add to my shop.But…When I turned to lay the welding gun down the wire tip poking out of the gun brushed the welders outer case (no trigger was pulled) and the first capacitor of the two blew. Something discharged to the case ground? I went into my parts stacks And couldn’t come up with another capacitor solution except for twelve 5k microfarad and that opens up another issue of space. Someone told me to get as close as I can on the high side to absorb the strange over-load. I put in a combination (all identical)  that added up to 80k microfarad & immediately blew the breaker when I started to weld.I will have to purchase the next set of caps so I want to figure this out before I repair it a third time. There is almost no other electronics involved when you take out wire-feed motor & circuitry. Does anyone know what else (from an electrical circuit perspective) I might check? If the tip was ‘cold’ does that mean the caps were fully charged when I stopped welding & discharged when I touched the case? Why would they not suffer a similar fate each time I started a new weld bead? Is there a reference chart on what caps to use for a given power?Thank you.
Reply:What make and model is the welder?
Reply:As drf255 asked - what make and model welder?Are your replacement caps rated for DC or AC?There should be a discharge resistor in the circuit across (+) to (-) to prevent what happened.  Is it in place?MM200 w/Spoolmatic 1Syncrowave 180SDBobcat 225G Plus - LP/NGMUTT Suitcase WirefeederWC-1S/Spoolmatic 1HF-251D-1PakMaster 100XL '68 Red Face Code #6633 projectStar Jet 21-110Save Second Base!
Reply:I am not sure what happened there. But only 3 things kill capacitors.1 over heating, 2 over voltage, 3 revised charges,  if its a polarized electrolytic. 4 physical damage, but pretty sure that isnt the case.Over volting tends to cause catastrophic, burning or blowing it apart.If your machines max OCV is under 40 volts you need to install one of my metal oxide varesistors.I specifically ordered them to protect mig machine capacitors from voltage spikes. Still got like 95 of them left.MOVs rapidly bleed off voltage above a certian level. Also what process do you like to run? If you are running gas sheilded solid wire you dont need a lot of capacitance and dont worry about finding an exact identical replacement. If you run flux core, this might be a good time to up grade your capacitor.old Miller spectrum 625 Lincoln SP-135 T, CO2+0.025 wireMiller model 250 and WP-18V torchCraftsman 100amp AC/DC and WP-17V torchCentury 115-004 HF arc stabilizerHome made 4 transformer spot welderHome made alternator welder
Reply:Originally Posted by mad welder 4I am not sure what happened there. But only 3 things kill capacitors.1 over heating, 2 over voltage, 3 revised charges,  if its a polarized electrolytic. 4 physical damage, but pretty sure that isnt the case.Over volting tends to cause catastrophic, burning or blowing it apart.If your machines max OCV is under 40 volts you need to install one of my metal oxide varesistors.I specifically ordered them to protect mig machine capacitors from voltage spikes. Still got like 95 of them left.MOVs rapidly bleed off voltage above a certian level.
Reply:I had a few blow in my older Miller mig a few years back, filled the shop with white smoke, scared the bejeeses out of me.
Reply:If the tip shorts out when it touches the case, you have other problems besides the capacitors. There is probably an internal short in the welder. This short may be causing the breaker to blow, that or the rectifier is bad.Additionally, there is a reason they use a bank of 6 caps rather than 1 or 2 larger ones. The caps in MIG welders experience high peak currents and thus a fair amount of internal heating. If you replace those 6 caps with 2 larger ones, they may have the same total capacitance but will soon overheat and fail.JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:Originally Posted by pin2hotFrom what I understand, high frequency can kill a capacitor that isn't desiged for it.  SCR commutating caps are made for that.  I once had to replace some failed caps in a rectifier system that were used for SCR commutation, but could not handle the high frequency component.
Reply:Still don't know what kind of welder,it may have no SCRs.The voltage,ripple,power dissipation and ESR all have to be accounted for ,depending on the type of output circuit.A blown rectifier circuit will put AC into the capacitors and make them blow and depolarize as shorted.Without knowing the specifics of the welder,it is all just speculation.
Reply:Check for a shorted diode.This will put AC on the capacitors.