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What are the symptoms of a bad capacitor.

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  • What are the symptoms of a bad capacitor.

    I have an issue on my Millermatic 175 220 volt welder. The problem showed up the other day when I went to weld some sheet metal. I had the setting on volts around 25 and the speed around 40. The machine acted like the wire speed was inconsistent to to slow. I tried several voltage and speed settings and nothing helped. I did a free run on the wire and it was coming out fine and would change speed as expected with the control dial. I've welded with these setting for years on this machine with no problems. I did some research on line and it appears to be the capacitor. I took the cover off and removed the diode bridge. I checked the diodes for continuity and they appear to be ok ,current in one direction and none in the other direction, but I was not able to check them individually only 4 at a time. I do not have a multimeter that I can check the cap because of how high uF it is. What does the community think is the issue? I've had this welder for years and other than a trigger replacement this is the first time it has let me down.

  • #2
    Unless one of them was physically blown, you,d need a meter that reads uF capacitances.

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    • #3
      Ye. My meter only goes to 20uF. The cap in my welder is 100,000uF. I was hoping that someone had experience a bad cap so they could tell me how it acted.

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      • #4
        I use a Fluke capacitance meter. Pretty easy after that. Board level, I have a Huntron scanner.

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        • #5
          Have you seen what the welder acts like if the big cap/s go bad? Or any idea what would cause my welder to act like it is?

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          • #6
            Are there any signs of fluid leaking, stains around the vent? You can usually smell a bad cap too. If it's bad, then it's either shorting out or not storing charge (open) at working voltage. Your open circuit voltage (OCV) would show low either case - OV if the cap is shorting, or about 70% of what the manual says if the cap is not storing charge. Section 3.3B in the manual has the OCV levels for the different voltage settings. OCV should be between 16V and 29.5V from Voltage Control 0 to 10. I have never seen the issue, but would think if your cap was bad, your wire would be stubbing and looking like it's too fast instead of looking like it's too slow. Does your OCV change with different voltage settings?

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            • #7
              The Cap looks fine. I took it out today and did a test today. I tried charging the cap by using the ohm setting on my multimeter. It's not much of a test but the cap would hold a charge. jjohn76 the manual you talk about, is that the owners manual or a repair manual. If it's a repair manual can I download it some where? Thanks for trying to help

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              • #8
                It's the owner's manual. I just looked at version D.

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                • #9
                  Well today I put the cap and diode bridge back in the mig. Fired it up and checked the OCV at multiple points. The voltage was in spec from setting 1 to 10 ranging from 16 to just over 30 volts DC. I also checked to see if there was any AC ripple hanging on the DC and there was 25mV on the low end to 34mV on the high voltage setting. Not sure if that is ok or not I know on IC circuits it is not. After doing these test and everything seemed ok I decided to try doing some welding and the thing welded fine on sheet metal and heavier material. Not sure what changed but for now it seems ok. Thanks jjohn76, cruizer for the help.

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                  • #10
                    Probably a corroded / loose connection on 1 or more of the bank of capacitors.

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                    • #11
                      Everything looked good because that was one of the things I thought of first. But something changed and that is probably what happened. Thanks again.

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                      • #12
                        I'm new to this forum.I have a miller 175 mig welder that is totally dead.The circuit board is fine (tried on friends welder)Tested the power switch,over temperature circuit ,resettable breaker and ohmed the fan motor they all check out.Just before it died the fan was barely spinning and the two led lights on the board were dim.I can hear the transformer humming but thats about all it does. I was wondering if the rectifier could cause this issue.Any help would be appreciated as im running out of ideas.

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                        • #13
                          Welcome! You probably should start a new thread if you want more people to see your post, with a title that reflects what you're working on. Also, please post the machine's serial number when you do - it lets us find the correct manual to look at.

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                          • #14
                            Originally posted by John exc View Post
                            ... ohmed the fan motor they all check out. Just before it died the fan was barely spinning and the two led lights on the board were dim...
                            Don't take offense, but we don't know you well enough to know your qualifications. What do you mean you 'ohmed the fan motor and it checked out?' What was your number, and your reasoning that it checked out? Because the symptom describes a bad fan, which also powers the circuit board.

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