Capacitor testing

SofaKing

SofaKing

Well-known member
I know there are some members with solid electrical backgrounds and I need some help troubleshooting my mig welder. It is a 100 amp unit, 110v. Lately it is not working well. It seems to not be generating a hot arc like it used to, it also feeds the wire slowly. The symptoms seem to have appeared abruptly between uses but I don’t use it very regularly. I don't see any loose connections and the main capacitor does not appear to be swollen or leaking.
Is it possible to check the capacitor with a multimeter sufficiently? How is that done? I googled but what I saw does not seem like it would be an accurate measure for this large a capacitor. It has been unplugged a week but I have not tried to unload it and am reluctant to short across it. Thanks.
 
You need a common analog multi tester to check it.
FIRST, put the meter on "DC VOLTS" scale. 100v or more.
Put the red lead on the "+" terminal of the cap and touch the black to the other terminal to see if there is any charge left in the cap.
If it shows anything over 100 volts, let it sit some more till it goes below 20. When it gets that low, you can short it. Shorting a large cap with a big charge in it can burn the leads off inside the cap.
If it shows no voltage, short the cap with an old screwdriver to be sure it is safe.

THEN, put the multimeter on a mid to high ohms scale. Put the red lead on the "+" terminal of the cap. put the black lead on the "-" terminal and note any meter movement.
If the cap is good, the meter will deflect up momentarily and slowly go back toward zero.
If it does not go clear back to zero, the cap is leaky. If it goes up and stays at some level, the cap is shorted.
If the capacitor terminals are not marked, try the test one way and then reverse the leads to get results.
 
SofaKing said:
...Is it possible to check the capacitor with a multimeter sufficiently? How is that done?
Click to expand...
Firstly you need a wiring diagram or schematic for the welder soas to determine the characteristics of this suspect capacitor. It's size in farads will determine if it can be checked with commonly available equipment. I sold my capacitor checker but even it wouldn't test large AC caps.

Do you have a schematic? Do you have the Make and Model of the welder? Let's go from there. It might be easier to swap a new cap into the unit rather than testing that nasty cap.
 
Also check weldingtipsandtricks.com, I have a millermatic 180 and the wire tension knob usually needs adjustment after heavy use.

If it is a new spool of wire the drive roll grooves need to match the wire diameter, cheaper wire may not be of the proper spec.
 
I got bored so with great trepidation I shorted across it, no trace of life. This was before anyone posted last night. I don’t have an analog meter. With it disconnected and digital meter on 20m ohm scale it will climb to .032. Reverse the leads there is a quick spike but it never drops to 0. It stays at .1. This sounds like what Pete described as “leaky” but I don’t know if using digital meter ruins the test. It is a 59000mfd 40v cap. It will be $100 to swap out. Only found one place that sells them so far.

I appreciate the help.
 
SofaKing said:
I got bored so with great trepidation I shorted across it, no trace of life. This was before anyone posted last night. I don’t have an analog meter. With it disconnected and digital meter on 20m ohm scale it will climb to .032. Reverse the leads there is a quick spike but it never drops to 0. It stays at .1. This sounds like what Pete described as “leaky” but I don’t know if using digital meter ruins the test. It is a 59000mfd 40v cap. It will be $100 to swap out. Only found one place that sells them so far.

I appreciate the help.
Click to expand...

A digital meter will not load the circuit enough to get a good reading, especially with a 59K mfd cap. With those readings, I would say it is ok though.
It is NOT shorted.
 
Quick follow-up, I read the thread on cleaning the feed wheels, tension and inspecting the cable housing suggested by rotor wrench and 39portlander. There was some schmutz and I cleaned everything, rotated the housing even though I couldn’t see any wear, and put it back together. No joy. The arc is still weak and unstable. I hate swapping parts for troubleshooting but don’t see an alternative at the moment.

Pete, I did not see your response until after I added this comment. I don’t have a wiring diagram as suggested by Red. Any ideas on how/where I can get this tested to nail it down?
 
Have you tried Digi-Key? I get most of my electronic components from them. They seem to have the best prices and fastest shipping. Their product selection is huge, and you have to know what you're doing just to get around their website, but it might be worth a try.
 
I have gotten those big value caps from Fair Radio Sales. They are usually under 5 bucks.
Sometimes you have to gang 2 or 3 up to get the value you need.
 
See > https://www.weldingandcutting.com/Capacitor-for-Century-Welders-p/s13490-109.htm



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59,000mf, 40Vdc Capacitor for Century (213-035-666) and other welders. 9SS13490-109
Description
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$86.33

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Just a follow-up, I replaced the capacitor, using the one shown above. This was the only result I found googling. The welder seems to be working ok but I am withholding judgement until I use it a while since it was intermittent. I didn’t try stacking smaller (cheaper) caps because if it didn’t work I am not smart enough to know if it was a welder problem or modification problem. One more thing, turns out there is a wiring diagram on the inside of the side panel. I didn’t notice until I was putting it back together, not sure if I am getting less observant or less interested. Thanks everyone.
 
When the welder at work got weak I found connections at the transformer had corrosion, aluminum windings to copper connections
 
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