Electronics question

I got the new pot installed in the foot pedal to my TIG machine today . Same 47k ohm rating as the original . But there's something weird going on , I'm not getting the amperage I set the dial for , it's lower than the setting . Not an earth shaking problem as I have never used the full amperage and I can control it now . Still , I'd rather get the amperage I have it set at instead of guessing . Snag

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Here is a sample wiring diagram and test procedure for a TIG foot pedal.

Pots can have different curves to their change in resistance on the sweeper. For something like that I would think a linear curve would be best, but it might be worth some research.

Its pretty easy to stick a meter on it and check total value and the sweeper value. I'd probably do that first.

Then I might check to see if the pot is at max and min values at max and min pedal travel. It might be possible to stick it in with the knob or ring not lined up correctly. If that happens you can break the pot in some cases by stomping on the pedal.

I had to search for that document as the link didn't work, probably the ').pdf?' on the end it didn't like. The pedal looks just like the Linemaster Varior used with my Hitachi TIG, I had to check that recently and found the 50k pot was reading about 100k and not very smooth. Now changed after locating a compatible direct replacement pot but still something is not quite right with my TIG so it's going into the supplier for testing. It took a while thinking about how to take the Varior pedal apart as Linemaster indicates it can be mounted with the 2 screw holes provided but no information about how to take the pedal apart to do so. In the end I figured out it had to have a spring loaded pivot pin like a large watch strap pin and that proved to be so, easy once you know how.

Pots can have different curves to their change in resistance on the sweeper. For something like that I would think a linear curve would be best, but it might be worth some research.

Its pretty easy to stick a meter on it and check total value and the sweeper value. I'd probably do that first.

Then I might check to see if the pot is at max and min values at max and min pedal travel. It might be possible to stick it in with the knob or ring not lined up correctly. If that happens you can break the pot in some cases by stomping on the pedal.

-------------------------------------------- The common nonlinear pot is an audio volume control.

Did the original pot give the amperage you set?

Does the new pot have the same mechanical travel as the original?

bob prohaska

I got a 404 error from Jim's link . Did you find the page ? My welder is an Everlast .

Try

This is a linear taper pot and I made sure I got it clocked correctly before I locked the cog pulley down . I have another , I'll check resistance values tomorrow . The original pedal broke the pot because the overtravel stop screw was missing . I repaired and used that pedal for a while . I asked Everlast customer service for a replacement pot and they sent me a whole new pedal - which is the one I just repaired . The old one still works and is here somewhere I put it so I'd remember where it was ... but that was 12 years ago , and my memory is the second shortest part of my anatomy .

Still no joy . No matter , I wired this new pot exactly the same as the old one . It works , just not calibrated to the amperage set on the display . I'll test it tomorrow to make sure it has the advertised values .

Yes and yes . Chances are slim it's a bad pot but I'll test it tomorrow and test the other one - I bought two .

When I click on the link it give me 404 and the URL is missing the ').pdf' at the end, I can add those characters manually and retry and it finds the document.

Have you installed the pot in the correct orientation. The illustration in the PDF looks like the Linemaster Varior I have and the pot could be installed so it is rotated if not careful and that could lead to it going solid at extremes of travel possibly damaging the pot. The one mine required was this one

There is a tab that engages a slot to prevent rotation . With the pedal all the way up I rotated the shaft to the internal stop then backed it off a tiny bit then locked the cog pulley down . At the other extreme I still have just a tiny bit of rotation left on the pot .

I just ordered a pot just like the one in your link but several bucks cheaper and closer to my location - same mfr . The ones I have are plastic shaft , and that was part of the reason I bought 2 . But at least I can weld aluminum a lot easier now .

Here's a good link to the pedal wiring diagram and test procedure .

Here is the same link from a different search:

I found the link last night and printed the info .

I checked the resistance on that new pot . And I got some odd readings ... I get 21k maxed out one side and 46k the other side . Outside terminal to outside I get the 47k plus/minus . THis may or may not have something to do with output not matching set point . The other pot measures 47k+- maxed out both ways . I'm having a hard time understanding the different values , AFAIK these all use a single wiper center contact so I should be getting the same reading from either side when maxed out .

I should note that the "other pot" mentioned above is the other new one . The old one has readings so unstable you can't tell what they are .

Has to be disconnected to get a good reading.

Sounds like a bad pot to me. Exactly _how_ it's bad is confusing to me as well. In any case, your expectation is correct and the pot is likely the culprit.

My first thought was that something got stuck under the wiper, but that would likely raise the resistance, not lower it. I can't come up with a simple way to cause what you're seeing. In fact, it's hard to come up with a _complicated_ way to obtain what you're seeing....

If you don't end up sending it back a dissection might be interesting. The vendor should certainly replace it, or at least offer a credit.

Nice work!

bob prohaska