Strange pedal behavior

[rcross]

Greetings,

My itig200T is coming on 6th month and I've been generally happy with the performance.

There is, however, a strange behavior I am seeing. When tapering off with the pedal at the end of weld, sometimes I get a spike of current towards the end, when I am easing up on the pedal. Say, I am getting down to 40A, then all of a sudden I get a full blast at 125A right before the arc extinguishes. Haven't had a way to look at the panel when this happens, so it is just my judgment of what is happening.

It is very annoying when that happens near the edge, because it pretty much guarantees a nice blow-through. It does not happen all the time, maybe twice every 5 welds.

To me, behavior is very similar to what would happen with dirty (scratchy) pot. So I looked inside the pedal.

The pedal (has FT-47K-CL on the back) has small caps soldered across pot pins to deal with scratchiness, I presume. I was surprised to see that the insides of the pedal are squeaky clean. Visually the pot does not appear to be of cheap variety and seems well sealed. The pedal is very well constructed. So no I am not really sure.

Settings: no pulse, peak at 125A, start 20A end 20A, 0s up/down slope, 2T, 0.5s pre flow, 10s post flow.

Looking for ideas on how to pinpoint the cause and/or avoid the issue. All suggestions much appreciated.

-rC

[christian]

Gee,

It seems like you've touched all of the proverbial bases. I mean, it sounded like your end amp setting might have been higher than the minimum, which would cause as much, undoubtedly.

Since those items apparently have a 6 month warranty, I'd send it in to Everlast for a repair or replacement. Give them a call for an RMA number.

Or I might just buy one of their new upgraded NOVA pedals. I mean, I bought an upgraded SSC pedal for my 2015 210EXT right after getting the welder, as it seemed a sensible upgrade, much like I also immediately upgraded to a Superflex torch hose/cable.

[Rambozo]

Looking for ideas on how to pinpoint the cause and/or avoid the issue. All suggestions much appreciated.

I think you called it, sounds like a bad spot in the pot to me, too. One way to pinpoint would be to test the pot with a meter that has a min max function or a DSO. Or just replace the pot and see if the problem is gone.
For a way to avoid the issue, set the machine to use the downslope function then you don't need to do a manual tapering off. (not a real fix, but you did ask how to avoid)

[rcross]

Yes, I tried the multimeter, even LCR meter, but those digital ones usually have a slow analog to digital converter, so it is rather hard to catch transient events with it. I really need to hook the DSO up.

For the downslope to work, I would need to just get off the pedal abruptly and then let the downslope to run its course, right?

Thank you for the suggestion.

[performance]

Just a cheap 10.00 needle meter would work better, more than likely.
Make sure you test both sides. Something is causing it to surge. Could be the machine, but most issues are in the pedal.

[rcross]

Thank you for the suggestion. I will try the analog multimeter too.

Do you happen to know the voltage that is applied across the pot when the pedal is connected to the machine? I assume it is quite a bit higher than my multimeter is putting out.

[performance]

I don't know the voltage off hand but it won't matter when you test the pins according to the plug pinout in the manual. I don't think it is higher than a few volts anyway.

[rcross]

The pot is bad. Analog meter is able to catch the problem. Sometimes when getting to the end where it is supposed to read 0 Ohm, it gets to infinity just before the very end.

The pot is carbon film type. I think I think, I will replace with cermet. $20 vs $5 part.

[rcross]

So the engineer in me just had to get to the bottom of this. Posting for the benefit of others and also because it is somewhat interesting. Everlast people, if this crosses some sort of ethical line or breaks rules, please take it down.

The fact which triggered this investigation was that the flaky spot was on just one side of the wiper. As in one half of the pot had the transient infinity reading, while other did not. If there was a bad spot on the carbon film track, both sides should see the transient. Nope, happens only between the wiper (contact 2) and contact 1 - refer to the picture below. The transient would not happen between 2 and 3. At least I could not reproduce.

Turns out, the crimping at the contact post 1 was to blame (right above the number 1 in the picture). It was not crimped with proper force, so when the wiper moved close to the spot marked with the red arrow, the pressure of the wiper would create enough movement in the film strip, that it would break contact with the post 1, sometimes. I re-crimped with neddlenose pliers and cannot reproduce the issue anymore. Both sides work properly now. Of course, the pot is garbage at this point.

The pot design looks like a reliable one - triple spring loaded wipers, 7/8 of a turn rotation, film rests against very rigid plastic body, etc. Yet the manufacturing is more complicated and can introduce weak points where one would not expect them.

I find it interesting, that $5 part is the weak link in the otherwise very well engineered and manufactured $150 pedal (the one I have looks like Nova/SSC pedals).

Here is the image:





-rC

[performance]

That isn't the NOVA pedal. There's a big difference. The NOVA pot is much more than $5. But that is a better pot than used to be in the pedals. The NOVA pot is a 2 million cycle pot from Japan.

[christian]

Hey,

Nice job troubleshooting, in catching that slight pot wiper inconsistency.

Are you going to now get a new pot for the pedal, or just use the repaired one, or consider a pedal upgrade?

So the engineer in me just had to get to the bottom of this... The pot design looks like a reliable one - triple spring loaded wipers, 7/8 of a turn rotation, film rests against very rigid plastic body, etc. Yet the manufacturing is more complicated and can introduce weak points where one would not expect them.

I find it interesting, that $5 part is the weak link in the otherwise very well engineered and manufactured $150 pedal (the one I have looks like Nova/SSC pedals).age:

[rcross]

My experience taught me, that once open, the pot has very short life span. Besides, my crimping might have done some damage to the strip coating and it could fail in a different way. So this pot goes into the scrap bin.

The pedal itself does not warrant an upgrade. I like the feel of it, the mechanical internals and the design. The pot is and will stay the weak link (no matter what the replacement). The only upgrade that would make sense would be to switch to a solid state device, but that would be an overkill.

That being said, there are two minor things I will change - use a keyed shaft pot (with a flat spot) and use proper set screw in the pinion instead of generic screw.

I have ordered a high durability cermet pots and a few carbon pots of a different design. Will try the cermet first, if that fails, will go back to the carbon.