Foot pedal mods for PA160-STH

[joshuab]

I've got a couple questions about mods to the foot pedal for the PA160-STH. Another thread got me thinking that it'd be nice to be able to cut the pedal out of circuit without having to disconnect it entirely when I switch over to stick mode. Ideally, this would happen automatically when the welder was in stick mode (hint hint: future feature request!) but that would probably require mucking about inside the welder, and might not be compatible with my warranty. I don't know, though--would it be as simple as jumping a pin of the pedal jack over to a pin of the TIG/switch button, causing a certain pin to be pulled to ground when the welder was in one mode or another? Hmm... maybe so. An alternative that would work without opening up the welder would be to install a breakout box on the cable or something like that. The breakout box could have an on/off switch and a pot on it for adjusting the max output, just like the pot that's currently on the pedal, but I would be able to adjust max output without having to bend over and pick up the pedal and push it with my hand.

The second mod that I'd like to make would be to decrease the sensitivity of the pedal's max-output pot towards the high end of its range. As I've mentioned previously, the pedal's max-output pot goes basically from 120 amps right to 160 amps and pegs there for the last 1/8" or so of the dial's travel. Mark has explained to me that the pedal's pot needs to be logarithmic because... resolution. Or something. Sorry--here's my ignorance showing. I don't really have a strong electrical background, but I can solder components together if given instructions! So I'm wondering if there was some mod I could make to the max-output pot to change its response curve to have less sensitivity at the top end. For example, I once read about how you could build a semi-logarithmic pot by putting some resistors in circuit with a linear pot in the right way. So maybe there's a way to change the response curve of this one.

Thanks!

[Rambozo]

Yes, I'm sure what you want could be done. You would have to check with Everlast about warranty issues. I would guess that you would be on your own there, as you might be able to mess things up. If the warranty is important to you, I'd stop as you mentioned that you don't have a lot of experience.
You can change the way the pot works, but it's a bit beyond the scope of forum posts. You would have to reverse engineer what is going on and you might have to open up the welder to get that info. I'm not even completely sure that just opening the pedal sense pins takes everything out of circuit. It might take out the panel amperage but leave the pedal parts in circuit, that could make your adjustments strange or non-functional. You have to think about the original design. The purpose of those pins is to bypass the panel control and route it to the pedal. A little testing would be required to determine that the reverse is true. So you might need a few more contacts on your switch to get the job done. I believe someone has done a breakout box like you mentioned, but I'm not sure for what model welder. You can search the forum for more info. It would be handy if switching to stick mode bypassed the pedal. However, I have stick welded once using a foot pedal for fine amp control. I was filling in a varying gap from a bunch of previously torch cut pieces way up under a roof where I really couldn't do much to improve the fit. The only machine available was a TIG and it turned out having the foot amp was kinda cool. HF start was nice, too. Just another tool in your belt for the odd job. I know Miller has a belt mounted wireless remote amp for pipeline stick welders so you don't have to climb out of a trench to switch your machine.

[joshuab]

That's a good point about cutting the panel amp control out, but maybe not cutting the pedal entirely out of circuit. Depending on the number of pins involved, it could still be doable to buy an multi-pole switch and just cut the lead entirely.

Regarding the pot, I'd love it if you had any search terms or links to more information.

[Rambozo]

That's a good point about cutting the panel amp control out, but maybe not cutting the pedal entirely out of circuit. Depending on the number of pins involved, it could still be doable to buy an multi-pole switch and just cut the lead entirely.

Regarding the pot, I'd love it if you had any search terms or links to more information.

There are both log and linear pots, and also solid state pots that can produce almost any curve you want. Post a schematic of the two pot pedal and I'll have a look at it.

[joshuab]

Post a schematic of the two pot pedal and I'll have a look at it.

I'll get back to you on this. Have to open the pedal up first. And have dinner with my family ;-)

[joshuab]

Well, who doesn't love data? Just for kicks, I measured the resistance on pins 4 and 5 of my pedal. Here's the results.



It occurs to me that one other useful thing would be to add the welder's power output onto the same graph, to see how it is mapping resistance to weld output. There's no reason to assume a linear relationship there.

[joshuab]

Okay. One more data point. This adds the front-panel output value, mapping it across the resistance values. The pedal was full-down for the front-panel series. The welder was plugged into 110v power for the exercise, so I could have it in the same room as my computer.



I think this really clearly demonstrates the issue. More than half of the dial's travel range is spent between 27 and 19 amps, which is only 10% of the usable range of the pedal (96 to 19 amps).

[joshuab]

Ok. I opened up the pedal and poked at it with a multimeter. Here's my wiring diagram.



To sum up: The gray wire goes to pin 5. The brown wire goes to pin 4. The green/yellow wire goes to pin 4, but only when the pedal is pushed. Hopefully, this is enough information for a person familiar with electronics to suss it out. I'm a bit surprised to find that the max-output pot doesn't have three wires going to it, but maybe it's just being used as a rheostat. Or maybe there's some nuance of electronics there that I'm missing--quite likely.

I have been thinking about the graph of amp output relative to potentiometer resistance. The pot seems to have basically a linear response. The amp output line seems basically logarithmic. It seems that if the max output pot could be caused to be inverse-log, then the amps output would become linear. Any intelligent people have thoughts about this?

[Rambozo]

Ok. I opened up the pedal and poked at it with a multimeter. Here's my wiring diagram.



To sum up: The gray wire goes to pin 5. The brown wire goes to pin 4. The green/yellow wire goes to pin 4, but only when the pedal is pushed. Hopefully, this is enough information for a person familiar with electronics to suss it out. I'm a bit surprised to find that the max-output pot doesn't have three wires going to it, but maybe it's just being used as a rheostat. Or maybe there's some nuance of electronics there that I'm missing--quite likely.

I have been thinking about the graph of amp output relative to potentiometer resistance. The pot seems to have basically a linear response. The amp output line seems basically logarithmic. It seems that if the max output pot could be caused to be inverse-log, then the amps output would become linear. Any intelligent people have thoughts about this?

Kinda hard to tell things from your diagram, maybe pictures would be better. You have two leads going to pin 4 and that isn't correct. Where the wires go on the pot are important. A pot has a fixed resistance between two leads, with a wiper that can sweep between them, connected to the third. That one photo of the max pot is an odd one, too. It has three leads but they are placed in a way that I would check everything with an ohm meter to verify just how it is constructed, it may be something a little out of the ordinary.

[joshuab]

... and here's the marking on the back of the pot, which I'm sure will be very informative to a knowledgeable person.