Foot Pedal 47k Vs 50k Potentiometer ?

[brianscott]

I'm converting an old Lincoln Foot pedal to use with my 225LX.

The only pot that I can find (that has the long stem) is 50k, my pedal uses a 47k.

Is the 50k OK to use?

[performance]

That's what SSC uses.

[acourtjester]

mouser.com has all the pots you want with 2" shafts just pick you ohms and look for a 50.8 mm shaft length.
http://www.mouser.com/catalog/catalogusd/646/860.pdf

have fun
Tom

[Rambozo]

As you can see 47K isn't all that common. For something like this the 50K should be fine, but if you want to turn your 50K into a 47K it's easy to do, just parallel a 750K or 800K fixed resistor across it and bingo one 47K pot.
In case you are curious the formula for resistors in parallel is (R1*R2)/(R1+R2)

[sportbike]

Does teh welder "look" at both the increasing and decreasing sides of the pot?

All three wires are connected, just wondering what the control logic is inside the welder.

At 1/3 travel, is it looking for 1/3 total resistance on one side and 2/3 on the other? How does it use the resistance data?

[Kempy]

As you can see 47K isn't all that common. For something like this the 50K should be fine, but if you want to turn your 50K into a 47K it's easy to do, just parallel a 750K or 800K fixed resistor across it and bingo one 47K pot.
In case you are curious the formula for resistors in parallel is (R1*R2)/(R1+R2)

I tried a 50K pot and it gave me max. amps but not low end amps just 20A and 251A at max. put 47K in and got 6 amps low end and 252A top end. But did not try 750K resistor maybe next time.

[acourtjester]

How the pot works is the top is attached with +5V the bottom is to ground. The middle wire will have a value equal to a voltage that reflects where it is in that range of voltage. This voltage is used by the controlling circuits of the welder or other equipment to change the output.
The +5 volts is just an example to show the effect of the pots control. You could say it acts like a throttle, turn it up to go faster down for slower.

[Rambozo]

I tried a 50K pot and it gave me max. amps but not low end amps just 20A and 251A at max. put 47K in and got 6 amps low end and 252A top end. But did not try 750K resistor maybe next time.

I have found with most foot pedals, they don't use the pots full travel. Makes sense as you don't want foot forces to be applied to a pot. So timing becomes critical. Either belt, gear, or rack and pinion mechanisms have course timing by moving teeth and often fine by rotating the gear on the pot shaft or rotating the pot. I would test the output with the bare pot and then figure out how you want it timed in the pedal. Also there is the switch, so that will take some travel. Different brands of pots have different amounts of "dead space" at each end of travel, too.

Does teh welder "look" at both the increasing and decreasing sides of the pot?
All three wires are connected, just wondering what the control logic is inside the welder.
At 1/3 travel, is it looking for 1/3 total resistance on one side and 2/3 on the other? How does it use the resistance data?

In my 140ST all three were connected. I forgot to note the voltages measured, but typically pots like this are wired as voltage dividers. One end to ground, one end to reference voltage and the wiper gives a signal that sweeps between those two extremes. For the digitals that would be fed to an AD converter and for analogs it might control an opamp. Just a guess and based on a little snooping around.

[sportbike]

In my 140ST all three were connected. I forgot to note the voltages measured, but typically pots like this are wired as voltage dividers. One end to ground, one end to reference voltage and the wiper gives a signal that sweeps between those two extremes. For the digitals that would be fed to an AD converter and for analogs it might control an opamp. Just a guess and based on a little snooping around.

I understand how Pots in them selves operate, I was just wondering how, if (or why) the welder uses both sides of the pot, or if they are even using them as dual sided pots.

If it is just a single ended voltage divider, lots of options exist on how it can be configured. If it is reading both the increasing side and the decreasing side, not so much.

For example, if it is just a single voltage divider, a resistor wired in parallel could be placed on a switch and make a PT185 a sort of high and low level control (0-25k on "high" and 0-13k on "low").

[Rambozo]

I understand how Pots in them selves operate, I was just wondering how, if (or why) the welder uses both sides of the pot, or if they are even using them as dual sided pots.

If it is just a single ended voltage divider, lots of options exist on how it can be configured. If it is reading both the increasing side and the decreasing side, not so much.

For example, if it is just a single voltage divider, a resistor wired in parallel could be placed on a switch and make a PT185 a sort of high and low level control (0-25k on "high" and 0-13k on "low").

I get ya, now. I don't have a 185, so I can't be sure, but I will poke around in mine and see what I can find. I want to do some more tweaking anyway.
If it's straight resistance like tuning for an RC circuit you could put your parallel resistor between the wiper and one end, if a voltage divider than across both ends. I thought about having the panel control being used like that, but decided it wasn't worth the effort as the machine already has a nice two stage control like you described. I just run it on 120V for the low setting and 240V for the high. Problem solved.

[sportbike]

I get ya, now. I don't have a 185, so I can't be sure, but I will poke around in mine and see what I can find. I want to do some more tweaking anyway.
If it's straight resistance like tuning for an RC circuit you could put your parallel resistor between the wiper and one end, if a voltage divider than across both ends. I thought about having the panel control being used like that, but decided it wasn't worth the effort as the machine already has a nice two stage control like you described. I just run it on 120V for the low setting and 240V for the high. Problem solved.

I was thinking about making a finger slider or thumbwheel type (like yours here: http://www.everlastgenerators.com/fo...7477#post47477 ), printing the parts on my 3d printer. I found a very small micro switch to use as a contactor.

[Kempy]

Rambozo reply to Kempy
I have found with most foot pedals, they don't use the pots full travel. Makes sense as you don't want foot forces to be applied to a pot. So timing becomes critical. Either belt, gear, or rack and pinion mechanisms have course timing by moving teeth and often fine by rotating the gear on the pot shaft or rotating the pot. I would test the output with the bare pot and then figure out how you want it timed in the pedal. Also there is the switch, so that will take some travel. Different brands of pots have different amounts of "dead space" at each end of travel, too.


On my pedal I put stops at both ends so I can get full use of the 47K pot and have the on/off switch work at the right time. I got the pedal from Lenval from studioface who showed how he made the VOX pedals work I got the aluminum one myself use my 47k not the one that came with the VOX pedal it is a very low profile, small, light and much softer travel in the pedal.

[brianscott]

Thanks all

Rambozo, do I need two 750K resistors on the pot?
Both connected to common and then the other ends connected to

[Rambozo]

Thanks all

Rambozo, do I need two 750K resistors on the pot?
Both connected to common and then the other ends connected to

If your machine uses all three leads, you probably have a voltage divider, so you can just use one resistor across the two outside terminals of your pot. Not having that machine I can't tell you for sure, but that would be the thing I would try. Did you try with just the 50K to see if you had the full range of the original pedal? As most pots are 5% or 10% tolerance, you should be close enough if the circuit was designed not to require calibration.

[everlastsupport]

Mark's post (#2) mentions a 50K should work fine since SSC makes pedals for us with a 50K POT and with tolerances being up to 10%, you can be up to 55K or down two 45K ANYWAY.

It should work fine. If you really need the pot closer, add 800K across the two outside tabs (the center is common and required nothing), but again, you might be on the low side of the tolerance. Maybe unsolder and measure it.

The pot has a high voltage on one end (say 5 VDC) and a ground on the other end (outer tabs), as the resistance changes (turning the shaft) the center tab will show to difference (variable voltage divider). The welder uses the center tab in respect to ground to set the amperage range.