Potentiometer Puzzle - TIG Pedal

Question

I'm looking at a brand new foot pedal for a Tungsten Inert Gas (TIG) welder. This foot pedal uses cascading potentiometers to send a signal to the welder control system. I'm getting unusual results when I verify the pedal using a Volt Ohm Meter. I'm measuring resistance between the wiper terminal and 'ground' terminal on the pot. Pedal off = 0 ohms, slight push 5K ohms ,push more... 10k up to 12.5k ohms at 3/4 full push. If I go beyond that however, the resistance reduces from 12.5k down to 1.6k

This is a brand new pedal (yup, off of ebay.) Its obviously never been used. The pot has a smooth shaft with a spur gear attached. The foot pedal has a 2" gear rack attached.. when you push the pedal down the rack moves and rotates the gear. The movement feels extremely smooth. I can't find the spec for the pot for that manufacturer but I can find other manufacturers with virtually the same part #. It looks to be a liner pot, 300deg of motion. I don't have a lot of practical experience with potentiometers.

Here are my questions. 1) Does the 'hard' stop on a typical single turn potentiometer control full range of travel (i.e. zero ohms at one end of stop, max resistance at the other end of the stop) 2) How robust is the hard stop internal to the potentiometer? (Clearly with a foot pedal input the functional foot pedal stop should be something mechanical external to the fragile potentiometer. ) If the external mechanical stop was not set, and you pushed the pedal hard, would the stop just break off or would there be other issues? --3) Do they build rheostats with 300 degrees of function but withOUT an internal stop in them?? (That would make sense for exactly this type of application.. Manufacturer to provide range of motion control external to the pot. Mfgr to ensure alignment of zero and max set points. If manufacturer allowed the pot to spin too far, the function is off, but there would be no permanent damage to the potentiometer.)

Part number: RV24YN20S B503 Mfgr: Hungyun.com

No, I've not taken this pedal apart to isolate the pot.. I don't want any 'warranty' hassles from the seller.

Yes, I can reduce pedal travel to lock at the max observed resistance. I'll do that if I'm sure this isn't a different type of 'quality' problem which will cause issues later.

So hey, is the answer to #3 = yes, and I should just adjust the hard stop and move on?

Many thanks. Zip.

Update to add schematic and address some of the questions below:

This type of pedal is found on some 'low end' welding machines.. The side of the pedal has a dial with which you set the max. amperage. The foot pedal potentiometer allows you to easily change output amperage from zero to max amperage. Obviously its a simple signal back to the welder's output control system. For troubleshooting this new pedal, I removed an inspection panel allowing me to see inside the pedal. I can see clearly every wire connection. Nothing is grounded to the pedal steel chassis. I was very careful to ensure that both pots are actuated CW (when viewed from end of shaft). I was also very careful to ensure I have the wire and their attachments EXACTLY per the schematic I just now drew on digikey (scheme it)

For troubleshooting I have a nice fluke V-O-M reading between the red wire and the yellow wire with the "dial at back of pedal" set to zero. The 'back of pedal' pot is a RV28P B103 (10K ohms).

I think I'm doing this right.. do you agree? The other thing that is baffling me... I can understand a variable resistance going from 0 to big. What I don't understand is 0 to big back down to a lot less than big. I've never taken a potentiometer apart. I can understand 0 --> max ohms --> max ohms -->max ohms --> zero but never 0 --> max ohms --> some middle value. Can anyone explain?

Answer 1

Given that you've said R7 is set to 0 ohms, the two fixed terminals of R6 are shorted together. When the wiper is at either extreme, its resistance to either fixed terminal will also be 0 ohms. When the resistance is 50%, both halves will be in parallel. Given that this appears to be a 50 k pot, we'd expect

$$R_{eq}=\frac{R_{max}}{2}||\frac{R_{max}}{2}=\frac{R_{max}}{4}=\frac{50000}{4}=12500$$

just as you measured.

Answer 2

Following your post of the schematic: Ah HAH. I think you may be getting thrown off by measuring the resistance to ground when two pots, and connections to Ground and +5V are involved (even though you no doubt had power off). Instead, with the pedal disconnected from the welder, attach a power supply (say +5V, or a 9V battery) across the power pins, and then measure the voltage coming out of the "variable signal".

I expect that with the rear pot adjusted to max, the pedal pot should adjust smoothly from zero volts to max.

By the way, is red really ground, with black +5V? That seems a little suspect.

Answer 3

The pot has an electrical angle that is less than the mechanical angle- a bit of motion at either end does nothing.

In your case, the mechanical angle is nominally 300° and the electrical angle is 280°, so the 10° at each end does nothing.

The stops only prevent the wiper from exceeding the mechanical angle. There are pots made without stops (often called 'servo' pots for their main application). They generally do not look like the type your reference, which is a copy of a Tokyo Cosmos panel mount potentiometer.

If you force the wiper past the stop (which is just a bent bit of metal from the tab) the wiper may be damaged as well as destroying the stop.

Your pot should be a 50K type with linear (JIS B) taper, as illustrated from this website- you can see the unused part of the mechanical angle at each end on the "B" taper:

A typical strength for a mechanical stop is 8 kg-cm. You can translate that to foot-pounds if you find that more intuitive. It's not much for foot pressure to overcome. I looked up the official procedure Bourns uses for testing stop strength, and they emphasize it's static stop strength. You (gently) apply the specified torque and the position cannot change more than 1° after 10 seconds. Reference: The Potentiometer Handbook, C.D. Todd 1975.

What should you do... the increase then decrease of resistance is suspicious and certainly not what you'd expect. How about a photo of the pot wiring. Could it be shorted from wiper to the "unused" end?

Edit: Okay, it's more clear now with the schematic. It's not really useful to measure the resistance, you should apply a voltage to the +5/0 and watch the voltage change at the output.

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This kind of circuit is a voltage divider designed to work into a high impedance (your multimeter will do).

With R7 cranked all the way up, you should see 0 to 5V at the yellow wire wrt the black wire as the pedal moves.

With R7 at about half rotation, you should see 0 to 2.5V (maybe a bit less) at the yellow wire wrt the black wire as the pedal moves.

With R7 all the way down, you should not see hardly any voltage at the yellow wire wrt the black wire as the pedal moves.

Answer 4

Q1: Yes pots usually have a stop at each end. Q2: The hard stop is implemented by a cast pin or bent sheet metal interacting with a protrusion on the shaft, or something like that. Ie: it's strong against turning of the knowb by hand, but probably not by a foot on a pedal. So yes, some additional end-stop protection is needed, like a mechanical stop on the pedal, or a spring in the drive. Q3: Lack of end stop. Certainly this could be built, and there are some pots intended to rotate 360 degrees. I'm not convinced that this is one of those though.

Are you sure that this pedal's end region isn't intended to correspond to some particular TIG function. Like, push the pedal all the way down to start, or stop, for example.

The other question mark is your comment about "cascading potentiometers". Is there more than one pot? How many pot outputs are there to the welder? If just one, then how do the pots relate?