I am in the process of restoring a wood lathe circa 1950 by using a treadmill motor to power it. I bought a 5k linear potentiometer (rotating) at radio shack to replace the slide potentiometer that was originally on the treadmill. I noticed the when the potentiometer is near the nearly all the way at the end of its range the motor actually slows down. If I back it off a bit then it will regain it's higher speed. I am not very informed about circuits, in fact, this is my first experience with trying to use a pot to control a piece of equipment. I also noticed that this also occurs when I hook up the original slide potentiometer to control the motor.
Can anyone explain what is going on and how to fix it? The potentiometer I added varies the voltage to the motor from 0 to 12 volts. It is a 2 1/2 hp dc motor and I have taken the original controls (circuit board) from the treadmill. Everything works fine until I get to the end of the pot range. Is this just a quirk of potentiometers, or is there some rational behind it. The original controls do have a some kind of limiting circuit, which I assume is to limit the current to avoid burning things out. I have rotated the limit switch on the original treadmill circuit board to it's highest position to get the greatest speed out of the motor. Perhaps this plays into the problem? Any ideas/explanations would be greatly appreciated.
Any chance you've mapped out the existing circuit and you could share the diagram? What markings are on the existing slide potentiometer, and which did you buy to replace it? Pots come in multiple types with the two most common ones being linear taper (equal movements of the control result in the same change in resistance regardless of where the control is in its range) or logrithmic taper where smaller changes occur early in the control's movement compared with the other end.
This wikipedia section shows the different markings for the tapers you find in newer and older controls.
Hi, have you double checked the connections to the pot, it sound like you may have the wiper and one end of the pot swapped.
Is the slider voltage changing at the high speed end when the motor slows down or does it continue to go up.
While this particular question is concerning potentiometers, I will be taking a reading off this pot to the Arduino board to display the % of motor capacity (based on information provided by members of this forum!). The Arduino board and LED display is already displaying the spindle rpm's and keeps track of the time since it is turned on. I plan to have it alternate the display between rpm's and the motor %. If you check my other questions and the reply's you can see how I was advised to measure the motor %. I realize that I could just put a scale behind the pot to see what the motor % is, but that would not be as cool as having the value displayed. Arduino boards are new to me, but the specs for the Arduino board state that it can take from 6 to 20 volts, but applying anymore than 12 is not recommended, so I don't understand the comment concerning "a world of hurt". In fact, I had 12 volts going to it when I originally hooked it up and it worked fine (I took the power off the red and black pot wires to a 2.1 mm barrel plug when I was originally testing it)
Before I go further, I want to make sure the pot is both working correctly and of the correct capacity. I have attached a photo of the slide pot that was on the treadmill and a copy of the treadmill schematic. I have not (because I'm not sure I have the ability) mapped the circuit. I did google the numbers on this pot, but the only thing that came up was one for sale on e-bay.
I took the three wires that went to the slide pot and attached them to the rotary pot I purchased at Radio Shack (for around three bucks). I want to switch to a rotary pot because it does not take up as much room in the box I plan to put everything in. Before I soldered it up, some weird things happened. The lathe would speed up without turning the dial and I found that if I pulled on the connections it would slow back down, so I surmised that it was acting funny because the connections were loose, so I soldered them.
I wired the red wire to the end terminal of the pot, the white (wiper, I believe) to the middle terminal and the black wire to the other end terminal. Everything seems to work OK, until you get to the end of the pot range. Then, it slows down and seems to pulse. I suppose one solution is to just not turn it up all the way, but I thought I would ask in case it is something simple that I am just not aware of. If that wiring doesn't seem right please let me know. Thanks for all your comments and observations.
OK, we need to distinguish between the supply voltage restriction for the Arduino, which is as you cite, and the permitted voltages to be applied to the input/ output pins which is strictly no more than its own internal Vcc voltage, generally 5V.
This means that if you wish to feed your potentiometer voltage into one of these ports (for analog measurements) you need to scale it using a resistive voltage divider. Using relatively high resistance values - greater than 10k - in series with the analog input will generally mean that if the resultant (divided) voltage happens to exceed 5V, the protective diodes on that input which prevent the voltage from exceeding the limits, will not be unduly stressed.
Now as to why the speed control behaves in an odd fashion, I was thinking that if this was something to do with the Arduino, it could have been due to feeding excess voltage into an input but it seems this is in fact, a behaviour of the circuit you have presently and therefore nothing to do with the Arduino - in which case we quite obviously cannot advise without having substantial details of that particular device.
It most certainly has nothing to do with the behaviour of potentiometers in general.
Your explanation of the difference between the supply voltage and permitted voltage to the pins is very well put. This clarifies the distinction in my mind.
I also believe you are correct in that the weird aberration I am experiencing at the top of the pot range has nothing to do with the pot or the Arduino board, but rather has something to do with the circuitry of the treadmill.
The advice I received on a previous post was also aligned with your advice below. I was advised, to use a 10k resistor from the wiper to analogue input 0. Then connect a 6K8 resistor from the analogue input to ground. Also,to connect the low end of the pot to ground.
The sketch would then use the map(0,1023,0,100) function (not sure if this is the correct terminology, perhaps it is a command) to convert to percent. I am not sure I understand how all this works, but I plan to try it to see what happens.
What eludes me is the map function reads the input from pin zero, and the resistors will reduce the voltage from the actual value (0 to 12 volts) to some value that is 5 volts at the maximum, but how do I know what parameters to use to get the map function to provide the correct % of motor speed. I suppose I can try it and then calibrate the values, or I can measure the value that the 10K resistor will reduce the max voltage of 12 volts to and use that as the upper bound value, or the value to substitute for the 1023 in the above listed map command. Does that make sense or am I not understanding how it all works?
