Two potentiometers in series help

[thebestg2002]

Hello all. I'm new to the forum and I look forward to hearing from you on a controls issue I need help resolving.

I have a customer who has an electrically controlled cylinder that is used to simply push caulk out of gun. The controller for the stepper motor has a 5V analog input that is used to control the speed with which it is dispensed. The original design included two 10K potentiometers. The wiper of each pot went through a relay contact (one n/o and one n/c contact of the same relay). The relay was controlled by a switch on the front of the main control panel. The intent of this control was to allow speed adjustment from a foot pedal or from a second pot located on the control panel depending on the switch position.

The customer now wants the control to work a little bit differently. He wants the pot on the control panel to set the max speed for each operator so that the foot pedal can be fully depressed.

Can I simply wire the wiper from the pot on the main control panel to the positive side of the second pot on the foot pedal? Then take the wiper from the foot pedal pot to the analog input.

For example: If the c/p pot was set for an output of 3V for operator #1, then the foot pedal pot would have a total range of 0-3V for that operator. If operator #2 likes it a little slower, then he/she could turn it down to 2V limiting the range of the foot pedal from 0-2V.

What am I missing here? Is there something I need to be concerned about? My circuit theory is a bit rusty.

Thanks for your help.

 

[ccurtis]

It would be immensely helpful if you could sketch out the entire wiring/schematic diagram (include resistor values if possible). I think your idea is correct, but you might have to cut a wire(s), like the supply voltage wire going to the "positive side of the second pot" and the wire presently wired to the control input. The devil is in the details.

 

[thebestg2002]

This is what I had in mind. Sorry for the crudeness of the sketch.
View attachment 65957

 

[ccurtis]

Yes, I think that will work for what you want to do.

 

[thebestg2002]

Thanks ccurtis. I'll test it next week. Is there anything I need to be concerned about or need to look for in particular? Is there anything I'm missing?

 

[ccurtis]

Two things are on my mind, about this. (1). I am a little concerned about the load the analog output drives, though it is probably not great enough to be a significant factor given that it is being driven by pots to begin with. (2). The pot on the right side in your diagram places a load on the pot of the left. This means that successive, equal increments of adjustment of a pot will not result in equal increments in the analog output. I am thinking that is not a significant issue either, as the control panel pot can still be set where you need it to be set. Just be aware that if you have the control panel pot calibrated with specific numbers/increments, the calibration will change.

 

[thebestg2002]

Thanks ccurtis. It just seems too simple which is why I asked the question in the first place. I don't think this needs to work with high precision. They simply want the operator to be able to select his/her max speed with the pot on the control panel so that he/she can use the foot pedal all the way to the floor if they'd like. It's more of a feel for them I believe.

 

[ccurtis]

My pleasure.