Help sizing a potentiometer

[ScottH]

To start, I'm a journeyman electrician but electronics gets me in trouble in a hurry. My issue is I have a full auto rubberband mini-gun that I designed and built (

) and I want to be able to vary the speed of the barrel assemblies. The drive motors are 6VDC cordless screwdrivers with the voltage currently bumped up to 12VDC and a draw of about 2.5A with just the barrels spinning and around 3A with the drag from the trigger mechanism engaged... that's for both motors. It doesn't have to be a pot. to add the resistance, I'm open to almost anything, but I just don't know all the options... kinda why I'm here!

I have the gun running at half speed from the video with a pulley change and if I can figure out a good way to control the motor speed then I'll just swap pulleys around and have it run twice as fast when it's wide open... and if the trigger mechanisms can cycle fast enough. It's shooting around 5000 rounds (bands) per minute in the video and the pleasure/pain ratio just isn't quite in sync with what I'd like. It takes around 30 minutes to load and it empties way too fast!

Any help with this would be greatly appreciated!

Scott

 

[Gryd3]

You could go with an electronic control method. Using PWM will allow you to use almost any potentiometer you want, and the PWM control for the motors will give you greater flexibility by allowing you to control more amperage more efficiently. Drill normally use this, it's where that tiny little squealing sound comes from when you slightly depress the trigger.

 

[ScottH]

Thanks! We use VFD's quite a bit at work and I never thought of applying that principal to my toy. I did some fast checking with Google and it definitely looks like a good possibility. It's time for some more research now!

 

[Gryd3]

Thanks! We use VFD's quite a bit at work and I never thought of applying that principal to my toy. I did some fast checking with Google and it definitely looks like a good possibility. It's time for some more research now!

Sounds good!
You can make your own voltage controlled PWM, then just use the center-tap from a potentiometer to feed it a reference voltage from 0-5V or what ever control voltage you want to use.
You could easily daisy chain multiple controllers together off the same control voltage too if you need to control multiple motors and can't fit them on one controller.

(Alternative solution is a linear device like a transistor, but they give off a lot of heat and need to be heat-sinked. They are much simpler to use though)

 

[BobK]

My rule is that if you need to worry about the power rating of a potentiometer, you should not be using a potentiometer that way. PWM is far superior and often cheaper than a potentiometer or rheostat with sufficient power handling capability.

Bob

 

[davenn]

totally agree with Bob on that one

a single pot is a bad way to control motor speed