DC Motor drive

[(*steve*)]

Or put the diac inside a diode bridge.

 

[eKretz]

Hmm... Duke, are you positive that the lead screws are both right hand thread? That doesn't make any sense for buttress thread. That thread form is designed for the load to push against the square (or nearly square) side of the thread. If both are right hand but rotating in opposite directions, one of the screws would have to be pushing against the angled side of the thread form unless it's a non-standard thread form. Got a picture of the lead screws?

 

[alsmith]

possible ready made solution- look at electric bike (ebike) and similar controllers. They are readily available for 24V, 36V 48V etc. and are available in various currents- what does your build draw? The speed is easily controlled on a bicycle (twist or thumb throttle, foot controllers are out there), maybe you could find either a built unit or a circuit to build. Just watch that you look at the right type DC controller not one for hall sensor motors.