is this a custom pot in this drill speed controller? How can I make is slower?

[dentaku]

I opened up the foot pedal speed controller for an old Foredom flexible shaft drill that's been in the basement for years. It's an amazingly powerful machine and works fine with standard Dremel accessories.
The foot pedal has a pot with a slot in it that accepts a twisted rod that "screws" into it as the foot pedal is depressed and it has a flat spot to turn the power off and on.
It's a ridiculously simple system that surprised me when I saw it.
It also has a triac and some pots and caps like I would expect.

Would the blue plastic part have been custom made for this application or is it a common old type of pot?

I measured the AC voltage coming out of it and slow is around 113V AC and full speed is 122V AC
Would it be simple to make the slowest speed be even slower?

[robrenz]

That looks like a standard old style trim pot to me I have many in my drawers that are that exact construction.  A flexible shaft is a very handy thing to have. I find it better than a dremel in many situations.

[dentaku]

Yea, I just checked in my bin of trimpots and I have a few very small ones that have a hole that goes straight through like the one in this speed controller.
The circuit board even had a hole designed into it to allow for the twisted rod to go through.

The funny thing is the flat spot on the blue plastic which unfortunately you can't really see well in my photo. It switches on and off the power to the drill by pushing two metal contact together (I guess you could call it a cam). Maybe they just took those standard trimpots and sanded a flat spot on them at assembly time.
Looking at the back of the board it looks like the metal tabs are connected to the ground wire. When the pedal is all the way up, ground gets disconnected.

Now that I've figured out how to change the collets and bits and found the speed controller might have some use for it. I might make a jig that lets me use it as a drill press. It's still a bit too fast to use as a regular drill but it has some scary amount of torque!

[amyk]

That looks like a standard triac-based phase-angle controller.

[dentaku]

Nice.
The wikepedia explanation is pretty good http://en.wikipedia.org/wiki/Phase-fired_controllers

That looks like a standard triac-based phase-angle controller.

[deth502]

im taken by the mechanics of it. ive never seen any such device implemented like that before. although it is so very basic and simple, id think there would be other readily available off the shelf methods that are equally-if not more so- simple. (like a simple "slider"linear pot.)

[dentaku]

Me too but it was quite tricky to re-assemble

im taken by the mechanics of it. ive never seen any such device implemented like that before. although it is so very basic and simple, id think there would be other readily available off the shelf methods that are equally-if not more so- simple. (like a simple "slider"linear pot.)

[SeanB]

Making it slower is very easy. The small white pot to the top of the blue variable one is the preset speed adjustment, it is used to set the maximum speed when the control is at the fastest position. It can either be turned to maximum resistance ( depending on the back side wiring either fully clockwise or fully anticlockwise, easy to see by turning to one end or the other and seeing where it goes the slowest) and if that is not enough add a resistor in series with it of around 100k which will make it slower. Will reduce the max speed as well though.

[dentaku]

Good. I might try that and put a pot on the outside of the foot pedal so I can adjust it.

Making it slower is very easy. The small white pot to the top of the blue variable one is the preset speed adjustment, it is used to set the maximum speed when the control is at the fastest position. It can either be turned to maximum resistance ( depending on the back side wiring either fully clockwise or fully anticlockwise, easy to see by turning to one end or the other and seeing where it goes the slowest) and if that is not enough add a resistor in series with it of around 100k which will make it slower. Will reduce the max speed as well though.