I'm on my second electronic variable speed control inside my Dremel model 395 tool. This one just crapped out with the same temperamental symptoms as the last one. I need to use the tool tomorrow night, and would like to bypass the internal variable speed circuitry to simplify it; maybe buy an external control later.
There aren't any wiring diagrams I could find on the Dremel site. I'm thinking if I had one for the model 275 tool (single speed) and the
395 (electronic variable speed), I might be able to make the 395 into a 275 by just jumpering some wires. I'll probaqbly need to at least retain the variable speed assembly, since the brushes fit into it.
Obvious, it's junk! Buy a new model dremel. or better yet, by one of those no name brands like I did, looks like it's made in the same place but cost much less! $29.00 for a complete set of cutters, stones with variable speed unit. I think it was "All Trade" or "TradeAll" or something like that.
Just take it apart, I'm pretty sure it's just a "potentiometer" (variable resistor) so it would only have 3 leads, one from the external wire to the pot., one from the pot to the motor, and one from the other external wire to the other lead of the motor.
Cut things off, plug the external leads directly to the motor.
You can make yourself something that would do the same with a variable light controller (those you have to control your house lights) Get one of these, one wall plug unit and one extension cord.
PWR IN (1) to VARIABLE (1) VARIABLE (2) to PWR OUT (1) PWR IN (2) to PWR OUT (2)
it would be an awesome idea to tape everything together to cover the connections once you're done so you don't get shocked everytime you want to slow it down or speed it up :)
(PS I also have one of the new Dremel units and it smokes too ... Uh.)
Unfortuantely it doesn't seem that simple. The speed control has what looks like a some kind of semiconductor/IC (3 leads), a diode, and maybe a fixed resistor in addition to the slide pot. I don't know if they're doing pulse width modulation or what.
Btw you said there was a IC ... how many pins does it has, can u give out the ID (in case it's a bridge or something so your motor would be DC (Uhm?!) and wouldnt be a great idea to plug it right into a wall outlet!
Of course it's not gonna be a pot, it would have to be far too big and burn up a lot of power. Instead they use what is essentially a light dimmer. The semiconductor you see is a triac, the diode is a diac to trigger it, if you just jumper together the right two pins on the triac, the motor will be forced on.
It sounds like you're describing a light dimmer circuit. See
for an example of a typical circuit. If your circuit is like that one, running a jumper wire across TR1 will get you going at full speed. I presume you can figure out which two of the three thyrister leads. And you do know your safety rules for working with mains powered equipment, right?
Most of the cordless drills these days use PWM power FeT drivers.
I modified a cordless drill with a mini PIC and Bridge to perform regulated torque control, auto reverse and then forward again until maximum torque was no longer peaking. Did this so that the drill would have a TAP mode in it. I stuck a mini pot on the back side of the handle to set the torque level.
if his dremel is also cordless, It may also be using it a PWM? who knows. how ever, with the part count, I'm guessing he's using a corded unit with a phase control.
None of those I've seen used an IC in the speed control. Also, he didn't mention a filter capacitor, so id may be a simple dimmer circuit. It's hard to tell from such a vague description. Part numbers would have been a big help.
BTW, have you seen the small DC powered clone at Harbor Freight? It runs on 12 VDC, and comes with an AC adapter. I was thinking about using one (or more, with different sized drills) with a homebrew CNC machine to drill PC boards.
Hmm, No, I haven't checked into Harbor Freight in some time how ever, I think you'll find that a lot of named brand tools we know are now being made by the same people that make the no-name brands from China. For example, I have a rotary tool that in all respects is a dremal. bu t the name isn't of course. As far as drills with PWM, the Craftsman 1/2 drive chuck cordless uses PWM driver board which is mounted as part of the trigger. the speed pot slider is on the board. It employs an IC chip with a logic level Power Fet.
We have some electric real movers that are still being modified by the manufacturer because they can't seem to get one to last any longer than
2 months in our shop. First they had drive problems where it wouldn't start half the time. This was an elaborate board with a micro driving what looked like a Mosfet H-bridge.
Any ways, we sent them back, the next set that came our way, they modified with the speed control in the handle of the unit. All they did was employ a speed control trigger slide switch from some existing cordless drill system. Those were very simply units, a single Power Fet with a 555 timer driving it. Not sure if it was variable freq pulsed or PWM? Anyways, those have a switch in the slide that initially connected the + batt lead to the Vcc and Drain of the Powerfet. the Minimum speed was too much on initial start. Those would burned them self's up in the switch.! oh well, so much for engineering.