Dremel Speed Stabilizer

The Dremel drill fitted with a mini drill-chuck on a stand is a great setup for drilling circuit boards.  The problem is that it does not do low speeds very well. My unit had a failed internal speed controller, so I just wired around it and connected it to a regulated 24 volt DC supply.  This worked OK, but I would like a slower speed when drilling boards.   Slowing down with a lower voltage, however, causes it to stall out completly when it begins drilling.  What I needed was a circuit to actually increase power to the motor under increasing load.

The circuit at right uses a logi-level N-channel FET driven directly by the PIC.  The PIC creates a variable PWM signal to power the  motor.
Motor current is sensed using a combined 0.5 ohm resitance to ground.  Since this voltage is only sensed during the power pulse, and the external voltage feed is well regulated, all of the change in the voltage is due to motor current.  This signal is filtered and fed to an A/D input.  As the voltage here increases, the PIC increases the power to the motor by increasing the pulse width.

The momentary push button is used to switch the power on and off for ease in changing drill bits.   The whole assembly is fastened to the side of the stand with double-stick tape.

As it is, this unit performs quite well.  It would work a bit better with maybe 30 or 35 volts feeding the system.

The board layout prints at 36% on my printer.  As usual, the current Source and Object files are provided.