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Reverse engineering: Narex drill speed regulator


The Narex drill consists of a brush motor, a direction switch, and a thyristor or triac based regulator. The direction switch is a lever above the trigger, selecting the polarity of the stator against the rotor.

The drill switch is housed in the trigger assembly and consists of two mechanical switches (one actuated at the very beginning of the trigger action, one at its very end). The first switch powers up the drill's electronics, the second one bypasses the speed regulator and lets the drill run on full power.

In the intermediate trigger position, the speed regulator is engaged. The regulation is done by a potentiometer, realized as a slider on a resistive layer deposited on the ceramic control board.

The control board is made as a ceramic sheet with silkscreened conductive traces and resistors. Some of the resistors are of fixed value, one is a variable (realized as a pair of traces with a conductive slider shorting them together), one is factory-trimmed (the elongated one under the layer of red protective coating), adjusting the electronics for a given mains voltage.

The control element is mounted on an aluminium heatsink (acting also as one of the contacts), and covered with a blob of soft silicone. It was therefore impossible to directly examine its nature. The blob has however only three terminals - two power, one control. A thyristor or a triac are therefore possible alternatives, a thyristor being slightly more likely.


Front of the control board

Back of the control board, heatsink

Disassembled trigger

Switch and the control board

Switch inside

Switch inside

Regulator in its housing

Drill handle

Schematics of the drill

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