How do we build a speed variator for mini-drills?

Reading time: 3 minute

The speed variator presented, intended for feeding mini-drills for drilling or milling wiring, offers a wide adjustment range and a high torque to the motor shaft.

Drill operation without speed variator

It is known that if you want to adjust the speed of a mini-drill for drilling wiring (or other fine mechanics), using a direct current source, if we are interested in a strong torque at low speeds, the results will be unsatisfactory.

A speed regulator with series transistor (the drill motor is powered by a transistor whose base current is variable) presents this desideratum. Another aspect is, at low speeds, difficult starting (due to the low current / voltage applied from the source). It thus becomes impossible to perform precision work (especially milling) with a drill at low speeds.

Electronic diagram of the speed variator with PWM control

Speed ​​variator operation

But what happens, if using the same method of powering a motor (with series regulating transistor - see the role of Q1 in the electronic diagram), the control of the bias current based on the regulating transistor is done by a method PWM - Duration pulse modulation?

The method involves attacking the base of the regulating transistor with rectangular pulses of constant frequency, but with the width of the variable high and low levels. To solve the problem we used a common oscillator with logic gates OR-NO (NOR), which can be found in the capsule of a circuit type 4001 (CD4001, HEF4001, MMC4001, etc.).

The oscillator, in the configuration of the electronic scheme (with gates U1D and U1B) offers a rectangular signal with frequency of 50Hz… 100Hz (directly dependent on the C4 value) and with variable filling factor, the durations of the high low levels being dependent on the position of R5's cursor (100k value).

How does the command work in PWM?

In order to achieve filling factor less than or greater than 50% at such an oscillator, at the entrance of the gates, the configuration R5-R6-R7-D4-D5 is used, which separates the two bearings.

Careful! Diodes D4 and D5 must be mounted correctly on the printed circuit board, otherwise the correct operation of the assembly is totally compromised.

Oscillator output, control with 50Hz signal, through R4, base of driver transistor Q2. The latter, in turn, controls the final transistor, series regulator, Q1, which supplies the motor with variable voltage, 0 / 12V, with the mentioned frequency, so in switching. This method provides a good control of the motor shaft speed even at low speeds and a satisfactory torque.

This eliminates slow starting at low speeds, because, regardless of the regulated speed in R5, the maximum value of the supply voltage is periodically applied to the motor, with constant frequency and variable duration.

For low current mini-drills (approx. 1A), transistor Q1, type TIP42 or BD912, does not require mounting on the radiator. For higher currents it is recommended to use the radiator.

The heating of the transistors is the lower, the more the rectified voltage is filtered, so with C3 the higher, over 4700uF. The values ​​of the components in the diagram are not critical. At low supply voltages R1 may be missing or diminished.

How to use

Printed circuit

Execution drawing

List of required components and other technical data

R1 = 220 Ohm;
R2, R6, R7 = 4.7k;
R3 = 1k;
R4 = 10k;
R5 = 100k potentiometer;
R8 = 180..200 Ohm;
C1 = 4.7 uF / 16V;
C2 = 100 nF;
C3 => 4700 uF / 25V;
C4 = 200nF (or 100 nF);
D1, D3 = 1N4004;
D2 = KBP204 (rectifier bridge 50-1000V / 2A);
D4, D5 = 1N4148;
Q1 = BD912 (or TIP42, BD910);
Q2 = BC327 (or 2N5401, 2N3906);
IC1 = CD4001;
2 x terminal contacts for supply and output;

  • Supply voltage: 9-12Vac or 9-16Vdc;
  • Recommended load current: 1A, without radiator on Q1;
  • Speed ​​adjustment ratio: 1/20;
  • Adjustment method: PWM.


Conex Club Magazine - no.6 - 2006

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