Attachment for microdrill control. Scheme, description

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Attachment for microdrill control. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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The device described below facilitates the process of drilling and countersinking holes in printed circuit boards with hand-held micro-drills made on the basis of DC electric motors with an operating voltage of 12 ... Radio", 27, No. 2009, pp. 9, 29), it is distinguished by a smooth start of the rotor, i.e., the absence of an initial "jerk", and, in addition, does not need a voltage regulator DA30.

The initial "jerk" here means a short-term operation of the engine at maximum speed when the supply voltage is applied, which occurs due to insufficient capacitance of the capacitor C2. If the drill is not fixed at this moment and, for example, lies on the desktop next to foreign objects, then it, having shifted due to a sharp acceleration of the rotor, can damage these objects with a drill or injure an accidentally substituted hand.

This shortcoming cannot be eliminated by simply increasing the capacity of said capacitor, since after drilling each hole, the engine will take too long to return to idle mode due to the fact that the time constant for discharging the capacitor here is greater than its charging time. In addition, the required capacitance of the capacitor C2 directly depends on the current transfer coefficient of the base of the transistor VT1, which negatively affects the repeatability of the device mentioned above. The KR142EN12 microcircuit stabilizer is not always at hand.

Attachment for microdrill control
Fig .1

Based on the above, a microdrill control device was developed, the schematic diagram of which is shown in Fig. 1 On diodes VD1, VD2, transistor VT1 and resistors R1, R3, a current stabilizer is made, the load of which facilitates the process of drilling and countersinking holes in printed circuit boards with hand-held microdrills made on the basis of DC electric motors with an operating voltage of 12 ... 27 V. From set-top box of a similar purpose, described in the article by S. Saglaev "Convenient microdrill" ("Radio", 2009, No. 9, pp. 29, 30), it is distinguished by a smooth start of the rotor, i.e., the absence of an initial "jerk", and, in addition to this does not need a voltage regulator DA1.

Trimmer resistor R4. A stable voltage from the resistor engine through the diode VD3 and a power amplifier assembled on transistors VT3, VT4 of different structures is supplied to the M1 electric motor and sets the speed of rotation of its rotor at low idle, and capacitors C1 and C2 provide a smooth start when the power is turned on.

Resistor R7 serves as a motor current sensor, and the current is proportional to the load on the drill. Diode VD5 limits the voltage drop across this resistor in drilling mode. The output current of the source, made on the transistor VT2, depends on the resistance of the resistor R5 and the voltage drop across the resistor R7.

With an increase in the load on the drill, the motor current and the voltage across the resistor R7 increase, which causes a current to appear in the collector circuit of the transistor VT2. The charging of capacitors C1 and C2 begins with a stable current. Diode VD3 closes by simply increasing the capacitance of the specified capacitor, since after drilling each hole, the engine will take too long to return to idle mode due to the fact that the time constant for discharging the capacitor here is longer than its charging time. In addition, the required capacitance of the capacitor C2 directly depends on the current transfer coefficient of the base of the transistor VT1, which negatively affects the repeatability of the device mentioned above. The KR142EN12 microcircuit stabilizer is not always at hand.

The linearly increasing voltage from the capacitors, as mentioned above, is fed through the current amplifier to the electric motor. As the capacitors charge, the voltage on the electric motor increases rapidly and becomes equal to the voltage of the power source minus the total voltage drop of about two volts across the VD5 diode and the open transistor VT4. The rotational speed of the drill increases to the working one.

After drilling the hole, the load on the motor drops, its current decreases and the transistor VT2 closes. Capacitor C2 begins to discharge through resistor R6, delaying the reduction of speed to idle for a time sufficient to remove the drill from the drilled hole. In order to then reduce the speed faster, the discharge circuit of the capacitor C2 by the VD4 diode is disconnected from the circuit of the capacitor C1, which is discharging more slowly. The drill is ready to drill another hole.

The prefix is mounted on a printed circuit board made of fiberglass with a thickness of 1 mm. The drawing of the board is shown in fig. 2.

Attachment for microdrill control
Fig. 2

In addition to the KD521A diodes indicated in the diagram, any low-power silicon diodes can be used. Low power transistors - also any of the indicated series. Transistor VT4 - any of the KT814, KT816 series (or a more powerful pnp structure, if the motor is designed for a higher current). The VD5 diode must also withstand the corresponding current.

It should be noted that the product of the current transfer coefficients of the base of transistors VT3 and VT4 must be at least 1000 at an output current of about 1 A. Capacitors are imported. Fixed resistors - MLT, S2-33 and others. Trimmer resistor R5 - SDR-Zva or SDR-Svv.

Establishing the attachment mainly consists in setting the tuning resistor R4 to the speed of the rotor of the electric motor at idle. Sometimes it becomes necessary to change the sensitivity of the attachment to a change in the load on the drill. This is easy to do with a selection of resistor R7. When using an electric motor other than that indicated in the diagram, it may be necessary to select capacitor C1 to achieve a smooth start of rotation of the drill and C2 to change the return time from drilling to idle mode.

To facilitate the thermal regime of the VT4 transistor, it should be installed on a heat sink in the form of a duralumin plate measuring 30x20 mm and 1 ... 2 mm thick.

Author: S. Glibin, Moscow; Publication: radioradar.net

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