AC power supply for screwdriver. Scheme, description

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AC power supply for screwdriver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Cordless screwdrivers are very popular among amateurs and professionals - reliable, light and powerful. But they have a significant drawback - the small capacity of the battery, the energy of which is enough for only half an hour of intensive work. This is followed by a forced break for 3 ... 4 hours to charge the battery. The solution to this problem is the use of a mains power supply, because most of the work is performed within walking distance from the mains.

The mains power supply of a screwdriver must be reliable, small-sized, light and convenient for storage and transportation. An additional requirement for the power supply, due to the specifics of its application, is a falling load characteristic, which prevents damage to the screwdriver's electric motor during overload.

Rice. 1 (click to enlarge)

All these requirements are met by the proposed device, the scheme of which is shown in Fig. 1 The basis of the power supply is an "electronic transformer" U1 with a rated output power of 60 W, designed to power lighting lamps with a voltage of 12 V. The frequency of its output voltage is several tens of kilohertz. Such a transformer can be purchased at electrical stores.

Transformer T1 provides additional galvanic isolation from the network and thus increases the electrical safety of the device. By changing the number of turns of its primary winding (I), you can select the output voltage of the unit. The increased leakage inductance contributes to the formation of a falling load characteristic. The secondary winding (II) with a tap from the middle ensures the operation of a full-wave rectifier on an assembly of two Schottky diodes VD1. Energy losses on diodes in such a rectifier are half that in a bridge one. The oxide capacitor C1 smooths out low-frequency ripples of the rectified voltage, and the ceramic capacitor C2 with a low self-inductance - high-frequency ones, which makes it easier for the capacitor C1 to work, given that the full-wave rectifier doubles the frequency of the pulses coming from the "electronic transformer" U1. Resistor R1 sets the current through the HL1 LED, which signals the supply of voltage to the screwdriver. Resistors R2-R7 - the minimum load of the "electronic transformer" U1, which significantly increases the reliability of its operation, since the idle mode is dangerous for it.

AC adapter for screwdriver

The mains power supply is placed in the case of the backup battery power supply, as shown in the photo (Fig. 2) In the middle of the case, an aluminum plate 3 mm thick is vertically installed. This is the chassis of the entire device, used as a common wire and heat sink of the VD1 diode assembly. Before installation, the heat-removing surface of the VD1 assembly is lubricated with KPT-8 paste. The assembly is fixed on a plate without an insulating gasket. Transformers and a power switch SB1 are installed on one side of the plate, and other parts are on the other.

Transformer T1 is wound on a K28x16x9 ring magnetic circuit made of M2000NMA ferrite. To avoid shorting the turns, the sharp edges of the magnetic circuit are rounded off with fine sandpaper. Then it is isolated, for which the FUM fluoroplastic tape is ideal. To increase the leakage inductance, one winding is placed opposite the other. The primary winding consists of 16 turns wound in two PEL or PEV-2 wires with a diameter of 0,8 mm. The secondary winding is wound with a bundle of four of the same wires and contains 12 turns. After winding, the beginning and end of each wire of the bundle are determined, then the wires are combined into pairs, each pair is connected in-phase in parallel, as a result of which halves of the secondary winding are formed. The beginning of one half is connected to the end of the other, receiving a tap of the secondary winding.

Schottky diode assembly VD1 - any with a maximum forward current of at least 5 A and a reverse voltage of at least 40 V, for example, KD636 with any letter index. In extreme cases, you can install two conventional silicon diodes KD213A or KD213B. Capacitor C1 - imported oxide, C2 - KM-5a, KM-56 or other ceramic.

Button SB1 - microswitch MPZ-1. It is undesirable to use a standard screwdriver switch instead, both for reasons of electrical safety and due to the fact that many screwdrivers have the switch combined with the motor speed controller. Button contacts SB1 - normally closed. The SB1 button pusher is made of a burned-out LED. At the bottom of the body of the proposed device part of the pusher protrudes outward. A spring is installed between the pusher and the SB1 button.

The device works like this. It is placed and fixed in the screwdriver body instead of the battery pack.

When a screwdriver with an attached power supply is on a stand or other flat surface, the pusher is pressed in. The force of pressing it through the spring is transmitted to the SB1 button, as a result of which it is pressed, its contacts are open. The power supply is disconnected from the mains.

When a screwdriver is taken to perform work, the spring depresses the pusher of the SB1 button; its convex part protrudes from the bottom of the housing. The button goes into an unpressed state, its contacts close and connect the power supply to the network.

Setting up the device consists in unwinding the turns of the primary winding of the transformer T1 until the required output voltage of 11 14 or 20 V is obtained, respectively, for a screwdriver with a rated voltage of 9 6 12 or 18 V.

Given the huge number of screwdrivers in operation, the author hopes that the proposed power supply will be in great demand, besides, it is cheap and assembled from available parts. Even a novice radio amateur can repeat it.

Author: K. Moroz

See other articles Section Power Supplies.

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