Charger for electric shaver. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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In the markets and stores selling household equipment, you can often find small-sized electric shavers (brands IRION - SHAVE, SUNNY, etc.) powered by a battery. The instructions that come with them do not indicate the country of origin, but judging by the Chinese XINHUIPAI batteries built into them, they are all of Asian origin. Such razors perform their direct functions perfectly, but their chargers do not provide enough current to properly charge batteries. The instructions for using the shavers say that their charging time is 16 hours, the case says 8 hours. In fact, chargers are not able to fully charge the battery even in 20 hours. As a result, the razor can only be used 2-4 times, after which it has to be recharged again. Why this happens and how to modify the charger is described in the article published below.

Schematic diagram of the electrical part of the IRION - SHAVE razor, compiled according to its printed circuit board, is shown in fig. 1. The razor can be powered from a mains voltage of 110 and 220 V. In the second case, it enters the transformer T1 through the resistor R1, and in the first case, through the closed contacts of the SA1 switch that shunt it. The connection of the shaver to the network is signaled by the lighting of the LED HL1.

A nickel-cadmium battery GВ1 is charged with a pulsating current with a frequency of 50 Hz, rectified by a diode VD1 switched on in a half-wave circuit. The average value of the battery charging current, measured at the beginning of its charging, is 35 mA. Motor M1 draws 230 mA.

What are the disadvantages of an electric shaver with such an electrical circuit?

Measurements taken while charging the battery showed that the voltage drop across the resistor R1 is 130 V, which means that the power dissipated by it is at least 0,8 watts. The shaver has a 0,5W resistor. As a result, it gets very hot. The temperature of the power transformer T1 also rises, although to a much lesser extent.

The battery charging mode is also not observed. The fact is that the standard charging tox should be 10% of the energy intensity, and since the razor has a 500 mAh battery, it must be charged with a current of 50 mA. The actual charging current, as mentioned above, is only 35 mA.

Eliminating these shortcomings is not difficult even for a beginner radio amateur. In order for the limiting resistor to overheat less, its power must be increased to 1 W, and the resistance reduced to 20 kOhm.

To increase the charging current, it is enough to replace the VD1 diode with a bridge rectifier (Fig. 2).

After such a simple modification, the battery will be charged with 10 kΩ 5 mA, and its full charge time will be 15 ... 16 hours. Diodes KD522B can be replaced by KD521, KD522, KD102, KD103 with any letter indices.

If possible, it is recommended to replace the Chinese-made battery with a more reliable one.

Unfortunately, the described refinement does not completely get rid of the overheating of the resistor R1, which will still dissipate relatively large power, heating the plastic case of the razor. It is not possible to exclude this resistor, since the T1 transformer is not designed for a voltage of 220 V. You can, of course, increase the number of turns of its primary winding, but this is a very troublesome business. To avoid these difficulties allows the charger, made in the form of a separate unit, assembled, for example, according to the scheme described in [1]. It not only eliminates the heat of the razor elements, but also speeds up the process of charging the battery.

On fig. 3 shows a diagram of another version of a simple charger that has the advantages listed above. On transistors \ / T1 and VT2, a charging current limiter is built. Chip DA1 stabilizes its output voltage.

(click to enlarge)

Transistor VT3 and LED HL1 act as an indicator of battery charging. As long as the voltage created by the charging current across the resistor R2 is sufficient to open the transistor VT3, the LED HL1 glows brightly. When the charger current drops to 10 ... 15 mA and the voltage across resistor R2 drops to 0,6 V, the VT3 transistor closes and the LED goes out, signaling the completion of the battery charging process.

In his electric shaver, the author replaced the GВ1 battery with a more capacious one (850 mAh) and, together with the M1 motor, the SA1 switch and the HL1 LED, placed it in the razor body on the printed circuit board that was there. The rest of the parts, with the exception of the fuse FU1, transformer T1 and capacitor C1, were mounted on a new printed circuit board (Fig. 4), placing it in a case made in the form of a plug of the BP4-1 power supply unit. The DA1 chip and the VT2 transistor must be equipped with small U-shaped heat sinks made of brass sheet. The VT3 transistor dissipates power less than 0,3 W and does not need a heat sink. The shaver can be connected to the charger using a stereo miniature connector and a three-wire cable.

Resistors MLT-0,125, capacitors K50-40 were used for installation. KP103 transistor - with any letter index. Instead of the KT972A transistor, it will do

KT972B or K829A, KT829B, KT829V with a current transfer ratio of at least 500 ... 1000, and instead of KT814G - KT814, KT816 with any letter indices. Diodes KD105B, KD105V and KD105G can work in the rectifier.

Transformer T1 can be used from the power supply BP4-1 or any other that provides voltage on the secondary winding 5...7 V at a load current of at least 600 mA.

After installation, by selecting a resistor P5, you need to set a voltage of 1,35 V at the output of the charger, and by selecting a resistor R1 - a short-circuit current of the order of 400 ... 450 mA. At the time of adjusting the output current, the terminals of the base and emitter of the KT814G transistor should be closed.

On fig. 5 shows the change in charging current during the first two hours of charging the battery. As can be seen from the figure, its time dependence practically does not differ from the theoretical one [2].

If the charger is not connected to the mains, you must not leave it connected to the razor, otherwise the battery will be discharged through resistors R4, R5 and the DA1 chip with a current of about 5 mA.

Literature

1. S. Alekseev, Chargers for Ni-Cd accumulators and batteries. - Radio, 1997, No. 1, p. 44-46; No. 2. p. 44-46.
2. M. Dorofeev. Charger option. - Radio, 1993, No. 2, p. 12,13.

Author: A. Shitov, Ivanovo

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