Charger for small items. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells
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Small-sized elements STs-21, STs-31 and others are used, for example, in modern electronic watches. For their recharging and partial restoration of working capacity, and hence, extending the service life, you can use the proposed charger (Fig. 1). It provides a charging current of 12 mA, sufficient to "refresh" the element in 1,5 ... 3 hours after connecting to the device.
Fig. 1
A rectifier is made on the diode matrix VD1, to which the mains voltage is supplied through the limiting resistor R1 and capacitor C1. Resistor R2 contributes to the discharge of the capacitor after the device is disconnected from the network. At the output of the rectifier, there is a smoothing capacitor C2 and a zener diode VD2, which limits the rectified voltage at 6,8 V. This is followed by a charging current source made on resistors R3, R4 and transistors VT1-VT3, and a charging end indicator consisting of a VT4 transistor and an LED HL).
As soon as the voltage on the element being charged rises to 2,2 V, part of the collector current of the transistor VT3 will flow through the indication circuit. The HL1 LED will light up and signal the end of the charging cycle.
Instead of transistors VT1, VT2, you can use two diodes connected in series with a forward voltage of 0,6 V and a reverse voltage of more than 20 V each, instead of VT4 - one such diode, and instead of a diode matrix - any diodes for a reverse voltage of at least 20 V and a rectified current more than 15 mA. The LED can be any other, with a constant forward voltage of about 1,6 V. Capacitor C1 - paper, for a rated voltage of at least 400 V, an oxide capacitor C2-K73-17 (K50-6 can be for a voltage of at least 15 V).
Parts of the device are mounted on printed circuit board (Fig. 2)which is placed in the case from polystyrene. A network plug XP1 is fixed on the case and contacts are installed for connecting the element.
Authors: V. Bondarev, A. Rukavishnikov, Moscow; Publication: N. Bolshakov, rf.atnn.ru
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