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There are many types of NiCd (nickel-cadmium) battery chargers on the market, but few can work with 12V cigarette lighters built into cars. Such a charger, for example, would be of interest to campers and travelers who do not have a 230 V AC outlet available. To meet the needs of these users, the charger could be designed to work from a cigarette lighter, but of course it would be much more interesting if it could also work from the home mains.
Furthermore, it is useful if several cells, for example 1 to 4, with different formats can be loaded simultaneously. Finally, another advantage is that the charger will automatically turn off after the battery or cells are fully charged. The charger described in this article does all of these things: it contains batteries or cells Type R6 and R14. Turn off after 2:30, 5 or 10 is guaranteed by 3-way switch S1. 2.5 hours for charging type R6 batteries (1/2 charge), 5 hours for fully charging type R6 batteries or half charging for type R14 batteries, and 10 hours for fully charging type R14 batteries.
LED D1 lights up while charging. Charging can be resumed after a set period of time, if desired, simply by turning the power off and then on. The interval is determined using counters IC1 and IC2, types 4060 and 4020. The 4060 has a built-in oscillator, the frequency is set to 932 Hz with the P1 preset and the help of a frequency meter. For various reasons, such as the value of the components used and the parasitic elements, the oscillator itself operates at a slightly higher frequency, around 1 kHz.
The signal frequency at the lever of P1 is divided by 214, so that the signal frequency at Q13 of IC1 is 0.056 Hz, or the equivalent of one pulse every 17.6 s. The signal at Q13 is fed to the input pin 10 of IC2. When switch S1 is in position 2 h 5 m (output Q10 of IC2), the divisor should be 210 (1024). However, contrary to what these numbers indicate, the dwell time is at half that of the Q10 output. To get a charge time of 2 h 30 m or 9000 s, which should correspond to half a cycle at the Q9 output of IC2, the period of the oscillator should be 9000 2/16.7 106 = 1,073 ms, respectively.
with a frequency of 932 Hz as mentioned earlier. When power is on, only the IC2 counter is reset, since a few seconds error can occur at IC1 doesn't make any sense. This arrangement simplifies the design. When the set time has elapsed, i.e. when charging is complete, diode D1 turns off. The load current is set by a Darlington T3 transistor, which is a classic current source design with negative feedback. The transistor tends to keep the emitter voltage at 1.3V, but this needs the help of a Zener diode, D2.
In this type of design the thermal stability is actually quite acceptable, since the temperature of the Zener diode, with the low current it draws and results in a low temperature rise, has practically no effect on the charging current. . Transistor T1 turns on or off and is used to power the LED on/off. Overloading at the output of counter IC1 should be avoided if it is necessary to absorb the total current (about 7 mA) consumed by the diode.
Transistor T2 interrupts charging when the time set by S1 expires by grounding Darlington T3. Diodes D3 D14 are connected in triplets to the terminals of the battery to be charged: D3 D5 to the terminals of the battery Bt1, D6 D8 to the terminals of Bt2, and so on. Diode D15 prevents battery charging in the event of a power failure. When using a vehicle charger, extreme care should be taken to ensure that any leakage voltage on the vehicle's power line does not adversely affect the operation of the charger.
The battery holder must be able to accommodate four sizes R6 (AM3; MN1500; SP/HP7; mignon) or R14 AM2; MN1400; SP/HP11; baby) battery. The length of these batteries, not their diameter, is the same (about 45 mm). When the charger is used at home, it can be powered through a suitable 15V power adapter. It consumes about 150 mA current. One last word of warning: it's possible the battery is connected to the wrong polarity to the charger. This can lead to very large discharge currents and even damage to the battery. Therefore, it is imperative to check the correct polarity of the battery before inserting the battery into the holder.