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TIMED_NiCd_CHARGER

2015-03-21 13:06  
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TIMED_NiCd_CHARGER
The primary supply is provided by 230 V through transformer T1, diodes D1 and D2, and reservoir capacitor C2. A supply for the timing circuit is provided through diode D3, with resistor R1 and zener diode D4 to keep it to a safe voltage for the CMOS devices used. These CMOS devices, IC1 and IC2, use so little current that the timer will keep going for up to 30 s if the power fails, preventing the charge period from being restarted by every minor glitch in the supply. IC1 is a 4060B 14-stage divider with built-in oscillator. Pressing switch S1 sets all its outputs low, so diode D5 is reverse-biased and the oscillator operates. With the component values shown, it runs at about 0.17 Hz, so the last output of the divider, pin 3, goes high after about 14 h. This applies forward bias to D5, which stops the oscillator. The first timer of the 7556 dual CMOS timer IC2 is connected to operate as an oscillator with a frequency of about 0.5 Hz with a duty cycle of about one to five. Pin 4 is an active low reset for this timer, so although the input to this from IC1 is low, its output is also low. The second timer is used as an inverter to convert this to a high output, which, via resistor R8, activates the output constant-current generator. When IC1 times out, the oscillator in IC2 starts running, and the current generator is then pulsed for about 400 ms every 2 s. NiCds have quite a high self-discharge rate: about 10 percent of capacity per week. If left on this charger, they will be kept fully topped up ready for use without overcharging, and by flashing in time with the current pulses, LED D10 will let the user know that the main charge period is complete