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# Universal battery charger

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In the circuit shown below there is a Universal battery charger from:
http://hem.spray.se/lythall/power/charger.htm
it is easy to understand and to implement, but i have some questions:

(1) Why u think the designer use the 4u7 cap.
(2) If LM358 is used as a comparator, why:
(a) using 820K resistor
(b) not using 741 single op-amp instead of this dual op-amps
(3) If I want to use this charger to charge a 9-volt square batt, how can i tune this circuit to fully charge my batt
(4) If I want to use a NPN trans type instead of BC557,what the modifications that must be done?
(5) If I want to charge atwo 1.5V batts in series (3V) using a 5V power supply, what to do other than changing zener to 3.3V?

thats all,
thanks.

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(1) Why u think the designer use the 4u7 cap.

The actual value doesn't matter. It filters noise from the battery if it is loose in the connector and provides a small delay.

(2) If LM358 is used as a comparator, why:
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audioguru: "The actual value doesn't matter. It filters noise from the battery if it is loose in the connector and provides a small delay."
Walid: it is good news that the cap value (4.7u) doesn't matter, but still i can't understand u, please explain it more for me, thanks.

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audioguru: "Without it then when the battery voltage reaches the threshold voltage, the circuit might oscillate."

Walid: (1) What the threshold voltage if the charged batt is, say, 9v
(2) is this op-amp oprate as a comparator. I never see a comparator with a feedback resistor.

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(1) What the threshold voltage if the charged batt is, say, 9v

The threshold voltage is adjusted with the pot.

(2) is this op-amp oprate as a comparator. I never see a comparator with a feedback resistor.

The opamp is definately used as a comparator. The 820k resistor provides a little positive feedback for snap-action switching which avoids oscillation at the threshold. The slow opamp that is used as a comparator probably won't oscillate but at the threshold it will amplify its noise and cause output fluctuations without hysteresis provided by a little positive feedback.
Comparator circuits commonly use positive feedback for hysteresis.

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