walid Posted November 18, 2005 Report Share Posted November 18, 2005 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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 19, 2005 Report Share Posted November 19, 2005 (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: Quote Link to comment Share on other sites More sharing options...
walid Posted November 22, 2005 Author Report Share Posted November 22, 2005 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. Quote Link to comment Share on other sites More sharing options...
walid Posted November 22, 2005 Author Report Share Posted November 22, 2005 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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 22, 2005 Report Share Posted November 22, 2005 (1) What the threshold voltage if the charged batt is, say, 9vThe 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. Quote Link to comment Share on other sites More sharing options...
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