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Pls help me explain this charger circuit

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Yosmany325

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Hello tengyy, this circuit is a little weird. Yes when you have a VCC voltage higher than battery's voltage it would charge through 1000 resistor, once VCC is lower than Vbat - Vdiode the battery will discharge through VCC to Gnd. The other part of the circuit will never work because to power the LED the PNP transistor should be at active or saturation region. To achieve this, should pass current through the transistor base and in this circuit this does not happen (unless the transistor let current flow from base to emitter which indicates a fault transistor).
 
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    tengyy

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tengyy

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Sorry ya.. maybe this will be easier to see , and thank you for your explanation
Solar+USB charger.JPG
 
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betwixt

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Looks like a schematic error to me, or something else is there but missing in the diagram. The battery charges as explained by Yosmany325 but the transistor never conducts. Either something else is connected to it's base pin or Q1 is both the wrong polarity and has it's collector and emitter reversed.

Note that the link you gave to the 2N5401 does not match the transistor in the schematic which is marked 2N5415.

Brian.
 

tengyy

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I am sorry, i just notice the first schematic should be the wrong one. this should be correct
Solar+USB charger.JPG
 

engineerpervez1

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This circuit has two parallel path, one for battery charging and one for LED glowing. battery charging may be work when the solar panel voltage will be more enough may be 18 to 26 volts but other part of LED circuit will never work as it needed -ve base voltage that it will never get.
I have checked this in Proteus. you can see the result when the applied voltage is 22 volts dc than 20 mili amp current is flowing through the battery for charging, but LED still not glowing.
 

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betwixt

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The new schematic makes a little sense but it still won't work.

The top transistor (Q5?) is wired wrongly to be a 5V regulator. It should be wired with the incoming voltage to the collector, +5V taken from the emitter, have the Zener diode from base to GND and the bias resistor from collector to base. Just about everything is wrong!

The LED circuit will light when the battery is on charge although most of the charging current will flow through the base-emitter junctions which is not a good idea. I'm not sure what D1, D2, D3 and D4 are there for, they seem to have no purpose. If they are to protect charging circuits when the power is off but a charged battery is still fitted, the 1.2V from the battery is low enough not to do damage anyway. There is no charge current regulation so either this is a failed attempt at designing a charger or the schematic copied from a poorly designed one.

Given the 5V to start with and the maximum current limited to about 3.2mA, it would make more sense to simply wire the LED in series with the battery.

Brian.
 

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