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LM317 battery charger

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jootachappal

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Hello,
I have been working on this charger circuit for quite some time this is a constant voltage battery charger . The problem is that i dont know that how does the ckt actually works especially the need of the transisitor BC547 .Whether the working of this transisitor is dependent on the AmpereHour rating of the battery or is it related to the output voltage .I came across this ckt form the site mentioned below:

https://www.vt52.com/diy/myprojects/other/charger/charger.htm

I"ll b extremely thankful if any one of the members can help me in this regard .

Ayesha
 

plusminus

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The transistor limits the maximum current to a lead-acid cellyou want to charge. (see text in your link)
When the cell is empty the current will be high, you have to limit this current to protect the cell for damaging. When the current drops (this is when the voltage of the cell rises ) the transistor goes open and the cell is charged with a constant voltage.
In the text you can see that the charge current is 1/10 of the Ah rating. The 1/10 current is the current which must be limited by the transistor. So if the current is to high (>1/10 of Ah) the voltage over the cell will drop to limit the current.
 

twidlerjohn

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But, please observe that the transistor senses voltage and not current. To sense current, there has to be a series-drop resistor.
 

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