# Help understanding simple solar charging circuit

Discussion in 'General Electronics Discussion' started by legoman, Oct 14, 2011.

1. ### legoman

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Oct 14, 2011
Hi,

I found this simple 12v solar charging circuit online. http://www.circuitdiagram.org/solar-battery-charger-circuit.html. Can someone please help me understand it. I understand the output adjusting part of the LM317 ic. What I don't understand are the conditions how the transistor is turned on and off. Isn't DC voltage an open circuit to the capacitor hence the transistor will alway be off? What conditions would exist, which would warrant the .5ohm 5watt resistor to be required?

Thanks

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2. ### duke37

5,364
772
Jan 9, 2011
I think that a link is missing.
A connection should be made from the junction of the 100R resistor and the 220nF capacitor to the battery negative.
When there is more than 0.6V across the 0.5R resistor, the transistor turns on and drops the output voltage so limiting the current to about 1.2A.

3. ### legoman

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Oct 14, 2011
OK, thanks. That makes a lot more sense. Could I substitute a 7 watt light bulb in place of the 5 Watt resistor? Would the resistance change in the light bulb with temperature vary too much?

4. ### OLIVE2222

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Oct 2, 2011
The resistance will vary a lot with the temp. don't remember exact values but certainly something like 10:1 ratio.
Concerning the charging circuit caps are also missing at the LM317 Input. you also drop precious energy in the diode.
Olivier

5. ### duke37

5,364
772
Jan 9, 2011
A 6V 7W bulb will be vey different to a 240V 7W bulb. In use the maximum voltage across the bulb will be 0.7V so a low voltage resistance measurement may be not far out.

The diode is necessary to stop battery discharge when the light is low. I do not know whether a capacitor is necessary on the input, there is one on the output for stability purposes.

6. ### duke37

5,364
772
Jan 9, 2011
Having another look at your circuit, the series resistor is much higher specified than necessary. The power in a resistor is V*V/R, i.e. 0.7*0.7/0.5 this comes down to about 1/4 W. A half watt resistor would be fine and a lot smaller and easier to fit than a bulb.

duke

7. ### Resqueline

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Jul 31, 2009
I get that a 0.6W resistor is needed even if the transistor turns on at only 0.55V (which it will do at the low collector current). The 1N4001 is not good for 1.2A btw..
But another question is if the current limiter circuit is neccessary at all. I'd say only if you're using a small battery (3.5Ah) with a large panel (>22W).
Solar panels are inherently current limited devices, so only if the panel gives a current higher than 1/3rd of the battery Ah rating would an extra limiting be desired.

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Oct 2, 2011
9. ### legoman

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Oct 14, 2011
A better choice of diode

Would a Schottky diode 5819 be a better choice?

10. ### OLIVE2222

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Oct 2, 2011
A BYV27 will give you more headroom. or anyone >=2Amp you can source

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Oct 14, 2011