# Help understanding simple solar charging circuit

#### legoman

Oct 14, 2011
5
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

#### Attachments

• tn_12v-solar-battery-charger-circuit.GIF
18.5 KB · Views: 15,251

#### duke37

Jan 9, 2011
5,364
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.

#### legoman

Oct 14, 2011
5
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?

#### OLIVE2222

Oct 2, 2011
690
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

#### duke37

Jan 9, 2011
5,364
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.

#### duke37

Jan 9, 2011
5,364
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

#### Resqueline

Jul 31, 2009
2,848
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.

#### legoman

Oct 14, 2011
5
A better choice of diode

Would a Schottky diode 5819 be a better choice?

#### OLIVE2222

Oct 2, 2011
690
A BYV27 will give you more headroom. or anyone >=2Amp you can source

#### legoman

Oct 14, 2011
5
Here are two simple solar charging circuits, I like the capacitance voltage doubling charge pump idea of one circuit. A have a solar panel that produces 22.5 volts and 110 mA. Which circuit would give me better efficiency or would put more current to the battery at any one time?
From this website: http://jaw.iinet.net.au/projects/solar.html. This is the second circuit:
from this website: http://electroschematics.com/4746/solar-charger-circuit/. Thanks

Replies
1
Views
3K
Replies
4
Views
2K
Replies
1
Views
1K
N
Replies
2
Views
1K
Captain Brazil
C
Replies
26
Views
1K