# solar charging circuit, need explaination

#### bug13

Joined Feb 13, 2012
2,002

I came across this circuit in this forum, and I don't understand it, I believe there is a missing link somewhere(some concept I don't know), can someone point me to the right direction? thanks

basicly I don't understand the part with a transformer - transistor connection.

#### jimkeith

Joined Oct 26, 2011
540
It is a self-oscillating boost regulator (actually non-regulator as there is no voltage feedback. It takes the low voltage solar cells 8 * 0.5V = 4V, uses this voltage to charge the primary transformer inductance--the secondary turns on the transistor harder so as to saturate the transistor (high current, low sat V zeltex transistor)--the current continues to increase until the transformer core saturates at which time the transistor drive substantially weakens and reduces the collector current, which reverses voltage polarity on both the primary and secondary--the transistor turns off and the inductance discharges into the higher battery voltage until the inductor is discharged--and then it starts over again--whew!

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#### bug13

Joined Feb 13, 2012
2,002
Thanks JimKeith, I got what you are saying, now I have new questions, what are those two capacitors do in this circuit?

#### nsaspook

Joined Aug 27, 2009
10,663
Thanks JimKeith, I got what you are saying, now I have new questions, what are those two capacitors do in this circuit?
Bypass filters for AC, at the frequency of the oscillator they are effectively a short.

#### jimkeith

Joined Oct 26, 2011
540
In addition to what nsaspook said, the input capacitor provides the peak current to saturate the transformer--the solar panel probably cannot do it--also keeps a steady DC current flow out of solar panel because the boost converter uses it in pulses so that during the transformer discharge cycle, the solar panel would be idle.

The 2nd capacitor provides a low impedance return for the transformer winding that drives the transistor.

An interesting experiment would involve trimming the transformer turns ratio for highest efficiency, and /or adjusting the base drive turns for the same.