## Solar Cell Charger Circuit

Tanya Said:

How do I transfer electricity through glass?

You can use two pot core halves:

http://sound.westhost.com/p69-f3.jpg

You will pick a core that is substantially larger than the thickness of the glass. You put the same number of turns on each one. The inductance of one coil should be 10-100 uH. Now you make the coil resonant by adding a small capacitor either in series or in parallel. A good resonance frequency would be between 50-250kHz. The lower end is actually better if you care about EMI/EMC because in the US the frequency range below 150kHz is not regulated and your design will cause a lot of interference.

Now you excite the primary coil with a MOSFET or bipolar power stage at its resonance frequency. The easiest way to make a resonant oscillator is to add a small coil to the primary and use it to drive the base(s) or gate(s) of the switching element(s). A better way would be to have a fixed frequency oscillator and tune the circuit.

The secondary coil, when in resonance, will pick up the strong magnetic field of the primary with very good efficiency. Now all you have to do is to rectify the secondary voltage and you are done.

There are a few critical issues with regulation, but I would have to write a tome to tell you all the details. Once you get started with this resonant converter, you will hit on them and then you can figure it out for yourself. Let me just say that you will need an active feedback loop from the secondary to the primary side. You can do it with an LED and a photo diode or transistor.

I would use Switchercad from Linear Technologies to model this circuit. Coupled inductors with a small coupling constant of 0.1-0.5 will do the trick just fine.

Good luck!

PS: The sizing of these inductors and circuits matters a lot if your power source is limited. You need to learn to calculate these circuits from first principles (energy stored in an inductor, current rise time for given voltage and inductance etc.) before you can successfully attempt to build this. What I have given you are zeroth order estimates for useful frequencies and inductor values. These are not enough to make this work on a given power budget. Switching supplies are notoriously scale dependent. If you are off by a factor of two, it will work poorly but still work. If you are off by a factor of ten, it won't even work. Don't guess! Calculate.

Randall Said:

What effects would it have if i constantly charge a battery?

Depends on the battery I suppose. All batteries have a limited lifetime (even capacitors). Does this circuit work 24hours a day? If so deign the circuit so that the draw from the batteries occurs at night and during the day you increase the duty of the solar cells so that you are not both powering the circuit with the batteries and running them at the same time. Day time (circuit 100% from solar cells, charge batteries, nightime circuit 100% battery). And to answer your question, most batteries that can be charged automatically shut off the charge mechanism when the charge has reached a sufficient level. Again, read the specifications for your particular battery and determine if that mechanism exists.

Brent Said:

How can I make a battery charge level indicator for 2 separate 4.5v battery packs?

You stand a better chance of getting an answer if you delete this question in Hobby's and Crafts and re-post it in Engineering.

Wingman

Arnold Said:

Is it ok to charge give a device 4.5-4.8 volts to charge its batteries when it runs on 5 volts?

No problem, less is fine more is dangerous. They have a portabhle solar recharger for laptops now which is really sorted.

## Discuss It!

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The working mechanism of the solar cell charger circuit is interesting. The circuit was bit confusing for me during the initial stage and I was able to have a better idea once I went through this discussion. You have been very helpful to explain the entire process out here.

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Solar Cell Charger Circuit

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