Tiny/Cheap Solar clipping circuit
SimonSolar2C
Registered Users Posts: 4 ✭
I'm building a small solar charger for my iPhone. Ive done some tests and it seems happy to use voltage 4-6.2v
My panel is 6.8 volts Voc
The iPhone does do some weird pulsing of drawing the power, and wont start until it sees a lower voltage.
So I want to keep this really simple, no switches or converters - just clip off the top of the voltage.
Ive tried a 6.2 volt zener across +/- but it gets too hot.
Any ideas?
My panel is 6.8 volts Voc
The iPhone does do some weird pulsing of drawing the power, and wont start until it sees a lower voltage.
So I want to keep this really simple, no switches or converters - just clip off the top of the voltage.
Ive tried a 6.2 volt zener across +/- but it gets too hot.
Any ideas?
Comments
I'd suggest a bigger zener. Digi-Key has a 10W zener for $30; that will work for a panel up to 10W. You can probably find one on Ebay for less.
Or if you want to get fancier, use a 7805 linear regulator (along with the recommended capacitors.) If you want lower dropout try a LM2940-5.0 (5 volts.) If you want an adjustable voltage AND low dropout try an LMS8117.
Is there a way to keep the voltage low without short circuit shunt?
Ideally the solar panel would sit at 7v when there is no load, but the voltage presented to a load would look like 6.2v, or lower.
When current gets drawn, the solar panel voltage comes down as the current draw increases.
Someone suggested an NPN transisitor would do that, but Im not convinced.
Here are two circuit options:
Is the NPN redundant here, or is it behaving like I want ( the load will 'see' 6.2 volts but panel sits at 7v so there is no big current being dissipated when no load is connected ) :
"Is there a way to keep the voltage low without short circuit shunt?"
There are two ways to do linear regulation with a solar panel.
1) Shunt (i.e. a zener.) With this you dissipate maximum power at no load.
2) Series (i.e. a regulator.) With this you dissipate maximum power during loaded operation.
Sadly whilst this produces 5v at the output, it does not pull the solar panel down at all, into its MPP, so little to no current is generated.
I only need to regulate the voltage when no current is being drawn, once the phone starts pulling power, I suppose I want to bypass the LDO regulator.
Im surprised I cant find a simple solution for this.
The problem with option 1) above, is that it seems to result in the 'system' settling at the zener voltage, and not transferring all the power available.
Pulling the panel voltage down will not increase the current output. But a larger solar panel (same voltage) will.
I am available for custom hardware/firmware development
If the input is 8 volts and the output is 5 volts and you're not getting enough current before the regulator folds back, get a larger regulator.
If the input is 5.3 volts and the output is 5 volts and you're not getting enough current, get a panel with a higher voltage.
Let's say you use a 5.6 volt zener. If you are drawing so much current that the voltage drops to 5V then you are getting all the power the panel is capable of at that voltage; the zener is out of the circuit at that point.
https://www.amazon.com/Anker174-Dual-Port-PowerIQx2122-Technology-Motorola/dp/B00B8M4IMK
Should work with an Vmp~18 volt solar panel (assuming enough sun and not too much USB load):
Since it is a switching power supply--Should be pretty efficient and not run hot (like a shunt regulator would).
-Bill