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I am starting a project and need help with the circuit designs as I am new to them. I want 18 LEDs powered by a 9v rechargeable battery. I also want to recharge the 9v with solar cells. The LEDs will only be on for about 1 minute a few times a day. Any suggestions?

The LEDs are 4.8mm(i guess 5mm) straw-hats, 3.0-3.2 forward voltage

You'll need a switch mode step up converter to step up from 9V to the total forward drop of the LED string (i.e. at 3V per LED that's 54V total).

This is probably best achieved with a flyback converter.

However, you don't want constant voltage out, you want a constant current to regulate the LED current.

Have a look at Anlog Devices, TI, Linear Technology, OnSemi and Diodes Inc., websites for info and apps notes on driving LED strings.

Solar PV also probably is best done with another switch mode supply to maximise charging from the low PV voltage and current.

Searching the same places will turn up useful info on PV battery charging.

Also have a look around CL. There are other people doing PV battery chargers.

Wow. I have no idea what any of that is. This project doesnt sound all that simple anymore.

@Signality: But why take all LEDs together to a string?

Regards, Sancho

@akramer08,

Sorry but SMPS's are pretty much de rigeur for anything solar/PV because they're so much more efficient than linear regulators.

Ditto using SMPS's to drive LEDs rather than linear regulators or simple resistors in series with LEDs.

I suppose if you have several PV panels then you can wire them in series to charge the batteries and if the LED only has a very low duty cycle then you can afford to be wasteful of power in driving them.

You could drive them all in parallel or maybe in 9 sets of two in series.

@Sancho_P,

Same current through each LED so if they're matched they're all the same brightness.

LED optical output is current not voltage driven.

:)

Can you just buy an SMPS? All the ones I have read about it looks like somebody just built them.

"Can you just buy an SMPS? "

Unlikely.

PV battery charging isn't a simple X volts in Y volts out job. Proper, high efficiency LED driving isn't either.

"Have a look at Analog Devices, TI, Linear Technology, OnSemi and Diodes Inc., websites for info and apps notes ... "

Based on that then see what you can get from places like:

http://uk.farnell.com/

http://cpc.farnell.com/

http://www.digikey.co.uk/

or your national equivalent.

I will look into that. Another question. Will I need some type of regulator between the solar cells and the 9v battery? I dont know if it would just keep trying to charge the battery even when its fully charged.

Not for the first time, I have this awful sense of Deja Vu again ...

https://www.circuitlab.com/forums/basic-electronics/topic/ndex62v3/9v-solar-powered-circuit-for-leds/#comment_3610

BTW:

like I said in the original post, I am completely new to circuits and wiring. This sounds too complicated for little reward. I will find a different project to work on.

Don't be too discouraged!

You can do this project without switch mode supplies: those solar powered garden and porch lamps are basically a PV panel, a resistor and a diode to charge 2 or 3 of NiMh cells in series, a light sensitive switch circuit and another resistor to drive a LED.

You can drive several LEDs, each with their own series resistor all connected in parallel off the battery, as I referred to earlier.

However, you do have to know a bit about how PVs work to see what max voltage, current and power you can get out of them to see how high a battery voltage you can hope to charge. From the battery voltage you can then see what max LED forward drop you can expect to drive: the resistor has to drop a volt or more to have any chance of controlling the max/min LED current so it doesn't blow the LED with a fully charged battery.

You can make the circuit more complicated by using a light senstive switch with a timer, adding constant LED current, trying to optimise the charging current out of the PV and so on but a basic circuit is not so complicated.

Have a search on CL for Learning Resources to get you started on the way to finding more about electronics.

if I had just a bit more info Imay be able to really confuse you,first what type of pv (photo voltaic) solar panel are we talking about? output volt amps also what are the requirements for your leds current wise?