I want to put together a string of 5mm LEDs that will be powered by by a 5V, 50ma solar panel I bought from Radio Shack.
Just playing around, I connected up to 11 white LEDs ( 7000mcd, 3.3V, 25ma each ) in parallel to the solar panel. Using my multimeter at peak lighting, voltage was about 2.7V and the current was 30ma. The lights have worked consistently and remained nice and bright without getting even remotely hot or even warm for over a week so far.
My questions to you are these:
(1) What would be the maximum number of these LEDs I could power with the 2.7V reading I’m actually getting from my solar panel and 25ma max? And how do you calculate this? This way, I can know how to adjust for different voltages, LEDs, and amperage.
(2) Would I even need a resistor(s) here? If so, how many and where would they be connected? I’m not sure if 1 resistor in series between the solar panel and the 1st LED would suffice. Or if I need individual resistors in series between each LED in the string and perhaps the solar panel.
FYI, to connect everything, I just used some old speaker wire I had leftover. It’s the kind that has 2 copper wires individually wrapped in plastic. Just split it down the middle so I would have individual wire strands. And sized the wires, then stripped off the ends to wrap around the LED leads. The final version will be more professional. This is only my “playing around” version right now.
Thanks everyone for your help!
Thanks everybody for all your help! I had a feeling I didn’t need any resistors because I’m dealing with very low voltage and current.
FYI, I did measure the voltage across each LED and it was pretty constant at 2.65-2.7V.
My ultimate goal is to make a solar string light that I can use for some small area lighting. So it’ll either be a thin ribbon type or I may put it inside a small plastic tube. Not sure yet.
And I’ll connect it to 2-4 rechargeable AA batteries, 2500ma Ni-MA ones. It depends upon which solar panel I’ll use. But the two I have, the Radio Shack and one from a garden solar light, are relatively low amperage and voltage. So I’m not concerned with overcharging.
I also have a voltage regulator I could add if need be. And will definitely be including a rectifier (blocking) diode so current won’t flow back into the solar panel. Plus, I have the photoresistor from the garden light to incorporate into the mix. And some 100 ohm resistors I could add if needed.
More Pages:
- Solar Panel: How Can I Set-up My Small Solar Panel To Charge AA Solar-rechargeable Btteries? (5/22/2011)
- Solar Lights: How Can I Power LED Light Strings With A Solar Panel? (6/24/2011)
- Solar Lights: Any Craft Or Hobby People Out There? (6/27/2011)
- Solar Products: In Terms Of Light Out-put,how Do Solar Landscape Lights Compare To Low Voltage Lights? (7/26/2011)
- Solar Panel: Can You Charge A Phone With A Small Solar Panel? (5/11/2011)
{ 3 comments… read them below or add one }
Normally I’d say you always need a resistor in series with each LED. In this case, the high internal resistance of the solar panel is providing that resistance.
Putting a bunch of LEDs in parallel without individual resistors normally is also a bad idea, unless the LEDs are very well matched. It appears that this is true in your case, otherwise you would see a lot of variation in brightness. But that can be explained by the fact that you have 11 in parallel, so each one is getting only 50/11 or 4.5 mA each, far from their rated 25 mA.
!) If your panel can only deliver 50 mA, and you want to run the LEDs at max brightness, 25 mA, you can only drive two. (50ma / 25ma = 2)
2) See comments above. You don’t need resistors as you are driving the LEDs with only 4.5 mA each, and the internal resistance of the panel is limiting the current. If you try to drive just 2 at 25 mA each, you would need individual resistors, about 2v/25ma = 80 ohms.
3) currents are so low that wire size doesn’t matter.
.
I’ll answer question 2 first.
Resistors consume power, so if you want to maximize the number LED’s you can connect then I would avoid using any resistors.
As for the first question. Anytime you draw current from a solar panel, the voltage will appear to drop. This is why it’s reading 2.7 volts instead of 5 volts. If you measure the open circuit voltage of the solar panel with sufficient light it will read close to 5 volts.
It’s hard to know how many LED’s exactly you can put in series unless you know the exact voltage drop of each one. 5volts – 2.7volts = 2.3 volts which is approx .2 volt drop for each one. So theoretically you could put 24 or 25 in series. But it may be possible that you don’t have to stop there. You may be able to connect another 24 in parallel with the first 24.
Try connecting 24 in series (or as many as you possibly can), then measure the current flow. If the current flow is less than the 30mA you originally measured, then I would predict you can put another row of LED’s in parallel with the first. Then if that works then measure your current again and keep adding rows of LED’s until the current approaches 50mA.
Good luck!
The other contributors have done a fine job of explaining the answer, I’m just intrigued why you want to convert sunlight into white LED light – it’s a very inefficient method of doing so. Unless you plan on storing it (in which case you will definitely need to consider resistors to limit the current of you use a battery).
You might as well use a light pipe or a fibre-optic bundle or similar.