Solar Powered Circuit

[gravymister]

Hi, I'm new to these forums and have a query about a circuit I'm trying to build, I don't have a huge background in electronics so I'm not entirely sure what I'm doing! Basically this year at glastonbury I want to power a small speaker, some lights and a phone charger using solar panels which I'll place (in a waterproof housing) on the roof of our gazebo!

I've sourced the majority of the electronics from Poundworld, you can get a keyring which has a 5v solar panel and powers 3x White led's. The good thing about these keyrings is unlike the solar panels from the lights outside they don't have a light dependant output, the electronics is pretty basic, I want to listen to the speaker, or charge a phone during the day.

I've attached a rough (and badly drawn) schematic and I was hoping people could give me some pointers of how to improve it, or whether it will work. I don't know much about the components used in the solar panel circuit, the batterys are all taped up.

The idea behind having a switch for each panel in parallel would be to increase or decrease the current as needed, i.e. if the speaker wasn't going to a loud enough volume, you could turn it up with a higher current.

 

[edman222]

Humm my first question would be if the solar cells even have enough current to power a speaker/amp directly ( or do you also have batteries there?)... I don't know if you could find this out from the manufacture of the panels. Also you should find out how much current your charger and speaker system draw and whether the solar cells can handle it, if they cant, it will turn your system off and on constantly, or not turn it on at all. You would need something called a capacitor to store some charge temporarily which would act as a reserve for the system to take current from. If you have a multimeter with a current function you can measure the requirements of the system and figure out if your panels would be up to it. Hope this helps a bit...

 

[gravymister]

I've wired one of the solar panels on to the speaker and it runs (its only a small speaker) but isn't that loud, this was in the dark so running off the battery more than the panel, when I wired another on in parallel it went louder. I managed to find a link to a website with the same keyring that I got from poundworld:
**broken link removed**

So its 5V, power 0.015W, P=V*I, so the current is 3ma? Thats pretty low if I'm correct?

With the capacitor, what kind of size would you think I need? and I would wire that in parallel wouldn't I?

I have a multimeter I don't know what I'm doing wrong but I can't pick up a current atall, I know it has to be in series to get a current but I just can't seem to get it to work unless the current is too small to pick up?

 

[gravymister]

Well I connected one up with the speaker (its only a small speaker) and it worked at a mid-low volume, I put another panel in parallel and it worked at a normal volume, this was running predominately off the battery as it was indoors at the time though. I found the solar panels I'm using:
**broken link removed**

0.015W/5V solar panel, so its pretty low, if I calculated it correctly I think the current is 3mA which is pathetic! I could be wrong though.

If I were to use some capacitors in parallel, what kind of capacitance would you recommend? I have a multimeter but I can't seem to get a current reading, I'm measuring it in series so I'm not sure whats wrong!

 

[carbonzit]

Yes, we need to know more:

• How much current (amps, or more likely milliamps (mA) the panels produce in bright sun;
  
• How much current each device draws (in mA)

This will determine if it'll work or not. Basically, you need enough "oomph" (highly technical term for current) to power each device. The more devices you put in parallel, the more current you need, while voltage stays the same.

From the way you drew the solar panel symbols, it looks like they have self-contained batteries and a diode. Is that correct? Schematic is not bad for a first try.

 

[gravymister]

Hi guys, thanks for the replies, i tried to respond before but I posted a link to the type of solar panel I was using and because I'm new they required moderation. So I'll respond without the link.

The information I found suggested the solar panels are 0.015w/5v amorphous silicon solar cell and the battery it charges is li-ion 3. 6v 40mah. P=V*I if I'm correct meaning the current from the panels is 3mA? which is less than I expected. I have a multimeter but I can't get a current reading I'm not sure why, I'm measuring in series but I can't get anything? Carbonzit: They do have a diode, which I presume is to stop the battery pushing current through the solar cell in reverse? the battery/diode are on a chip, I basically cut off the LED and soldered wires on each terminal.

As far as the devices go, I'm not entirely sure how much current they draw, how would I measure this? The speaker is a small one (from the pringles speaker promotion) and it doesn't need to be really loud, I tried it with just one panel and it worked alright indoors (so using the battery mainly) but it was quite quiet, with two in parallel it was a fine volume, but I don't know how long it would be able to sustain this.

I would only use one device at a time but the phone charger transformer has a rating of 5v/500mA so is that unachievable to power it using these panels?

I used a weird program I found online to help draw the schematic, I did a bit of electronics in my degree but nothing substantial and I still find it pretty confusing!

 

[carbonzit]

I would only use one device at a time but the phone charger transformer has a rating of 5v/500mA so is that unachievable to power it using these panels?

I don't want to insult your intelligence, so I leave it to you to answer this question.

(IOW, do the math, as they say.)

 

[gravymister]

Haha yeah fair play, but if the battery is rated for 40mAh that would mean that it could supply 500ma for 0.08 hours, or 5 mins? so 3 in paralell could charge for 15 minutes, and if they were being charged at the same time as discharging by the sun? or does that not work?

 

[carbonzit]

A battery cannot supply more current than its rating, so 40mA is what you get, tops. You could draw 20mA for 2 hours, for example.

You should also "derate" things conservativly; in other words, be pessimistic when it comes to calculating sources and loads. Batteries wear out, the sun goes behind clouds, etc.

 

[gravymister]

Ah so I'd need at least 13 solar panels! Thats definitely unrealistic. The speaker can be powered because I've tried that, is the circuit ok? would a capacitor in parallel help? I can't believe the solar panels are so weak, I think I had too high expectations, I suppose they were only a pound each!

 

[audioguru]

The smallest battery for my electric RC airplanes is 120mAh and it is tiny. Yours is 1/3rd of it.
Yours might be able to produce 200mA for 6 minutes but the little amplifier doesn't use a current as high as 200mA.

The supply voltage for the speaker amplifier will average 3.6V so the power into an 8 ohm speaker at clipping will be 0.04W which is as loud an an earphone laying on a table. The amplifier will use a max battery current of only 16mA.

For a gazebo you need 2W to 20W. Its amplifier needs many more volts and amps.

 

[carbonzit]

You need to shop around, and pay attention to the specs for any panels you're interested in.

Remember, you're basically budgeting your power. You have so much to "spend" (whatever the panels in parallel will produce), and you can distribute it among your devices, if there's enough to go around.

Here in the US, we can get pretty cheap solar panels from Harbor Freight. Surplus electronics dealers are another source.

 

[gravymister]

Well you see originally I just got a couple of cheap panels to play around with and my plans have grown and spiralled out of control! I think I'm just getting a little ambitious. One of my friends has some old solar lights she doesn't want so I'm going to harvest the panels/battery's from them in the hope they might boost this a bit.

With regards to Audioguru's comment, if I were to hook up some better rechargeable battery's from an old digital camera to be charged by the solar panel would that be better as they would have a better mAh rating?


I have a look at my phone battery and its Li-Polymer 3.6V 960mAh, although when plugged into the mains it is charged with 500mAh, if I were to plug it into to my three parallel panels at 160mAh, would it still charge but at a slower rate? I know I'm probably asking obvious questions but I'm just a little clueless.

 

[gravymister]

Also, what gauge wire should I be using for this, there will be quite long links between the panels and switchs/speaker, I know larger wire has less resistance but will it make a significant difference in this application?

 

[audioguru]

A battery cannot supply more current than its rating, so 40mA is what you get, tops. You could draw 20mA for 2 hours, for example.

No.
Lithium-Polymer 3.6V rechargeable battery cells are rated in mAh, not in mA. The 40mAh cell produces 40mA for 1 hour or 75mA for 30 minutes or 140mA for 15 minutes etc.

My 120mAh cells can supply 25 times their rating. So their max continuous current is 3A. Their peak allowed current is 6A.

 

[carbonzit]

No.
Lithium-Polymer 3.6V rechargeable battery cells are rated in mAh, not in mA. The 40mAh cell produces 40mA for 1 hour or 75mA for 30 minutes or 140mA for 15 minutes etc.

My 120mAh cells can supply 25 times their rating. So their max continuous current is 3A. Their peak allowed current is 6A.

Well, the point is there is a limit to how much power you can (safely) get out of any battery/cell. I was wrong about the 40mAh business, but correct in general in that batteries cannot provide infinite power.

To the O.P., if you're going any distance, then yes, use heavier-gauge wire. With low voltage circuits, resistance (I2R) losses are worse than with household AC.

And you can get more current than your solar panels will provide if you use them to charge batteries. You can stack batteries up in parallel until you have enough for to drive your loads. However, it's not quite as simple as just wiring everything together in parallel. You should have a controller to regulate the rate of charge. There are lots of circuits out there that do this; some of them might actually work.

 

[audioguru]

The Pringles Speaker is a crappy little 2" speaker in a Pringles potato chips can. One ad says it includes an amplifier.
It probably sounds very "tinny".

 

[gravymister]

@carbonzit: With regards to the charge controller, that sounds complicated, I was hoping to keep things simple, I was under the impression when using a trickle charge for batterys its not too much of a problem if its not regulated? The panels aren't powerful so at this low current would it be required? Also if I were to put lots of batterys in paralell could I blow the speaker with too much current?

@audioguru: Yeah its a rubbish little speaker, but Its only to listen to Worthy FM the festival radio station whilst at Glastonbury as it announces secret acts and other things happening during the festival, its not to supply music, we have a variety of acoustic instruments to do that! The advantage of it being small is that it requires less power which is why I chose it.

 

[carbonzit]

Also if I were to put lots of batterys in paralell could I blow the speaker with too much current?

No. Common misconception: the battery doesn't "push" current through your speaker (and by "speaker" I ASS-U-ME you mean a powered speaker with a self-contained amplifier, correct?). The amplifier draws power from the battery. It'll only drink until it's satisfied, to use an awkward metaphor.

The things that need current limiting so they don't draw too much current are LEDs and other non-linear devices, which draw more current as they heat up and heat up as they draw more current: you can see where that would go ...

 

[gravymister]

Ah right I see what you mean, thats great thank you! and yes I did mean speaker with self contained amplifier! Its going to be sunny tomorrow so I'll set up the three in parallel and see what I can do, hopefully I'll be able to measure the current as well.