[sdiy] OT: solar lighting

[Harry Bissell]

OT - WTF
inline

----- Original Message -----
From: John Mahoney <jmahoney at gate.net>
To: synth diy <synth-diy at dropmix.xs4all.nl>
Sent: Wed, 17 Feb 2010 18:17:39 -0500 (EST)
Subject: [sdiy] OT: solar lighting

I'll accept info or pointers to a proper forum, thanks.

Since I live in "The Sunshine State", I want to make some solar 
powered lighting for inside the house. Electronic Goldmine sells a 
3.6V 750mAh NiMH battery (from cell phones), a 3.6V 70mA solar panel, 
and a white LED that can take 3.6V @ 30mA.

I've read that you don't need to regulate the charging current if 
it's only 10% of the battery's current rating.

True in general for NiCd batteries. You might want to go even less than that
for a continuous charge.

 Is 750mAh (milliamp 
hours) the applicable rating, or do I need to know the maximum 
*continuous* current the battery can produce?

That is the 1X rating, the one you use. You probably want to discharge at only
1/10 of that rate, or so.

Do I need to regulate the solar panel's voltage? If so, would a 3.6V 
zener work? Should I use a 4V solar panel (Goldmine has those, too) 
and a 3.6V zener "regulator" with the 3.6V battery?

Yuck. Solar panel and a shunt regulator ??? Be aware that solar panels put out almost
a constant voltage, but vary greatly in current. They make only .7V per cell (woah,
just like a diode)

You probably want to have a shottky diode in series with the cell output, to prevent
any reverse leakage.

Finally, do I need a current-limiting resistor for the LED? If so, is 
120 ohms the right value? (3.6V / 0.030A = 120)

If its a white LED, you probably get just about the right voltage. I'd probably look into making a buck-boost converter (switcher) that can match the battery voltage to whatever the cell is most
happy with.

At the least you might want a "polyfuse" between the panel and the battery, and or the LED.
Abnormal currents would make that go high impedance.

LOOK inot what National Semiconductor has just introduced for solar cell arrays. They make a switcher that maximizes the current from each panel in a series array. Think of this, current
flow is equal everywhere in a series circuit... so the least powerful cell regulates the current
for all the ~better~ ones.

Oh... I know this wouldn't produce a lot of light. This is a test, 
and I'd scale things up if it worked well.

Go for the best efficiency you can get. Probably you'll be looking at switching regulators
soon.

H^) harry (who has been looking into switching regulators)
.

Thanks for any on- or off-list help!

John

Links to the above-referenced products:
battery: 
<http://www.goldmine-elec-products.com/prodinfo.asp?number=G15613>http://www.goldmine-elec-products.com/prodinfo.asp?number=G15613 

solar panel: 
<http://www.goldmine-elec-products.com/prodinfo.asp?number=G16394>http://www.goldmine-elec-products.com/prodinfo.asp?number=G16394 

LED: 
<http://www.goldmine-elec-products.com/prodinfo.asp?number=G17255>http://www.goldmine-elec-products.com/prodinfo.asp?number=G17255 


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-- 
Harry Bissell & Nora Abdullah 4eva