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Circuit: change AC emergency light to DC solar light?

Posted: Wed Oct 02, 2013 2:01 pm UTC
by webgiant
I want to be able to build a set of on at dusk, off at dawn, solar charged lights. I found this circuit, which uses AC power to charge a battery and turns on a light when the AC power shuts off. I feel extremely lucky to have found this circuit, because when I Google "diy solar charged light DC", I get a million hits for "buy our pre-made solar garden light!" and no DIY DC-only circuits.

I want to have a larger battery than the basic 1.2V NiCd battery found in solar garden lights, so that I can also have a larger LED array with much brighter LED lights. I have the resistor calculations down for the LEDs.

What I want to know is if I can simply swap out the transformer and full-wave bridge rectifier, which produce an AC-DC 9V 500mA DC input, for a 9V 500mA Solar Panel input. Is it really that easy, or do I have to remove or change something in the circuit to make it work with pure solar DC electricity?

Also, what would need to change to make the circuit use a smaller battery size? I'd like to use the same basic structure for a "Sun Jar" type light, but it wouldn't need (or be able to use) more than a 3.6V (3 - 1.2V AAA) battery pack.

Re: Circuit: change AC emergency light to DC solar light?

Posted: Wed Oct 02, 2013 5:47 pm UTC
by Wnderer
webgiant wrote:What I want to know is if I can simply swap out the transformer and full-wave bridge rectifier, which produce an AC-DC 9V 500mA DC input, for a 9V 500mA Solar Panel input. Is it really that easy, or do I have to remove or change something in the circuit to make it work with pure solar DC electricity?


Yes.

webgiant wrote:Also, what would need to change to make the circuit use a smaller battery size? I'd like to use the same basic structure for a "Sun Jar" type light, but it wouldn't need (or be able to use) more than a 3.6V (3 - 1.2V AAA) battery pack.


This is trickier. First you need to know the voltage drop and current requirements of the white LEDs. If the LEDs have a voltage drop greater than 3.6V your sunk. If the voltage drop is less than 3.6V than you need to change the 100 ohm resistors for the correct current (R = (3.6 - Vled)/Iled). Next you have to change the 6.8 volt zener diode. The transistor base-emitter drop + the 6.8 volts zener is turning off the charger when the battery voltage hits around 7.2 volts. You need to find out what the maximum voltage on the 3.6 V battery is and replace the 6.8 zener with a zener diode about 0.4 less than that. You also need to change the output voltage of the LM317 by adjusting the value of the 2.2k pot. The LM317 datasheet is here.

http://www.ti.com/lit/ds/symlink/lm317.pdf

It looks like you need to lower the pot to around 400 ohms to get 4 volts, so the 2.2k pot is fine but you can use a lower value like 1k if you want. You don't want to put too high a voltage on the battery or LEDs so remove that 16 ohm resistor and then apply power and adjust the pot while measuring the voltage at the LM317 output. Then turn off the power and reinstall the 16 ohm resistor and the circuit should work.

Re: Circuit: change AC emergency light to DC solar light?

Posted: Wed Oct 02, 2013 10:18 pm UTC
by Link
I'd be careful with this circuit; many types of battery are quite tricky to charge, with a risk of explosion if you do it incorrectly. The voltage difference between the fully charged state and nearly-empty state may also not be large to enough that you can just pop a Zener in there and make it work nicely and predictably.

If I were to design something like this, I'd probably go for a switching regulator (which, depending on how you do it, might even let you get away with a battery voltage that's actually lower than the LEDs' forward voltage) and a dedicated battery management IC, although that may be over-engineering it a bit.

Re: Circuit: change AC emergency light to DC solar light?

Posted: Wed Oct 02, 2013 11:51 pm UTC
by webgiant
Thanks for all of the responses. I knew I should have majored in electronics.

Re: Circuit: change AC emergency light to DC solar light?

Posted: Thu Oct 03, 2013 12:32 pm UTC
by PM 2Ring
You may have more success doing a search for "solar charger circuit".
Or just buy a solar charger... :)

Re: Circuit: change AC emergency light to DC solar light?

Posted: Thu Oct 03, 2013 2:59 pm UTC
by webgiant
PM 2Ring wrote:You may have more success doing a search for "solar charger circuit".
Or just buy a solar charger... :)

Thank you for the Google keyword idea, I shall try it immediately. Hopefully I will find the website I forgot to bookmark which had a much more detailed "how to do it" description which built up this type of circuit in easy to follow steps...(...bingo! also the Joule Thief circuit).

Buying a solar charger is going to be the solution of last resort. The point is to know how to build solar lighting and how to resize if necessary for larger solar panels or larger battery banks. All the parts may come from the same country as the solar garden lights come from as well, but there's just something special about being able to build this stuff yourself.