Solar powered LED light needs help

Question

I just got a outdoor solar powered LED lamp to illuminate our house number at night. It is supposed to charge up its battery during the day, and automatically turn the LED on at dusk. The problem I'm discovering is that the solar panel on this lamp doesn't get enough natural light to charge it so that it can power the LED even for a few hours at night. I like the light fixture well enough so I'm thinking of keeping it but giving it some help.

I have a door bell and opener circuit located under the lamp that runs off a transformer. I was thinking about giving the LED lamp some help from this circuit. Here are the specs for the LED lamp and the doorbell circuit:

LED lamp: SMD 5630 LED 0.5 W, 3.2V 1000mAh Li-ion battery

Doorbell transformer output: 16V AC 30VA (but I measure 18.25V AC at the output)

Would this be a complicated exercise? I have some soldering skills but I need help figuring out how to step down the voltage and amperage to feed the light from the doorbell. I'm assuming that with the proper step down I can wire in the power from the doorbell to the same place the little solar panel is attached to augment the same charging circuit in the light?

Any help would be greatly appreciated!

Back of PV and battery (blue and yellow wires from PV, blue and red from battery

Circuit board. The IC is labeled: 8182 4K154

During the day the battery charges from the PV, at dusk the LED switches on and is powered from Battery. Could I maintain the switching capability and just power the LED with appropriate 3.2V power supply? Will the PV be attempting to charge my power supply during the day?! I don't want to cut out the PV since it appears to be the dawn/dusk switch as well...

Answer 1

I picked up a bunch of doorbell transformers (just for my junkbox) at $1 each, a while back. About the same as you describe. I'd probably use a bridge rectifier followed by a ripple capacitor to get the basic DC part done in preparation for an LM2596 buck converter. (You can get these for under a dollar on ebay, if you don't want a fancy voltage and current display, and they can easily handle your LED requirements. Most include potentiometers to use in setting the voltage and current limit, too.) Once you get the voltage into the right vicinity (say 5 V), you can then drop the rest of the voltage with a current limiting resistor in the usual way.

The first thing to do is to test your transformer under load. Just to make sure it works, mostly. I use one of those big 25 W resistors -- \$100\:\Omega\$ if you have one around? Load it down and see what it does. Chances are, it's fine though. But I like to know what the transformer does under load just to be sure.

But that's what I'd probably do (partly because I have a small box of buck converter boards laying around, too.)

Answer 2

Bare minimum, a diode ac full wave rectifier, maybe a step down regulator and a lipo charger circuit. The solar and battery part of the lamp is likely just a solar cell, a diode and the battery pack. Dumb and depends on the low output of the solar cell, matching the charged voltage of the battery.

The LED half has a LDR circuit as part of it that turns on automatically, so no extra modification is really needed. At that point though, you might as well ditch the solar cell and battery and directly wire it to the AC wire. Or run a wire for low voltage outdoor lighting which is often 12V DC.

Highly depends on the actual wiring and circuitry of your lamp.

Answer 3

• Assuming 12h LED duration, current is ~156mA (0.5W/3.2V)
• thus 1900 mAh is needed (156mA*12h)
• you are getting a few hours with a 1000 mA LiPo battery charged only to 3.2V

• thus not getting full capacity of up to 6h (minus series losses from 3.7 to 3.2 or ~14% (0.5/3.7)

  • If 16Vac Tfmr is unloaded, it is reading 14% higher due rated V at 86% efficiency.
  • the transformer can supply <2A @30VA @16Vac only to a linear load and not to a bridge rectified battery but a series R can make it more linear yet lossy with < 15% efficiency 3.5/24V in powering the LED.

  • thus another wire pair from a full bridge diode at transformer or single wire to bridge and series R at load must be wired to charge the battery using current load for expected voltage R drop.

  • Using 16Vdc avg to 3.2Vdc LEDs with up to 156mA

  • with no PV, R=82 Ω drawing 2W (12.5*0.156A)
  • however when LED is off, Vbat continues to charge up during day time with no overvoltage protection.

Thus you can leave LED on all the time, and choose a 5W resistor for cooler operation or buy a proper supply or add more complexity to regulate LiPo charger.

Unfortunately both your PV and storage battery are under-designed to meet the load requirements.

One simple solution is power the battery and LED from a 3.3V source such as tap into an ATX PSU with wire pair to series Sch. diode to LiPo, or buy one ~3.2Vdc >150mA wall supply to feed circuit.