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 Good Ideas done Badly

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Bad LED Path Light II  (February 17, 2011)
My wife knows I love LED products.  A while back she spotted a solar powered LED path light at the dollar store.  Stores like that are flooded with discontinued or surplus products taken off the shelf at other stores and sold at the dollar store for deep discounts.  After removing the plastic insulator from the battery terminal, allowing the battery to be connected to the path light circuit, I stuck the thing in the ground in a sunny part of my front lawn.  I let the light absorb some sunlight all day.
 
Later that evening I noticed the light did not turn on.  I covered the solar panel with my hand and saw that the light still did not turn on.  It was time to crack open the thing and see why it was not working.  Sure enough, a wire to the LED had broken off from the circuit board. After re-soldering the wire, the thing sprung into life.  But, I was pretty disappointed by the light the LED produced.  Here are the overall specs for this device.
 
The glass covered solar cell area measured 1.6” x 1.6” for a total area of 2.5 square inches.  I measured an open circuit voltage of 3v from the solar panel and a short circuit current of 25ma.  It appeared that the solar panel was a thin film amorphous type.  Even at peak efficiency, the total power generated by this path light solar panel was a dismal 75mw.  Sunlight produces a power flux of about 100 watts per square foot or about 700mw per square inch.  With 2.5 square inches of area, about 1.75 watts worth of sunlight hits the solar panel. If the solar panel was generating only 75mw, that says that this solar panel has an efficiency of only 4%.  Good solar cells can easily reach an efficiency of 15%.  Some of the best units exceed 20%.  So, had the manufacture used a better quality solar cell technology, I would have seen more than three times the power generated as this unit.
Inside the path light assembly I saw a single AAA NiCd rechargeable battery with a marked rating of 300ma-hours.  I thought NiCd batteries were not being used in products like this anymore, but here is one that does.  An AAA NiMH battery would have been a better choice.  Also, most quality AAA NiMH cells have a rating closer to 900ma-hours, which is three times the rating of this battery.
Path Light 3v Solar Panel  
Path Light Circuit Board and Battery      AAA NiMH Cells
In addition to the LED, there were only two other components on the light’s circuit board. I saw a 4 pin custom IC and an inductor.  This tells me that the inductor is used to boost the 1.2v from the battery to the higher voltage needed by the white LED.  I was surprised that there were no capacitors or any other components, which would make a more robust circuit design.
During lamp operation, I measured a current of 12ma from the 1.2v battery.  This meant that the system consumed about 15mw of power.  Such a power level was pretty small.  If we were to assume an 80% converter efficiency, the 15mw of power from the battery would mean 12mw was routed to the LED.  If the typical forward voltage for a white LED was about 3.2v, that would put the LED drive current at only 3.7ma.  No wonder this thing was dim.  Much more light would have been produced if the LED received about 20ma of current.
If a typical sunny day has about 6 hours of sunlight and the battery received 25ma of charging current from the solar panel, then the most that the solar panel could do in a day is 150ma-hours.  Winter nights can often be 16 hours long, so if the circuit were to draw 12ma for 16 hours, that would total up to 192 ma-hours. That current drain exceeds the milliamp hours pumped into the battery. Only if the nighttime were 12 hours long, would the total milliamp-hours of battery charging current exceed the milliamp-hours of outgoing battery power.  In summary, although this light worked, it is dim and most likely would not make it through the night before running out of juice. Have you ever looked at path lights you own early in the morning, before sunrise?  Are they still working then?  I would bet that many do not.
I found it puzzling why the solar panel selected for this design generated 3v, when during charging, the NiCd battery only needs about 1.36v.  It seems that fully half of the available power from the solar panel was being thrown away.  A three cell panel, with an open voltage of about 1.5v would have made more sense.
Also, I did not see any CdS photocell as a daylight sensor.  This design must use the solar panel itself as a light sensor.  That seems like a good idea; perhaps the only good thing about this overall product.
 

Better Design?

If it were me, I would have taken a different, albeit more expensive, route.  I would have started with a three AAA cell NiMH battery pack with an 800ma hour rating.  That would put the available voltage at 3.6v and the battery pack should be able to store about 2.8 watt-hours.  I would use a 10 cell solar panel, which would crank out about 4 volts, which is just right for the three NiMH cells.  I would make sure that the 10 solar cells nearly filled the available 2.5 square inches of space on the panel. They should pump about 50ma of current into the battery.  Over a period of 6 hours, the 300 milliamp-hours of energy pumped into the battery.  That should exceed what was taken out at night.  There would then be some left over in the battery to take care of cloudy days. I would use the battery voltage to drive the 3.2v white LED with a constant current of about 20ma.  Like the path light my wife bought, I would turn on the LED when the voltage across the solar cell reached a certain voltage.  The result would be a much brighter and more practical path light, which would run through the night.


March 2011     Issue 15

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Envelope
Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
New Products What the World
needs Now
Wily Widget


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