Circuits designed by David Johnson, P.E.
Last Updated on:
Monday, December 25, 2017 02:08 PM
List of Dave's Circuit Designs
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Modified Cheap Solar LED Path Light (May
Many of the small solar LED path lights on the
market today use a single 1.2v NiMH
cell to power a white LED. The telltale giveaway on these lights is the
4 cell solar panel on top of the light to charge the battery. Inside these lights, a simple “flyback”
voltage converter circuit is used to boost the 1.2v from the NiMH
cell to over 3 volts, to drive one or more
white LEDs. Many of the converter circuits use a propriety IC in conjunction with an inductor. In one of
the lights I bought for a dollar, a 470uH coil was used to produce the needed higher LED voltage.
I measured the DC battery current drawn from one such light
at a low 2ma. Figuring a conversion efficiency of 80%, that put the 3v LED current at about 700ua. This is a
very low LED current. No wonder these things are not very bright.
|Duracell rechargeable 1000mAh
|2v 150ma solar panel
The battery in the dollar light was an AAA NiMH
cell, with a
rating of only 150ma-hour. I measured the current produced by the 4 cell solar panel on a bright sunny day at a
disappointing 35 milliamps. If you figure that the average sun lit day lasts 6 hours, then the panel would pump
only about 210ma-hours of energy into the battery. Yes, the solar panel would charge the 150ma-hour battery but
I wanted a brighter light.
|Typical Single NiMH
Cell Solar Lights
|The first thing I did was remove the low milliamp-hour
rated AAA cell and replaced it with a standard 1000ma-hour NiMHAAA cell.
I then replaced the small 4
cell solar panel with a larger 4 cell unit, which produced 150ma in the sun. Next, I simply changed the
470uH coil on the circuit board to a 22uH coil. The lower inductance increased the battery current to
40ma when the LED light was on, which translated to about 13ma LED current. The light was much
brighter. The 150ma from the solar panel should charge the battery in about 6 hours.
The larger 1000ma
cell should keep the light going through a long night. With just a couple simple mods, the light
was now much more useful.