A couple of days ago, I was in a store here in the UK (in the garden department looking at seeds and bulbs) when some 'solar lights' caught my eye. The very cheap
ones didn't look up to much, but the £2 ones had a decent solar cell on them so I bought one. I was curious as to how something with a CE sticker saying it had
a 1.2V battery inside was able to light a white LED that would need a forward voltage of around 3V to illuminate.
Here are some photographs as I disassembled it.
There's not a lot to it. The battery is a 1.2V Ni-MH cell. What at first glance might appear a transistor is actually an integrated circuit (three legs good,
four legs better!) and there's a single passive component that you might, at first glance, mistake for a resistor.
The cell voltage was very low and it would only power the LED for a few seconds, so I put it on a window sill in the sun for a day. By the evening, the cell was
charged and the terminal voltage (off load) was up to 1.32V.
As you might expect, to get the 1.3V up to the 3V needed to power the LED requires a switching circuit and the device that looks like a resistor is actually a choke (coil).
Here's a schematic
Here are some waveforms. The yellow trace is the connection between the coil and the LED. The blue trace is the current through the LED. A switch to ground in
the IC waits for the coil to ramp up to about 18mA, then lets it go, allowing it to run into the LED. The average LED current looks to be about 4mA overall.
If I calculate (roughly) the coil value from the waveforms
L = V x dT / dI = 1.32V x 2.4uS / 18mA = 176uH.
That agrees with the coil markings: brown-grey-brown would be 180.
I haven't looked at the solar cell and the charging side of things as of yet.
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