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Author: jc2048
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Solar Garden Light

jc2048

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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  • Hi Jon,

    Great analysis of a good mystery. I too have bought these lights but I never realized that they were small switching buck circuits. I made the assumption that the only purpose of the IC was to switch between charge and illuminate when the ambient light drops below a threshold. Tonight I will be going back to look at the similar circuits that I have accumulated. The Dollar Tree store here often has these for $1.00 a piece. It is hard to buy batteries that cheap.

    John

  • in reply to jw0752

    It's a boost converter, not a buck. Normally, a boost converter would have the diode feeding an output capacitor and topping it up each cycle with the energy from the coil (with feedback necessary to keep the voltage steady). In this case there's no need for the capacitor because the current through the diode (LED) is what we are interested in and not generating a fixed voltage. It doesn't matter at all that it falls to zero because our eyes happily integrate the light energy to give the average (at least they do when it occurs 215,000 times a second). No feedback is necessary because the LED start current will be the value the coil current has reached at the end of the switch 'on' period (that value may be determined by a current sense in the IC or probably, more simply, just having a fixed period for the 'on' time - the rate at which the coil current ramps is a function of the inductance). The output voltage you can see in the scope trace is then just the LED forward voltage at whatever current is flowing.

     

    I'd be interested to hear if the ones you have operate in the same way.

  • in reply to jc2048

    Of Course you are correct and I misspoke. It has to be a boost not a buck. Time willing I will check out and show some picture of the ones I have tonight.

    John

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