OK, so following Dave's sage advice as soon as I got home from Ikea today with a new Solviden solar light (on sale for £3!) I immediately proceeded to tear it down without turning it on
I wanted to see how it charged the included AA 1.2v Ni-MH battery and turned on the 3 LEDs only when it was dark. What I found left me intrigued, which I guess comes down to my inexperience.
There's only one tiny PCB with the control circuit, two solar panels connected in series, one switch, the AA Ni-MH 1.2v battery (1000mAh) and a separate board containing a cluster of 3 LEDs connected in parallel.
I measured the voltage going to LED cluster with the mutimeter and showed something around 1.2v. Oddly enough it didn't matter if the light was shining or not, the voltage to the LEDs was always around 1.2v.. however the LEDs seemed to "know"
. A current measurement confirmed this, when there was light shining on the solar panels the current to the LEDs would drop to 0, otherwise there was about 5mA.
How can we have a stable voltage but 0 current? Seemed sorcery to me.
So I connected my oscilloscope to the LEDs board and something very odd showed up. When it's dark and the LED goes on, I see a stable 1.08-1.4v line, OK makes sense... but when there's light, the solar panels are charging the battery and the LEDs turn off their input becomes these very odd waveforms at about 110kHz.
So I guess this why my multimeter is still showing voltage, but the LEDs don't light up?
I've traced the schematic of the PCB, hopefully it's correct to the best of my knowledge. There's one IC on the board which I don't know what it is, it has 4 legs and no markings other than "0116".
Can anyone explain what's going on and why they decided to do it this way? It's way too smart for me but I'm keen to understand :-)
Thanks!