On 25/01/2013 09:14, Ian Norton wrote:
> And what are we looking at?!
> For those of us who weren't involved with the original conversation between
> the two of you....
A Joule Thief. Basically it's a primitive flyback boost converter that
allows you to run a LED off a 1.5V battery that otherwise would be
classed as dead - they can work down to 0.5V or below depending on how
well made they are made (i.e. not this one ;-). The scope trace shows an
input voltage of 1.1V but if you put the battery in a torch it doesn't
even glow. The output is a 850Khz 3.4V waveform with a 30% duty cycle
that's enough to light a LED at a reasonable brightness.
Alex, I found some info on the intertubes that suggested sticking a 10nf
cap from between the resistor/coil junction and ground. The one I used
is a honking big mains-rated one but adding it pushes the frequency up
from 550Khz to 850khz, increases the duty cycle and the peak output
voltage so the LED is noticeably brighter. The circuit works OK without
it but it does make an improvement. There's that and a whole load of
other plumbing detail ;-) at