On Dec 27, 10:10*pm, "Michael A. Terrell"
wrote:
" wrote:

On Dec 27, 12:54 am, "Michael A. Terrell"
wrote:
" wrote:

I recently was given two strings of LED blue/white Christmas lights..
The bulbes are in series, with 35 lights in each string. *There are
also a couple of lumps in the series lines. *They appear to be just
resistors, not diodes, as I measure the same value (~500 ohms) with
either polarity of my multimeter. * My guess is that the resistors are
there just to decrease the current so that the LEDs are not running
overrrated. *I am thinking of putting/rewiring the two strings
directly in series and then reducing the series resistors until I get
the same overall brightness of the new 70-light string. *Has anyone
done any experimenting like this?

* *Leave it alone. The forward voltage drop on white or blue LEDs is
higher than red or green, so that's why they chose *35 LEDs per string.
If you get too close to the actual line voltage and use lower
resistance, any spike will cause a high current surge through the
string. *If you aren't capable and willing to design and build a
constant current boost supply, you are wasting your time.

Good point about surge limiting, altho I am not sure if LEDs are any
more susceptible than tungsten filament lamps.

* *Tungsten has a time constant, and the resistance goes up as the
voltage increases. *LEDs are current operated at a lot more constant
voltage. *A few volt rise in the power line rasies the current flow.
Harmonics and spikes on the power line will cause higher peak current
flow in the LEDs. It would be like operating a Zener very close to it's
knee voltage with only a very low resistance to limit the current.


The variation in atom size in III-V semiconductors creates more
defects than a homogeneous semiconductor like silicon would have,
because of the strained lattice. High currents will propagate those
defects, creating areas where the steady-state current will bypass the
p-n junctions (so-called dark current, because it doesn't produce
light). So the useful life of the LED string would be unnecessarily
reduced.