 |
| |
35 blue/white Phillips LEDS Christmas Lights in a string, operating directly
off of 120V AC. Does anybody know what that sort of thing typically uses for
current limiting? There are no visible series resistors. |
| |
 |
| |
-I don't know how it _is_ done, but one possibility would be to use two (or more) different kinds of LEDs in the string, such that the total
drop across all the LEDs is 120V. Just like a silicon diode (not LED, of course) has ~0.7V across it for
any reasonable forward current, an LED will have a constant voltage drop
across it, largely independent of the current through the LED. The drop across the LED will depend only (or at least largely) on the
material the LED is made of. I _think_ the drop across GaAsP and GaAlAs
LEDs is around 1.3V, while I _think_ the drop across GaN LEDs is around 1.7V. The LED Christmas light strings I've seen have been 60 LEDs, not 35, so
unless the makers are using a type of LED I'm not thinking of right now (or unless my recollection of the voltage drop across various types of
LEDs is wrong), the pure LED diode drops probably isn't the entire
story. It's also possible that the series resistor is built into the LED
itself, during the manufacturing process, by varying the doping
concentrations in the LED semiconductor materials in specific regions. A search of the US Patent Office records might be illuminating, no pun
intended. -Both blue and white LEDs have a Vf between 3 and 4
volts - that is, each LED will drop the voltage
by something around 3.5 volts. That tempts one
to conclude that it is a simple series string (35 * 3.5 = 122.5 volts) with a diode in series
to block reverse voltage to the string. Regarding visible series resistors - there are LEDs
available with built in resistors. I got a string of 100 LEDs and disassembled it.
There are 5 colors: red, yellow, blue, green and
amber. The ambers have a built in resistor. Each
"substring" of those colors drops ~24 volts, so 5
substrings (25 LEDs) in series drops ~120 volts.
By arranging the strings such that they are in
parallel & antiparallel, a configuration of 100
bulbs works nicely. |
| |
 |
|
| |
| |
