LED Christmas lights

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

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.

Bob Pownall

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.

Ed

Yes, but the peak line voltage is 160, not 122. So, the LEDs do nothing until the voltage rises above 122, and then burn out when it rises to

What happens when the line voltage reaches a peak of 160 volts?

-Bill

That's work perfectly 120 times per second

the problem is it's AC and the rest of the time the volttage will be too high or too low.

Nope. The 35 bulb strings work, and continue working, however they are configured. I have not taken one apart so I don't _know_ how they are configured, which was why I said "it tempts one to conclude" etc. But the same principle regarding burning out applies as it does to the 100 LED string I dissassembled. See below.

The bulbs keep on working. Thousands of these things are sold, and work, and keep on working. The 100 bulb set is rated at 8 watts, meaning ~ 16 mA per LED substring of 25. So at peak, the LEDs should be running at a bit under 24 mA - no problem.

Ed

At 160 volts peak, 25 series LEDs will drop 6.4 volts each. Also consider the LEDs are totally off when the voltage is less than 122 which amounts to about half the cycle time.

So, the RMS current is probably 34 mA, and the peak current is around

But that may be doable. I've run red LEDs at 200 mA without burning them out. They start to get dim and hot as the current gets too high.

-Bill

Take another look at my post. The amber LEDs have a built in series resistor. Voltage drop across the LEDs is not uniform, in any event. I tested each color at ~ 20 mA DC and, as I recall, got the following drops: red & yellow ~2, blue and green ~ 3.5, and amber w/resistor ~ 14. I still have the LEDs, so if you want exact figures let me know & I'll test again.

Ed