LED Christmas lights

[David Harmon]

35 blue/white LEDS 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.

 

[Bob Pownall]

35 blue/white LEDS 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.

Bob Pownall

 

[ehsjr]

35 blue/white LEDS 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.

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

 

[Bill Bowden]

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.

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
160.

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

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

-Bill

 

[jasen]

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.

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.

 

[neon]

Forget about LEDS voltage except that it must be there to provide the current thrugh. you may sring as many as you like in series from 120v ac provided that the current is limited to the LEDS specs.

 

[ehsjr]

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
160.

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.

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

-Bill

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

 

[Bill Bowden]

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
51 mA. But that's for full wave operation. If the LEDs are in a
antiparallel setup, the peak current will be twice as high, or 102mA
for the same 2 watt power level.

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

 

[ehsjr]

At 160 volts peak, 25 series LEDs will drop 6.4 volts each.

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