# Help with LED rope lights.

#### reno12469

Joined Jan 17, 2009
47
Does any one know how LED rope lights work? Ive been looking every where and i cant find it. And does anyone have a schematic?

#### mik3

Joined Feb 4, 2008
4,843
Does any one know how LED rope lights work? Ive been looking every where and i cant find it. And does anyone have a schematic?
What do you mean by LED rope light?

#### reno12469

Joined Jan 17, 2009
47
What do you mean by LED rope light?
Like this. I know that they are either wired in parallel or in series but websites say never to use parallel.

#### mik3

Joined Feb 4, 2008
4,843
I think these string LEDs are wired in parallel inside the tube in case on goes off the other will stay on. By parallel, you mean that you want to connect more strings at the end of the first string?

#### reno12469

Joined Jan 17, 2009
47
This is what i mean

And make it into a long strip

#### SgtWookie

Joined Jul 17, 2007
22,210
The LEDs would most likely be wired in strings of three, with a current limiter; either an active semiconductor device, or more likely a simple resistor.
60/3 = 20 strings
400mA / 20 = 20mA, which is a typical LED current.
Blue LEDs typically have a Vf of between 3.2v to 3.8v.
3.4v x 3 = 10.2v total.
So, the resistor would have to drop 1.8v across itself allowing 20mA to pass.
1.8v/20mA = 90 Ohms. 91 Ohms would be the closest standard value.

See the attached schematic.

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#### mik3

Joined Feb 4, 2008
4,843
The idea of using one resistor for all the leds is not a good idea. If One leds goes off ten the other will receive more current and will burn too. Or the voltage of the leds, even if they have the same specifications, can vary during their operation slightly and causing the current in other leds to increase and burn them. You can parallel many leds as you want if you use one resistor for each led and if your supply can provide the required current.

#### reno12469

Joined Jan 17, 2009
47
The LEDs would most likely be wired in strings of three, with a current limiter; either an active semiconductor device, or more likely a simple resistor.
60/3 = 20 strings
400mA / 20 = 20mA, which is a typical LED current.
Blue LEDs typically have a Vf of between 3.2v to 3.8v.
3.4v x 3 = 10.2v total.
So, the resistor would have to drop 1.8v across itself allowing 20mA to pass.
1.8v/20mA = 90 Ohms.
so your saying every 3 leds put a resistor?

#### SgtWookie

Joined Jul 17, 2007
22,210
so your saying every 3 leds put a resistor?
See the schematic attached to my prior post.

As Mik3 suggested, this schematic:

#### reno12469

Joined Jan 17, 2009
47
Thank you.

#### mik3

Joined Feb 4, 2008
4,843
Sgt's idea works well. However, if one led of the three in a branch goes off, then the whole branch won't light. If you use one resistor for each led this problem will be eliminated but you increase the components and cost. Its your choice.

#### italo

Joined Nov 20, 2005
205
Wrong assumption if one fail_ how short? then there is no more light. If it fails open then there is 1/3 more power available. He shows a source of 3v so for LEDS in parallel that is proper design. What i don't understand how can you get a ROPE of only 3 LEDS.

#### reno12469

Joined Jan 17, 2009
47
I think ill go with a resistor every LED. How many do you think i can put on with a 12v dc car battery?

#### mik3

Joined Feb 4, 2008
4,843
I think ill go with a resistor every LED. How many do you think i can put on with a 12v dc car battery?
If each led needs 20mA to work then you can put up to 1000 leds! However, if you use Sgt's idea you can put up to 3000 leds with the same power consumption. Its a compromise between with what I mentioned before.

#### reno12469

Joined Jan 17, 2009
47
So with Sgt's design i would have to run a positve and a negative wire all the way down the strip?

#### mik3

Joined Feb 4, 2008
4,843
So with Sgt's design i would have to run a positive and a negative wire all the way down the strip?
The answer is yes, you have to run a positive and negative wire along the whole string.

#### reno12469

Joined Jan 17, 2009
47
Thank you.

#### mik3

Joined Feb 4, 2008
4,843

#### Audioguru

Joined Dec 20, 2007
11,251
Manufacturers of LED lights measure and sort the LEDs into groups that have identical forward voltages so that they can be safely connected in parallel.

But this rope light probably has 3 blue LEDs in series and other strings of 3 LEDs in series with all the strings in parallel and they are powered from a single resistor.

#### bertus

Joined Apr 5, 2008
20,326
Hello,

Take a look at this PDF on grouping leds.

Greetings,
Bertus

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