Resistors in LED Rope Light

[ThaiWay]

My 220VDC LED Rope Light can be cut only every two meters, but I want to make a few one meter sections.

A two meter section has six groups of 12 LED's in series, each group followed by a 300 Ohm resistor in series.  (I think the third band is brown, but it could be a dull red  )

I tried just cutting at one meter and making the proper connections, but as I expected, the LED's burned brighter, and then after about 15 seconds the resistors all fried.  

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I assume that adding an appropriate resistor in series would get me back to normal brightness and no smoke, but I have no clue.  I get the gist pretty easily and follow directions well, but  I'm a dummie when it comes to the theory.   I know I'll get the help I need here.

Thanks, John

[Hauvega]

What color is it?  Determines the led forward voltage.  The current is usually pretty constant about 20ma.

Eric

[ThaiWay]

It's white (clear) rope, in this case. 

So, now, each new one meter section has 36 LED's.  But one has three resistors, the other two.  I presume that means I would need a different value resistor for each section.

[Hauvega]

Since the voltage is dc then the formula is        V/I = R

  (Voltage(mains) -   (Sum of Forward Voltage of leds in series))

take that result and divide by the current = Resistor  value.

Then you square the current and multiply by the resistance to obtain the power dissipated.
and go to the next higher size wattage ie 1/4, 1/2 etc. 

The number of leds in series is biggest part of the calculation.

Eric

[ThaiWay]

Thanks Eric, but that's mostly greek to me!

[Hauvega]

If I had some of that type of rope light, then I would have brought out the Xacto knife to take it apart to find out the value of the resistor. 


Eric

[ThaiWay]

...A two meter section has six groups of 12 LED's in series, each group followed by a 300 Ohm resistor in series....

You mean the 300 Ohm one?  (now verified with an ohmmeter

[dmaccole]

Thanks Eric, but that's mostly greek to me!

Or, you could visit

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and plug in the numbers. White LEDs typically have 3.2v forward voltage (YMMV).

\dmc

[Hauvega]

I'm thinking since 300 ohm running 72 white leds that is about 3 volt per led.  I calculated the resistor value to be about 5600 ohm 3 watt for 36 leds (using assumed info). 

Eric

[ThaiWay]

Or, you could visit

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Thanks dmc... but:

"...The wizard thinks your source voltage is too high to be practical."  (220V)

Eric... really appreciate your help!  But doesn't the 300 ohm R just run 12 LED's?  (since there's a 300 between each group of 12 in the 72 LED/2m section.)  Sorry but I'm still a bit confused.

John

[n1ist]

Is each resistor+group of 12 LEDs connected across the line, so they are in parallel, or is some number or resistor+12 LED groups connected in series?

By itself, 12 white LEDs in series will drop about 43 volts.  If the groups are in parallel, the resistor would have to drop 220-43=177 volts.  For a 300R resistor, the current would be 177/300 = 0.6A which is not possible (those little LEDs usually want 10-20mA).  Also, the resistor would have to dissipate over 100 watts...

So you have some sort of series/parallel combo.
/mike

[Hauvega]

How many wires at the 1m cut end?  Does the resistor look like a 1/4w or 1/2w?  Is this light rope advertised on the web?  I know I would have already disassemble it already.

Eric

[ThaiWay]

Thanks Mike, Eric.  To answer your questions...

A 2-meter section of the rope has two continuous wires from end to end... "+" and "-"

Between these wires runs a series of 6 groups of 12 LED's separated by a 300 Ohm Resistor (all 72 LED's + 5 resistors are in series.)  At one end, the series is soldered to the "+" and at the other end the series is soldered to the "-"  220VDC can be applied to the two continuous wires at either end.

So, if I cut at one meter, I have the + and - wires, and one wire for the series (which in my original test I soldered to the appropriate wire to complete the circuit.)  I end up with a one-meter length of 36 LED's and 3 resistors, and another length of 36 LED's and 2 resistors. 

The resistors look like 1/4 Watt like we use in LE's.  I've never seen it on the web, and the mfg Lanhe Light in Guangdong has no website that I can find.

Thanks guys... if this isn't clear, I'll draw something up.

John

[Randy_A]

Don't know if this is the web site for them or not...

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,Randy

[Hauvega]

Is this rectified ac?  It changes the game rules.  See this forum thread

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The total voltage for 72 leds at 3.5 each is 252 volts.  Total resistance is about 1500 ohms.  The current is about 20ma, then the voltage across the total resistance is about 30v.  The total voltage is about 282v. 

Pure dc is no problem.  Rectified ac into dc changes things a bit.

Running new numbers for 1m with 3 resistors needs a 6900 ohm resistor in series and the 1m with 2 resistors needs about 7200 ohms.

Trying to compensate for the missing part of the string.

Eric