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anicholas
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Join Date: Jun 2007
Location: Wake Forest, NC USA (in Central NC, God's country)
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Good Answers: 2

Transformer for 12 V LED Rope Lights

02/19/2008 8:05 AM

Suppose I have a 50' string of LED rope lights (lights at 1" intervals) for a low level lighting application (I think 120 V would be too bright). How do I choose a transformer or other voltage converter to provide the necessary 12 V? Wattage is unknown, but is this necessary to know for LED's?

Thanks for your help.

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Anonymous Poster
#1

Re: Transformer for 12 V LED Rope Lights

02/19/2008 9:19 AM

LEDs are really current controlled. The forward drop of an LED will vary some from LED to LED and with Temperature. You also have to look at series and parallel configurations. If you assume 1.2V per LED in series at 1" intervals on a 50' strand you will have 600LEDs and you will need 720VDC to drive them.

Are you building the LED strand or is it already provided?

If you are building it then you can control the series and parallel configurations, if it is already built you need to deal with what you have.

You also need to control the current because very slight changes in voltage and temperature can equate to very large changes in current. You will need a few extra volts to insert a current limiting circuit.
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HUX
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Join Date: Jan 2007
Location: UK S.Northants
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#2

Re: Transformer for 12 V LED Rope Lights

02/19/2008 9:29 AM

50' is 600" = 600 leds. Since you said "the necessary 12V" I will assume they are all in parallel or series / parallel and have appropriate series resistors to limit the current so they don't go bang. All 600 in parallel at 20 mA per led gives a total of 12A, if they were in strings of three (3.6 V * 3 = 10.4 V and a series resistor would do it) then that would be 4A. The second is the most likely option so in this case you want a transformer with 50VA secondary rating.

I am also assuming that they are supplied by a full wave rectifier, but you could split the string in two and put a single diode in series with each leg and have the diodes light alternately, needing only 25VA.

It is important to have a real diode as well as the LEDs in the circuit as the peak inverse voltage is likely to blow them if you don't have one.

Is there no info with these or are you making your own?

parallel processing! I posted mine to find there was one just done; this is an edit.
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bhankiii
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Engineering Fields - Electrical Engineering - New Member

Join Date: Sep 2006
Location: El Lago, Texas, USA
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#3

Re: Transformer for 12 V LED Rope Lights

02/19/2008 10:29 AM

I have used outdoor lighting transformers - like you can get at the hardware store. They put out 12VAC, at 100, 200 or 500W, depending on which one you get. Use a full bridge rectifier and some hefty capacitors to get your DC.
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anicholas
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Join Date: Jun 2007
Location: Wake Forest, NC USA (in Central NC, God's country)
Posts: 64
Good Answers: 2
#4

Re: Transformer for 12 V LED Rope Lights

02/19/2008 11:39 AM

OK, here is the supplier information (Meng Meng, USA, New York):

LED 2-WIRE 50'12V 3/8" WHITE ROPE LIGHT

No Reserve

  • Highly Durable Solid PVC Tube and Sub-Miniature LED Bulbs
  • 3/8" Diameter,1" Spacing,540 LED Bulbs, 30 Watts Only
  • Much Brighter, Much Less Energy Consumed
  • Flexible But Sturdy to Bend to Shapes
  • Cuttable Every Three(3) Feet
  • Easily Installed
  • Used Indoor or Outdoor
  • For Decorations and Illuminations
  • Anywhere Soft Incandescent Lighting Used and Flexible
  • Corners,Celling,Staircases,Decks,Driveways, Boat,Galley,Clubs,Party




Kits Included:

  • Brand new LED 50' rope light
  • 1 Power connectors
  • 1 Safety caps
  • Questions to:mengmengusainc@gmail.com


This string has a rectangular plastic box in-line near the beginning; supplier states that this is normally sold to be run from a 12 V battery source. Maybe this will help define things better.

Thanks.

anicholas

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elnav
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#7
In reply to #4

Re: Transformer for 12 V LED Rope Lights

02/20/2008 12:38 AM

The company has already given you al lthe information you need. One clue is the "cutable every three feet." Each 3 foot segment has the diodes wires in seris to safey connect across a 12V source. They also tell you the 50 foot string consumes 30 watts. The No reserve means you are not limited. Filament bulb rope lights have a reserve or limitation of so many feet to keep the total current below the ampacity of the two parallel wires.
In North America this is usually 15A because the two parallel wires are 14Ga AWG.

The 50' @12V total 30Watts means the total string will consume 30 watts. So if you plan to power it as a continuous light rope, you need to provide a 30 watt @ 12V supply. In round numbers a 3 amp supply wil work. A 2 amp supply will be over loaded and pop a fuse. The rope light should have small scissor symbols marked at every 3 foot intervals. You MUST cut exactly on these scissor marks, if yo uintend to cut shorter lenghts. Cut anywhere else and you destroy the series string in that segment.

The various segments are in effect parallel wired in relation to each other between the two big conductors you can see with the naked eye. Close examination will reveal a tap off lead going to the first LED in the series string. The lead from the other side of that first LED goes to the lead of the next LED not back to the main power leads.

If the two conductors are polarity sensitive, they are usually tinned and bare copper to distinguish one from the other. More sophisticated light strings wil have a full wave rectifier bridge inserted into each segment. No matter which way you connect power or even with 12V AC the string will ight up. Be careful when connecting up your first string.

The rectangular box at the end of the 50' string suggest it was intended for use as one continuous piece and this box may contain the connector and bridge rectifier for safety. I would guess it is molded to make it splash and possible water proof for outdoor use. The mention of cut points and the inclusion of an end ( safety) cap also suggest they expect you may have to cut it to fit in a space less than 50 feet if necessary.

Wonderful product. I have been using rope lights for over 10 years now. It comes in various voltage ratings. Each voltage has a unique cut lenght since each series string has a different number of lighs to drop the voltage across each light or LED.

hope this helps.

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anicholas
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Join Date: Jun 2007
Location: Wake Forest, NC USA (in Central NC, God's country)
Posts: 64
Good Answers: 2
#9
In reply to #7

Re: Transformer for 12 V LED Rope Lights

02/20/2008 6:22 AM

Thanks so much for the great answer! I think that it says it all.

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Anonymous Poster
#11
In reply to #4

Re: Transformer for 12 V LED Rope Lights

06/02/2008 7:32 PM

I just bought one of these from Meng-Meng and I think he is mistaken about the cuttability. Mine has scissor marks every 12 LED's, which is close to a foot. I'm going to do some experimenting . . . .
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Instru
Member

Join Date: Dec 2007
Posts: 5
#5

Re: Transformer for 12 V LED Rope Lights

02/19/2008 11:09 PM

Hi ,

Could anyone suggest a company who manufactures similar stuff which can be used in Hazardous areas. I am on the lookout for Low Level (escape route identifier ) lighting for a semi submercible rig for Brazilian waters.

Thanx in advance for your help.
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Anonymous Poster
#6
In reply to #5

Re: Transformer for 12 V LED Rope Lights

02/19/2008 11:29 PM

Try the Aero industry. I presume the lights on the floor of aircraft obey some safety standard
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elnav
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Location: Western Canada
Posts: 1863
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#10
In reply to #5

Re: Transformer for 12 V LED Rope Lights

02/20/2008 10:51 AM

Electroluminescent displays are used in the marine industry as back lighting in a lot of instrument applications. For hazardous locations you need to have a product rated as intrinsically safe by the Standards body having juristiction over your particular application. The offshore rigs used off the Brazilian coast could be certified by ABS, Lloyds, DNV, or Germanischer Lloyd. Those are the guys you have to satisfy concerning the suitability. I run into the same problem al lthe time. A product I want to use is not always certified by the applicable Standards Group.

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omar_beyaty
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Join Date: Feb 2008
Posts: 4
#8

Re: Transformer for 12 V LED Rope Lights

02/20/2008 3:22 AM

i think you can use a simple cct contains a filter bank of acapacitor parallel with resistor and whole bank in series between the source 120vac or 220 vac..etc , and between the leds clusters .(the output of filter bank will be in 20s vac) you should add a half wave rec (two diods) with a series resistor to diode(s) . the capacitor in filter bank is 0.1 micro at 400vac and the res. at 1mega or vary its value descendingly as you wish to obtain the sufficient current (by simple calculation) to leds bulk , thats generally, in more specific , you must divide the 600 led to 4 or 6 groups seperatedly , and put a (one filter bank + one half wave rectifier + one resistor for current limiting to diods) to each leg , the added element is in small size if prepared correctly together ,you see , we eliminate the use of transformer which take a big place and take more care to calculate its specification to load demands. hope this help you, thnx
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anicholas (2); Anonymous Poster (3); bhankiii (1); elnav (2); HUX (1); Instru (1); omar_beyaty (1)

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