LED Lights and Resistor Help

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  #1  
Old 11-26-15, 06:15 AM
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LED Lights and Resistor Help

I built a snowflake out of wood and wanted to run LED Christmas lights through it. So I drilled 102 holes and bought 2 strand of Christmas lights, one was a 100 count strand and the other was a 50 count strand.

I snipped off 102 lights and crimped both wires coming out of them. I want to wire them all together with a plug on the end. I know how to properly splice and wire them all together but the issue I am having is what resistor to use to keep the voltage down so they don't burn out.

Now, the 100 count strand of Christmas led lights says it is 7.92 watts and 0.066 Amps. I figured with the 2 additional lights (102 total) I am at 8.0784 Watts and 0.06732 Amps. At 120 Volts, the OHM calculator says it is 1782.53119 OHMS.

This is where I get confused, lol.

Now, there was a resistor on the 100 strand lights, so I was wondering with just 2 additional lights (102 total instead of 100), cant I just use that resistor? Or will they burn out? Also, how do I wire in the resistor? The resistor on the Christmas lights is sealed but has a wire coming out of both ends. Do I need to hook that up to the power or negative wire near the plug? Or somewhere in the middle, at the end?

I would greatly appreciate any suggestions you can provide.

Thanks!!
 
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Old 11-26-15, 07:11 AM
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Rather than reinventing the wheel, why didn't you just drill 100 holes? If they are LED, then the additional two lights won't matter much. Why not just plug them in to a receptacle and see how they work? There is no "negative" in alternating current, which I am assuming you are using. Don't get caught up in Ohms, watts, and amps. Use Ohm's law pie chart if you need a warm and fuzzy. Name:  Ohm's_law_formula_wheel.JPG
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  #3  
Old 11-26-15, 07:24 AM
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Why not just use the 100 lamp strand and skip two holes. Who besides you will ever know? If it bothers you do, another snowflake with 100 holes. Good luck.
 
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Old 11-26-15, 07:50 AM
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And, two extra bulbs is only a 2% difference. Not much on something that can probably operate at plus or minus 10 or 20 percent.
 
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Old 11-26-15, 07:51 AM
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lol, I understand what you are saying, but 102 holes was just what it came out to. I chose not to use a strand of lights because the spacing was way off, it would have never worked. way to many different angles and turns and such to use a strand of lights. so my question is - how can i get these to work without burning them up? heres a picture....Name:  lights.jpg
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Old 11-26-15, 08:41 AM
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There are several issues at play here. LED's are polarity sensitive.

Were all those LED's in one continuous series string ?
What does that "resistor" look like..... and did you try measuring it ?

Most light strings are wires in multiple series strings.
For example with regular bulbs... a 50 light set uses approx 2v bulbs.
A 100 light string uses the same bulbs but in a second loop.

I haven't played with any of those sets yet but LED's run on approx 2v so I'm thinking you have(had) a dual series string. 100 LED's in a series string would need 200v to operate.
 
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Old 11-26-15, 09:11 AM
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yes, these lights were all from one strand of Christmas lights, 100 count, led and the box says they are rated at 7.92 watts.

I took the 100 lights and snipped them all off. The resistor is encased in a green tube shaped thing, what you would normally see on a Christmas light strand.

The Christmas light strand had 3 wires. So you're saying I need 200Volts? I don't understand when they were on a strand that normally up to 30 strands can be hooked together on a 120V outlet. Isnt a wall outlet normally 120V?

Im just looking for some information on how to wire these together with a plug.
 
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Old 11-26-15, 10:06 AM
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And yes, i understand there is no negative/positive, i understand the two polarity types. Just need to know what to do to make this all light up properly
 
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Old 11-26-15, 10:18 AM
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The resistor you refer to, is it a resistor or the fuse for the strand? Often there is a small housed fuse that is changeable should it blow out. I think with LED's your 120 volt set up will not recognize the additional 2 bulbs. I have a problem with having 102 junction boxes, one at each bulb, which would be required by code to make your connections enclosed, but accessible. As Pete says, you will need to determine the polarity of the LEDs and make sure you connect all of them the same way.
 
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Old 11-26-15, 10:32 AM
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when i hook up an LED to a 9v it burns out, but when i hook it up with a resistor it works. see picName:  image1.jpg
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Old 11-26-15, 10:47 AM
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Actually.... an LED is considered to be a device that is not polarity sensitive when it comes to AC use but it does have a + and - side. If the wiring at the socked isn't ID'ed you may need to use a AA battery to test each light to mark the wiring.

You will be required to connect the LED's like my diagram shows. + from the first one to the - of the next and so on.

In the diagram below. Imagine A is 50 bulbs and B is another 50 bulbs. You'll see each set of 50 bulbs is treated as it's own self contained circuit or loop. A set of 50 would be just A. A hundred light set would be A + B. A 150 light set would be A + B + C.

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An LED runs on approx. 2 volts..... so yes.... a 9v battery will burn it out. Are there two of those resistors in the light set ?
Do you have an ohmmeter to measure the resistance of that device ?
 
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Old 11-26-15, 02:17 PM
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Yes the Christmas light strand had 2 resistors. I understand hooking up + to the next - and so on. So you're saying I need to do 2 sets of 51 (102 total)? I understand the concept of what you're saying but I'm a visual guy, lol, any chance anyone would be willing to draw up a quick diagram on how I need to wire this? I do have the 2 resistors that came on the strand of 100 lights. If someone draws up an easy to understand diagram for me I will Paypal you $50. Lol. So just to confirm - I do know how to determine the two polarity's (anode and cathode) so i need to hook these up with anode to cathode to anode to cathode and so on? Where would the two resistor go? And how do I split up two sections of lights into one plug? Again, a custom diagram would be soooo helpful and I'll pay you!!
 
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Old 11-26-15, 03:49 PM
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I drew you a diagram. Do you need to see something with all 102 lights on it.

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pretend there are 102 LED's there.
 
  #14  
Old 11-26-15, 03:53 PM
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PJ, i'm not an electrician so alot of that is greek to me, lol. Are those black bars the resistors?
 
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Old 11-26-15, 04:05 PM
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The black dots are the LED's. The green bar is the resistor. You are putting 51 LED's and the one resistor in one loop times two loops.

The polarity of each LED is important but once they are in one loop... the ends have no polarity.
 
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Old 11-26-15, 04:34 PM
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PJ finished his diagram before I did. They both show the same just drawn different.

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  #17  
Old 11-26-15, 04:36 PM
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Very nice Ray.... very festive.
 
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Old 11-28-15, 08:46 PM
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It's a little more complicated than that. The following could enter into the problem:

- It is better to use intact strings and hide the unused LEDs in a light-tight box (or put them in a different display). I have a Christmas star in my window that uses 36 lights in one group of lights and 22 in the other group (independently controlled). I put the remaining lights of two 50-lights strings in a box in the back of the display.

- In strands with more than one series, half of the series strings light on the positive swing of the AC and the other half light on the negative swing.

- Some of this "resistors" are full wave rectifiers, voltage doublers, constant current devices, or resistor diode combinations.

- Different colors of LED drop different amounts of voltage. Red and orange LEDs usually drop 1.3 V, yellow and green ones usually drop 2 volts, and 4 volts for white and blue LEDs.

- Something is needed to prevent conduction in the reverse direction. Most LEDs can tolerate only about 5 volts in the wrong direction before they blow out.

- Most LED circuits like this use 20 mA of current.

- To calculate the resistor for a series of LEDs, add up the forward voltages (voltage drops) of a series you actually have.. Then subtract that from 170 volts (the peak voltage of 120 V AC power). This gets the voltage needed to be absorbed by the resistor. Now divide that voltage by .02 amps (20 mA) to get the resistance, and multiply that voltage by .02 to find the power rating needed for the resistor.

- Note that, if too many LEDs are in the series, the time current flows (when the voltage exceeds the sum of the forward voltages of all of the LEDs) will be shorter, and the lamps will be dim.
 

Last edited by MidiMagic; 11-28-15 at 08:53 PM. Reason: fix typos
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