Create a 50 Lite LED string In Parallel

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jack0987

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As in previous threads, my outdoor LED light strings mostly fail during the winter season. Only about 1/3 remain lit after about 4 months. Last year, I filled each socket with silicone paste to no avail.

I chose the socketed ones so that I can make them multi-colored.

I see no other option but to cut them apart and rewire each LED in parallel and use a low voltage DC power supply. Strings of 50 lites would be ideal for me.

Since adding a resistor for each LED would be rather tedious, the hope is to power entire the string at the right voltage. This is where I need help.

5 volt USB power supplies are fairly cheap and easy to come by but are to high for the LEDs. Maybe I could use them with one 2 or 3 watt resistor.

Please offer your thoughts.

 
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You drive LED's with current, not voltage - so a 5V supply and suitable resistor would work, but the resistor is going to get HOT.

50 LED's at 20mA each?, gives 1A - guess 'roughly' at 2.5V voltage drop across the LED's, so you require a 2.5 ohm resistor - and it will dissipate 2.5W. I would suggest 10W or greater for the resistor, and it will probably still get seriously warm.
 
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How about a power supply with a lower voltage output?

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Find out exactly what the LED's need, and then pick a supply voltage high enough to give a 'reasonable' drop across a resistor - too small a drop and you don't have enough range for changes, such as caused by temperature or voltage variations - essentially the higher the resistor value, the closer to 'constant' the current flow becomes (but the greater the dissipation in the resistor).

Historically a 'constant' current source was simply a high value resistor fed from an unregulated high voltage source.

You're really better off with a proper constant current source, rather than just a resistor - something like this perhaps:

 
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You mentioned "multi-colored". But different color LEDs have very different forward voltages. Maybe 1.8V for red and maybe 3.3V or more or blue. Then in parallel the red LEDs will hog all the current and burn out but the blue LEDs will not light up until all the red LEDs have burned out. Then the blue LEDs get all the current and also burn out.

Even the same color LEDs in parallel cause burn-out problems because each LED has a different forward voltage unless they are all tested, measured then grouped.
 
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Designing strings of LEDs is more complicated that meets the eye. Unless you have your heart set on mastering this art it might be better to just buy the sets that are now cheaply available and save your talents for other things. Just a thought.

When I have made large arrays of LED (50 to 250) I first figured out what I can use as a power supply. Taking that into account (mainly voltage) I made several strings of series connected LEDs, with each string having a resitor. I left enough voltage across the resistor to make sure that variations in individual LED voltage drops had a small effect on the voltage across the resistor.



 
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Wow! Interesting. I use a specific color sequence.

Blue-Blue-White-Blue-Blue-Purple-White

Maybe the only way is to have a resistor for each one.
Maybe I could wire the string as three segments. One segment for each color.
 
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LOL. Thanks so much. I am glad I asked for help.
I really do want them in parallel, though.
 
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jack0987 said:
LOL. Thanks so much. I am glad I asked for help.
I really do want them in parallel, though.
Click to expand...

It's the worst possible way, electrically - but if you're mixing different colours, then it's essential that you split them to their separate colours, and feed each colour through it's own resistor (or constant current).

As a 'bonus', this will also reduce the dissipation in the individual resistors - if you had 3W dissipation in the single resistor, you should now have about 1W in each of the three resistors.
 
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Don't take things so literally -you are supposed to learn something useful from the example. I

t all depends on what you can power it with. A single resistor can set the current for an entire string if you know how much voltage your power source supplies and the voltage drop of your LEDs. The drawback with that approach is that all the LEDs in a series string will have the same current through them. That might be good, or maybe not.

A very popular way to drive uncharacterized is with a constant current power supply, which you buy ready-made.
 
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In addition to the color of an LED that sets its forward voltage is the range of the voltage with each color because the voltage cannot be made accurately.

Then you must have enough voltage across the current-limiting resistor so that the range of an LED's forward voltage does not cause the current in some of them to be too high and burn out the LED or cause the current to be too low and make dim light.
Here is one of hundreds of voltage graphs on the internet:
 

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You can put LEDs in parallel if you buy hundreds of them then test and group them into groups with a similar forward voltage.

If you buy a few blue ones like in the voltage list I posted and one has a voltage of 2.48V and another has a forward voltage of 3.7V, then in parallel the 3.7V one will produce no light because the lower voltage LED prevents it from getting its needed voltage. Then the lower voltage LED will have the current of two or more LEDs and will quickly burn out.
 
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I am only using blue, white, and purple LEDs. From the information I can gather, the forward voltages of the three are in the range of 3.0V to 3.4V. The current for each is the same at 20 mA. To cut down on the wattage per resistor, I could wire 2 or even 3 resistors in parallel.

Maybe not ideal, but my hope was to wire them all together in sort of a one size fits all arrangement. A slight difference in brightness will be OK. The string I showed in my original post was for a box of 50 blue. They also make multi-colored strings.

I would like to do strings of 50. So, 50 x 20mA = 1000mA or 1A. I can get a 4A DC power supply for about $8. Not a constant current type but should do the trick or get a larger one (say 20A at 5VDC) for all of the five or six strings I usually use.
 
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If you parallel different colours then it's VERY likely the lower voltage ones will take all the current.

The current for them isn't the same - it doesn't even apply - it's entirely dependent on what current you feed it with, either constant current or just a resistor. If you're wanting the current to be the same (and why wouldn't you?), then you need to feed them separately, and have individual resistors for each colour.
 
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If I use a constant current source, Do I still need resistors?
 
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