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Advice for soldering long chain of RGB LEDs

seanspotatobusiness

Sep 11, 2012
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I have many 5 mm RGB LEDs that I want to solder together into a long chain. I want to make three chains of about 30 cm length. They will be hidden inside some frosted acrylic tubing so it doesn't need to look too pretty but it does need to be pretty straight.

In this picture you can see the method I've used to bend the legs outwards, then downwards before soldering each LED to the next. I guess I can just proceed like this but it's hard to keep the thing going straight and I wanted to check whether anyone knew of an easier or tidier way.

 

Martaine2005

May 12, 2015
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Hi,
You maybe better to have a wooden template.
Or a wood dowel.
leave the 'common' straight and bend the other legs out. Then use the 'common' on the wood dowel keeping it all tidy and straight.

Martin
 

Gryd3

Jun 25, 2014
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I hate to bust your bubble... but this type of connection is typically frowned upon.

LEDs operate happily on a constant current power supply.
LEDs wired in series will all have the same amount of current flowing through them.

LEDs wired in parallel may have different currents flowing through them, leading to the failure of one ore more LEDs... at which point, the constant current power supply is still outputting at 100% even though it should be turned down due to the quantity of LEDs declining... This has a downward spiral affect which will then cause more LEDs to fault because they are being fed too much current.

To get around this, you will need to operate the LEDs with a constant voltage power supply that is tuned to output at the same voltage as the forward voltage drop of the LEDs... but even then, you run the risk of unbalanced LEDs being brighter or dimmer than the rest...
Ideally, when using multiple LEDs in parallel as you are doing, you will want to have a resistor for each parallel element... which in this case would be a resistor for each LED.
You can put the resistor on the common lead, unless you plan to power more than one color at a time, then you will need a resistor for each color for each LED. (If you use one resistor, you can use PWM and simply cycle between the 3 colors to give the illusion that more than one colour is lit.)

At this point, it may be of your best interest to get your hands on some small SMD resistors or some small resistors and placing them on the common lead of each LED. You can try to continue on this path, but it may not give you the best results.
 

seanspotatobusiness

Sep 11, 2012
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Sep 11, 2012
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Hi, thanks for your advice. I'm actually intending to use one of these controllers from China. They seem to output a constant voltage and I was going to use a single high-power resistor (one for each colour) attached to a heatsink. There will be three columns of LEDs in series. I've done a similar project before where I used high-power LEDs (~1 W per colour) and it seems to work okay.

 

Gryd3

Jun 25, 2014
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I've done a similar project before where I used high-power LEDs (~1 W per colour) and it seems to work okay.
Well... to be fair, many manufacturers power an LED without any resistors by tying it directory to a battery. It works well enough for that case.
Your layout will work without a resistor on each led IF and only if the LEDs are balanced. Two leds in parallel, whichever one has a lower forward voltage will conduct and light first. As the voltage rises, the other one will light up as well and all will appear normal. (perhaps one is slightly brighter than the other).
However, because in this example they are not balanced, the led with a lower forward voltage will be conducting more current than the other LEDs and will run warmer... this changes the characteristics of the LEDs and can cause them to become even more unbalanced. This can lead to an LED failing... and because they are wired in parallel with a single resistor sized to allow current for BOTH, the remaining LED is now dealing with twice it's intended current.

So... will it work? Yes, most likely.
Is it an ideal design consideration by any standard? No. Good enough for an experiment, but not much else.
If you intend for this project to last a considerable length of time, or to have any kind of robustness... please consider looking into buying a pack of surface mount resistors that you can use as current limiters to protect your LEDs.
 
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