LED Brightness Problem

[Jasontron]

Hi,
So I'm in the process of making an LED string blade using around 80-90 LEDs
wired together.

The LED specifications are:
FV: 3.2~3.4V
FC: 20mA
mcd: 12,000-14,000
Viewing angle: 120-140 degrees

I'm using a 3.7V 5.3Ah Li-ion battery to power this, and am also using a
200ohm 2 watt resistor for the LEDs.

This is all well and good, except for the fact that the LEDs really aren't that bright.
After being put in the blade with a diffuser, I can barely see the light in a lit room.

Is there something I'm doing wrong with the setup? Are the LEDs the wrong kind? If the
LEDs are wrong, is there any you guys know of that would work better?

Thanks a bunch,

Jason

[Silver Serpent]

How are they wired? Multiple segments, or all of them in parallel? If segments, how many LEDs per segment?

I'm curious how you calculated your resistor value. That's much higher than you'd need for even a single LED at those ratings, let alone 80-90 of them.

[Jasontron]

Thanks a lot for the quick reply,
First of all, I'm wiring the LED string in parallel.
Second of all, I'm not sure what I was thinking doing the resistor the way I did.
I used the calculator you provided and saw that I would need an 18ohm 1/4watt resistor
For each LED in the string. Is there a way I can just use one resistor for the whole string?

Thanks,

Jason

[Silver Serpent]

Yes. If you're wiring them in parallel, you add up the mA of all the LEDs in each segment. So, 20mA * however many LEDs = total current for that segment. Treat that segment as a single LED for purposes of resistor calculation, and each segment will get a single resistor.

Please note that most people who build LED string blades will make 5-6 segments, so that they can get the extend/retract scrolling effect for their blades. You'll need an appropriate soundboard or LED controller to handle this.

If you wire up 80 20mA LEDs in parallel, you'll be needing about 1600 mA of current. If you wire them up in segments of 20 LEDs each, then each segment will only need 400 mA of current. The total for the whole strip will be the same, but you're likely to run into heat issues if you're pumping 1600 mA through your wire as opposed to 400 mA.

[Jasontron]

Alright, so I entered those values in the calculator (this time for 1000 mA because I'm thinking of doing two segments of 50) and the calculator said I'd need a 1ohm 2 watt resistor. That's great, but if I wire the two segments in series I'll need 6.8V not 3.4, correct? If that's the case, I'll need another 3.7V li-Ion battery. Is this all right? Or am I going crazy.

Thanks for all the help!

Jason

[Silver Serpent]

Or wire the two segments in parallel. You'll end up with a total drain on the battery of 2000mA, perfectly fine for an 18650 sized li-ion. Each segment can be considered as a single LED, 1000mA each.

[Jasontron]

Thanks!

You've been a big help!

Jason

[Jasontron]

Oh, and one more quick thing,
I was looking up what people said about wiring LEDs in parallel with one resistor, and there was a lot of talk about it being a bad idea.
Here is the link to the this thread: http://electronics.stackexchange.com...-parallel-leds
Is this a problem applicable to me?

Thanks for all the help

Jason

[Silver Serpent]

It's something you need to be aware of, but it's a bigger problem with the higher-current LEDs.

[Obi1]

(Jedi mind trick: "after reading this, you will forget who wrote it. You will not curse my name if the idea I present does not work for you - disclaimer end.")

Actually I've built my share of LED string blades and also modified some toy blades for more brightness.

In case of blue LEDs with max 3.4V, I would not even consider a resistor with a 18650. These LEDs will definitely survive even the 4.2V max voltage this Akku can throw at them. I have a blue one with 2-serial set-up, 2x18650 and so far no issue. Same with a 3-serial red one (3x2.4Vmax=7.2V, but 2x18650 can go up to 8.4 max).
For some Hasbro toy sabers I bypasses all the resistors in the blade. They work perfectly and so much brighter than with the supressed set-up.

Of course all on your own risk...

P.s.: if you have a CF LS, you can even set the drive strength, so no resistor is needed.