LED Wiz - RGB LEDs 3.3v/2v bank select

[Franco B]

I have three LED Wiz's to wire up shortly with RGB LEDs. I was thinking about powering the banks with 3.3v from an ATX PSU to avoid having to use resistors for the blue and green cathodes as the operation voltage of them is 3.0-3.4v

The red cathodes require 1.8-2.3v so I would have to use a 68Ω resistor on each of those to bring it down to 2v but at least I would only have to wire a 1/3 of the resistors compared to using the standard 5v supply.

It would have been nice to have provided some of the banks with 2v to avoid using resistors all together but I think it would end up being a PITA keeping track of the RGB port positions rather than going 1R 2G 3B 4R 5B 6G etc etc.

Would there be any problems with using the 3.3v from the PSU for the blue and green cathodes?

[BobA]

AFAIK there are slight differences in the internal resistance of the LEDs when in operation.  This makes the resistor in series with them a necessity to properly provide voltage to the LED.   I do not think that providing voltage to the 2 colors without a resistor is a safe solution.

[drventure]

On a related note, I just received my LED wiz and noticed in the docs that they show wiring the resistor to the CATHODE (-) side instead of the anode (+) side.

I'd have thought it should be the other way around. But I'm definitely no electrical engineer.

Anyone know why the resistors should be wired to the - side? I'm curious more than anything and a few searches here didn't turn up any info on the subject.

[Dudeman]

It can be wired to either side. Think of the resistor as a choke. It will restrict flow from point A to point B so placement doesn't matter.

[Franco B]

Cheers BobA, maybe Randy could confirm.

drventure, you do need to solder the resistor to the cathode and then connect that to the numbered terminal.

On a side note if you are using RGB LEDs you need to buy common anode so that you have a cathode for the red, green and blue elements.

I actually wired my 1st LED Wiz up the same way as you thought using the anode. What a complete waste of time that was 

Are you sure Dudeman? It didn't work for me.

[drventure]

Hey Franco

Thanks for the confirmation. I was planning on following the dirs  , just curious why that's the way it is.

But yeah, I definitely know they work work if you get em backwards, and I had no idea about the common anode in the rgb LEDs, but I just happened to get lucky I guess and I picked up the right kind!

I was hoping I could control some ELWire directly via the LEDWiz, but, after some emails with Randy, looks like that won't happen. I'll have to drive them through a relay of some sort.

But I was kind of expecting that so no big deal.

[Dudeman]

Are you sure Dudeman? It didn't work for me.

Positive. I've done it... on purpose and by accident.

Here's one way:
http://www.theledlight.com/ledcircuits.html

Here's the other:
http://www.nycresistor.com/2008/05/04/handy-dandy-led-resistor-calculator/


With RGB LEDs, I "assume" (we know where that leads) that you need to use seperate resistors for each cathode because each color requires a different voltage. I say assume because I've never used one.

Red 1.63 < ΔV < 2.03
Green 2.18 < ΔV < 4.0
Blue 2.48 < ΔV < 3.7

[RandyT]

With RGB LEDs, I "assume" (we know where that leads) that you need to use seperate resistors for each cathode because each color requires a different voltage. I say assume because I've never used one.

Red 1.63 < ΔV < 2.03
Green 2.18 < ΔV < 4.0
Blue 2.48 < ΔV < 3.7

This is true.  If a resistor could not be placed on either side of the LED, then at least one type of RGB LED could not exist.  But they do, in fact, come in both common cathode and common anode configurations.

RandyT

[Franco B]

Could you confirm this Randy?:

Would there be any problems with using the 3.3v from the PSU (to the LED Wiz banks) for the blue and green cathodes (using no resistors)?

AFAIK there are slight differences in the internal resistance of the LEDs when in operation.  This makes the resistor in series with them a necessity to properly provide voltage to the LED.   I do not think that providing voltage to the 2 colors without a resistor is a safe solution.

[RandyT]

Could you confirm this Randy?:

Would there be any problems with using the 3.3v from the PSU (to the LED Wiz banks) for the blue and green cathodes (using no resistors)?

Everything I have ever read has stated that one should use some sort of resistor to limit current, even if the voltage is the same as required by the LED.  The thing is, the calculations show a 0 ohm resistor when using these numbers, so that is essentially a jumper, not a resistor.  Therefore, I think that you would be safe omitting the resistor, so long as the supply is well regulated, and the LED's are operating within the stated spec. 

One way to look at this is (apologies in advance to the EE folks):  If the LED is rated for 4v which, with an appropriate resistor in circuit, causes it to glow brightly, and you are feeding it less voltage without a resistor (3.3v), which you can see causes it to glow less brightly, then you probably are going to get longer than rated life from the LED, not shorter.

RandyT

[Entropy42]

I just wanted to add some info to this thread, even though its really old, since other people may come across it in a google search like I did.  You should not use a 3.3V power supply without a resistor unless your power supply is current limited.  Without an external resistor, the LEDs will become extremely sensitive to small variations in source voltage, i.e. 0.05V could increase LED current by 50%.
More info here:
http://electronics.stackexchange.com/questions/12865/is-a-current-limiting-resistor-required-for-leds-if-the-forward-voltage-and-supp

[RandyT]

I just wanted to add some info to this thread, even though its really old, since other people may come across it in a google search like I did.  You should not use a 3.3V power supply without a resistor unless your power supply is current limited.  Without an external resistor, the LEDs will become extremely sensitive to small variations in source voltage, i.e. 0.05V could increase LED current by 50%.
More info here:
http://electronics.stackexchange.com/questions/12865/is-a-current-limiting-resistor-required-for-leds-if-the-forward-voltage-and-supp

What you say is true, and it is always a good idea to include a resistor.  However, if the LED is rated at 4v, and the power supply is regulated 3.3, there would need to be a large and very unlikely power deviation to approach the limitation of the LED.  The link you provided questioned whether an LED and power source at the same voltage negated the resistor requirement.  In that case, it is always much safer to include the resistor as the supply outputs are never exact.