Couple of LED / Resistor Questions.

[Hippogriff]

Hi all,

I'm new here and I am probably guessing that "LED / Resistor" questions are quite common. Hopefully I'm not going over too much old ground but a view from some folk in the know on here would be very helpful to me.

I have a lovely RGB star LED that I want to control with a PIC. That's all working fine, but I want to make sure I have the LED as bright as possible without it exhibiting its deathwish. The LED has forward voltage figures of Red = 2.5 volts, Green = 3.8 volts, Blue = 3.8 volts and a maximum forward current of 350 mA. The power supply is 5 volts.

I figured, using the equation, that I should be able to use resistors of around 7 Ohms, 3 Ohms and 3 Ohms for red, green and blue respectively. Going up a bit, for safety, I then figured on 10 Ohms, 5 Ohms and 5 Ohms.

Are those the lowest value resistors that I could use to ensure this LED is the brightest it can be from a 5 volt power supply? I only ask because the circuit that I was initially following used 5 Ohm, 10 Ohm and 15 Ohm respectively - I think that was designed to give a "warm" colour when the LED was white!

Anyway... I feel relatively comfortable with that equation. You work it out and then add a little bit for safety - that seems fine. I'm just looking for confirmation there.

However, what I don't understand is the wattage of said resistors and whether it even matters. I have available to me some 0.6 watt resistors and some 1 watt resistors.

If I choose to use the 0.6 watt resistors or the 1 watt resistors, will I affect the brightness of the LED or will it have no bearing whatsoever? I've tried to check with my eyes... but, ummmm , I can't seem to tell - so I'm wondering if anyone can tell me what the real story here is.

I'd appreciate any advice that anyone on the forum can give - obviously I'm only starting out with this stuff - so assuming I know little would be good.

 

[mbarazeen]

rating of the resistors only to tell the capacity of them (the discipation of heat it can stand), it doesnt influence on the brightness.

you have 5V and your maximum current is 350mA, the resistor you will connect red LED will have more voltage across ie 2.5V and wil discipate 2.5x.35 = .875W

1W resistors will serve for your need

 

[audioguru]

If the circuit is the Mood Light project at Electronics-Lab then the resistor values were wrong which caused the red to burn out. Since the red is not as bright as the green then the author increased its current too much to make it appear brighter.
He selected the currents to make the colour white when all LEDs were at max current.

In my Mood Light circuit I selected the resistor values like you so that each colour can go to its max rated current to appear as bright as possible. Then my circuit randomly ramps each colour's current up and down and pure white appears randomly along with every colour and brightness.

 

[Hippogriff]

Thanks for that.

So, it is probably working as bright as can be then, with these resistors (Ω and wattage) where wattage seems to make no discernible difference.

In fact, I selected 10Ω / 1 watt for all three LEDs (R, G and B) in the end, because I then managed to cut out two resistors by placing a single 10Ω resistor between +5v and the common anode... whereas, before, I had the +5v connecting directly to the common anode and then three different resistors coming out the other side of the LED.

It all seems fine and I'm quite happy.

It is a mood lamp, yes, but not from Electronics-Lab (which I've not heard of) so I might take a gander there... always good to see what others have done.

I would like it to be even brighter, though!

 

[audioguru]

Since you use a single current-limiting resistor for all 3 colours then if more than one colour is turned on, only the lowest voltage one (the red one) will light.

 

[Hippogriff]

Nah, mate - they're all lit and are fading away very nicely.

I put the single 10Ω resistor between the +5V and the common anode of the RGB LED - previously it was...

+5V - RGB LED anode - split into 3 at cathodes - three separate resistors.

But now it is...

+5V - single 10Ω resistor - RGB LED anode - split into 3 at cathodes - no need for resistors here.

Not sure that the 10Ω value resistor is perfect for all of red, green and blue (I admit that) but I'm getting a pretty good wash of colours... when all are on at full power the white looks kinda... well, blue-white, I'd say. Looks a bit cool, instead of warm.

Not a problem.

 

[Willbe]

If the tolerance on the voltages is +/- 1v you can expect the current/brightness to vary accordingly. Go with a higher supply voltage so you can more nearly approximate a current source.

 

[Hippogriff]

Nice idea.

I think - correct me if I'm wrong - if I chose to do that then I would need to add a voltage regulator and a couple of capacitors to my circuit because the PIC will probably burn out at anything over +5V?

I've used RGB LED strips before (bloody cool things!) and they required 12V, so I had to do this when controlling them via a PIC.

Never thought about trying to pump more into this RGB LED though.

Anyway... it's all soldered-up now and is currently being glued into the base of the lamp. Maybe for the next one I might experiment with power supplies.

I just wish my soldering was better... more precise and dependable, I mean.

Thanks for the responses!