I would like to make an array of 16 -25 RGB LEDs with max forward voltage 2.1, 3.3 and 3.2 respectively each with a forward current of 20 mA.
I'm wondering what board would be best or if I would need to do a separate power circuit?
Thanks!
Any idea how you want to connect them? Multiplexed MxN array? Individually to
shift registers? Are these individual colours or a 4-lead RGB led (and if the latter,
common anode or common cathode?)
Connected in parallel, I want them to all change together.
4-lead common cathode.
Firstly you can't just connect them in parallel, you'll need a current limiting resistor for every
single LED, one per colour - sometimes you can get away with paralleling, but only when they
are thermally matched and from the same batch (this is unlikely to be the case here). These
resistors set the current for each colour (slightly different values for the different colours
because of different forward voltages if you want accurate current matching).
You then have all the cathodes to ground, the red anodes (via their resistors) to a high-side
switch, ditto blue and green.
Each high-side switch would be either a p-channel logic-level MOSFET or a PNP switching
transistor capable of the current load.
25 times 20mA is 0.5A, which isn't too tricky, times the 3 colours means the supply has to
handle 1.5A minimum.
I've only had an introductory circuits class so I can honestly say I don't understand half of what you just said.
Could you possibly provide some kind of drawing? I'm really not sure what you're trying to convey.
I also don't see why the following wouldn't work...
Thank your for your help! I'm glad one of us knows whats going on... haha
tylerkenepp:
I also don't see why the following wouldn't work...
Because all LEDs are different and if you only have one resistor then some will be brighter than others.
There's even a small possibility that one of them might take far too much current and die.
You need one resistor per LED.
Ok, that makes sense, so what about this...
Other than being obnoxious to solder would this work? I know @MarkT said that they need to be thermally matched? I know resistances vary from resistor to resistor of the same type and resistance but I can't imagine that they have that much variance?
they need to be thermally matched
Only if you want to try using a single resistor for all the R LEDs etc. Not advisable.
As you have it there is no need to match anything more than just selecting a reasonable value based on the forward voltage of the different colour LEDs.
Rob
So what I'm getting is that my second drawing would work?
If so we're back to which board to use and if I need a separate power source.
If most boards only output 50mA that's not gonna cut it for 16-25 LED's so I'm stuck again...
25x20 = 500mA per colour, yes you will need a separate power supply.
Before going any further, what do you need them to do?
Do you need individual control over the colours?
Have you already got the LEDs? If not common anode might be easier to drive.
Rob
Yes if you want to switch and control them from an Arduino you need to think about
switching device(s) - for common cathode high-side switches give individual control
to the colours, high-side switches can be PNP or logic-level p-channel MOSFET, but
you need to choose devices that can handle the current and power dissipation.
@Graynomad
I just want to be able to turn them on and off and change the color all at once all together.
Nothing is done individually, I may do that eventually but for now its all as one.
I haven't ordered anything yet.
@MarkT
I still don't know what you're talking about.
Like I've said, I've only taken one circuits class. I don't know what a MOSFET is and therefore what you said means almost nothing... sorry. If you could elaborate a little as to what they are and why I would need them that would be helpful.
Also just so it's known, I'm planning on controlling these via bluetooth from my phone. Seeing as that is just the type of input I hope that won't change much, if anything.
However with that I've been looking at the Bluno since it already has bluetooth built in to a UNO.
So if I'm using that, and feel free to suggest something else, how would I power the 16 (I just decided I only need 16) RGB leds?
@MarkT
I still don't know what you're talking about.
Like I've said, I've only taken one circuits class. I don't know what a MOSFET is and therefore what you said means almost nothing... sorry.
You really don't know enough to tackle this project do you. It is hard to give advice when you can understand so little.
Read this:-
The load mentioned in it is your LED chain. You do not need the diode across the load for LEDs.
@Grumpy_Mike
You're absolutely right in saying that I don't know enough to tackle this project, that's why I'm asking so many questions, some of which may seem stupid to experienced users. Thank you for the link! I'll definitely read it and see how I can put that to use. I meant no disrespect to @MarkT I just didn't know what he was talking about and therefore couldn't use the information provided without some elaboration or external links.
that's why I'm asking so many questions, some of which may seem stupid to experienced users.
No it is not that, it is that you do not know enough to understand the answers therefore you are not getting what you should from doing the project.
I would advise that you start off doing something more basic first and try and understand that. The trick to learning is to take small steps. If the steps are too large then you get lost.
Try some of the tutorials in the learning section of the site. Learn about transistors and LEDs before trying to tackle what you want to do.
It is rather like asking how to do a hand break turn in a car without knowing how to put the car into gear.