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common anode rgb leds in series?

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danrogers

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Hi all, I have some rgb leds to use, both superflux and standard 5mm leds. When I have made rgb arrays in the past I have allways used separate colour leds, and wired them in groups of 3 in series with either 180Ω or 270Ω resistors from 12v supply.

I tryed using a 380Ω or 470Ω im not too sure, to drop the voltage from 12v down to something near the foward voltage but the resistors were getting to hot and it seemed like an inefficient way of doing things.

My question is, how is it possible to wire up RGB leds in groups of three with common anodes? Maybe im being a bit dense but I cant think if it would be possible or not?

thanks in advance
 
As far as I can see, your problem is getting the LEDs being too hot to handle. Use 9V box batteries to power up an LM7805 regulator and run it up through 670 ohms for each LED, it doesn't matter which is commoned. That way they don't suffer from overcurrent, as I presume the current rating of your 12V battery (what are you using? A wall-wart? Yuasa?) must be quite high, not LED-friendly, I'd like to put it.
 
Thanks for your reply. Im using a circuit that takes 12v on the supply, which i was powering with a 500mA wall adapter. The leds are controlled by 3 mosfets which switch the 12v supply through.

That is the problem as I because i need to be supplying 12v to them?

cheers!
 
just to clarify, If someone could draw up a quick schematic showing how to wire up 3x common anode RGB leds in series it would be very much apprieiciated! :)
 
The leds are controlled by 3 mosfets which switch the 12v supply through.
That is the problem as I because i need to be supplying 12v to them?
Why can't you use a 6V wall adapter instead? If you must use a 12V supply, the only way to drop the voltage without a lot of heat is to use a switching regulator to buck the voltage down to 5-6V. If the RGB LEDs emitted colour was static/unchanging, it would be easy to wire them in series.
 
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Hi there, as far as I know the PIC that runs the 'fets requires 12v then dropped down to 5v i think.

I just cant think of a way to make this work :( I dont suppose I could use a regulator after the 'fets could i?

So you dont think there is a way to wire each colour of the 3 RGB leds in series then?

Thanks for your time
 
Hi there, as far as I know the PIC that runs the 'fets requires 12v then dropped down to 5v i think.
A schematic of the driving circuitry may help us find a solution for you that is not obvious at this point.
If your FETs are logic level ones (5V gate voltage), you could eliminate the PICs 5V regulator and drive the entire project from a power supply like this one:
https://www.dipmicro.com/store/DC-0512


I just cant think of a way to make this work :( I dont suppose I could use a regulator after the 'fets could i?
You could use a 5V regulator to feed VLED but, unless it is a switching regulator, you'll just be moving the heat from the resistors to the regulator.


So you dont think there is a way to wire each colour of the 3 RGB leds in series then?
Not unless you get a RGB LED like this one with 6 leads:
https://www.electro-tech-online.com/custompdfs/2010/01/OVSTRGBBCR8PDF.pdf
 
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Hi again, thanks for the info on the leds, thats what I was thinking and unfortuniately only have 4 pin ones.

That being said, makes me want to work out a way to drop this voltage down to use single leds!

Here is the data sheet for the mosfets if that is any help? https://pdf1.alldatasheet.com/datasheet-pdf/view/171118/STMICROELECTRONICS/P36NF06L.html

The whole circuit is designed to run from 12-15v and i dont have a schematic for it BUT.. have just made a crude layout diagram from a photograph as its only a one sided pcb.

I hope it helps for those of you that are clever enough to understand the circuit from it, i guess there must be some way of switching a lower voltage direct to the leds and leaving the 12v supply? By the way it powers common anode rgb arrays.

226-RGBLAYOUT.jpg


thanks for your time
 
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You could easily convert it to work on 5V and use the ac adapter I posted above or a similar one.
Or add a 5V switching regulator circuit to power just the LEDs.
Or you could simply just use higher wattage resistors to dissipate the heat.
 
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What could I do to? would it just be a case of separating that earth and cutting the track from the circuit supply one, then applying a lower voltage to the supply com?

thanks for your help
 
What could I do to?
If you mean modifying the circuit to use the ac adapter I posted above, then you would add a jumper and remove the zener. Once this "purple mod" is done, you can only run the board from 5V and the on board LED will not light up. 12V will destroy it!
 

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Thanks so much for taking the time to do that! Very much appreciated!

I’m in the UK so might not purchase that exact 5v adapter, but I guess any 5v switch mode supply would do? Could I use a 12v-5v dc-dc if there is such a thing available?

Thanks again for your efforts, I’m looking forward to trying the 'purple mod' :)
 
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I’m in the UK so might not purchase that exact 5v adapter, but I guess any 5v switch mode supply would do?
Yep. Any 5V regulated supply should work as long as it has enough current capability for the number of LEDs you intend to drive at once.


Could I use a 12v-5v dc-dc if there is such a thing available?
Yes, and they are available. If you used one, you wouldn't need to modify the driver PCB as you could feed 12V to the PCB and 5V to the LEDs.
Here is one from Farnell, but you may be able to find a similar one cheaper elsewhere.
 
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