RGB LEDs in series - turn Green on

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Discussion Overview

The discussion revolves around the challenge of controlling the colors of high-power RGB LEDs with a common anode configuration when connected in series to a 15V power supply. Participants explore various methods for switching the green and red colors independently, while addressing concerns about heat dissipation and circuit design.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes using six transistors, specifically P-channel MOSFETs, to control each color of the RGB LEDs separately.
  • Another participant suggests a simpler approach of connecting the green and red LEDs in series to a single switch at ground, but this is challenged due to the common anode configuration.
  • A later reply acknowledges the common anode issue and proposes connecting the LEDs in parallel instead, tying the anodes together and using resistors for each color.
  • Concerns are raised about heat generation in resistors when dealing with high current (0.4A), leading to a discussion about the heating of transistors in the circuit.
  • One participant questions the wiring of the transistors and suggests measuring Vgs and Vds for troubleshooting.

Areas of Agreement / Disagreement

Participants express differing views on the best method to control the RGB LEDs, with no consensus reached on a definitive solution. The discussion remains unresolved regarding the optimal circuit design and the issues with heat dissipation.

Contextual Notes

Limitations include potential misunderstandings about the common anode configuration and the implications of using high-power LEDs. The discussion also highlights unresolved questions about the proper wiring of transistors and the effects of current on component heating.

meereck
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Hello,
I have a high power RGB LED with common anode.
I want to have 3 of them in series, and put them on 15V power supply.
And I want to switch green color on, and red color as well (something like a normal traffic light)

Here is the problem:
Given that three LEDs are in series, and are common anode, how can I switch color separately?
I was thinking about placing 6 transistors between them (on cathode of each LED - e.g. 1LED= 2 transistors for each color). I assume P-channel MOSFET (i.e. IRF9250) can do the job?
However, this is quite bad solution, does anyone have anything better?


I hope I explained it well,
I will post a drawing later on.
Thank you in advance,
M.
 
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here is the schema, sorry for making it by hand.
hopefully it is understandable.
All I want is to switch between all green and all red.
 

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Last edited:
All I want is to switch between all green and all red.

Why do you need six switches? You could connect the green LEDs in series to one switch at the ground, and likewise with the reds.
 
Thank you for the reply.
unfortunalely, that's can't be done since RGB LEDs are "common anode".
If I did what you suggest, green and red LEDs would light at the same time.
Dont forget that left and right diodes are in one package (denoted as slashed rectangle on the drawing), having common anode.
Or am I missing something?
Cheers
 
meereck said:
unfortunalely, that's can't be done since RGB LEDs are "common anode".

Never mind, I missed that. The next simplest option would be to go parallel. Tie all the anodes together and connect them to plus of the power supply. Then connect the greens together to a resistor and to the on/off switch and the ground. Same with the reds.
 
you are right, but the leds are high-power (current 0.4A) so the resistor would get hot pretty much.
What I tried to do is the following scheme. But T3 is getting very hot when the current 0.4A flows through it. T1 is ok - they both are connected in the same manner (at least i think).
Does anybody know what the problem is? I probably miss something. Are the transistors wired properly? The gate of the resistor is still on +15V. When I ground it, the transistor should switch on, am I right?

thanks in advance,
regards M.
 

Attachments

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you might get some insight if you measure Vgs and Vds on each
 

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