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I am a complete beginner at designing circuits.

I was thinking of building an aquarium light. As plants need a full spectrum of light to grow properly, this is the selection of colours I came up with by watching many YouTube videos about this topic. I also wanted to adjust each colour separately with an Arduino or esp8266.

The first problem was to control these LEDs based on colour. Which I think I have solved with help from the internet. Please check if there are any problems with the Mosfet driver section.

I am planning on powering these LEDs with a voltage and current-controllable buck converter. (The LEDs are 1W each. They are bead-type LEDs commonly used in horticulture) Now the problem is the red and the yellow LEDs have a lower forward voltage than the rest and there is also a smaller number of yellow LEDs. So, my questions are:

  1. Won't there be a lower resistance path through the red and yellow LEDs for the current to flow? Which in turn will make them brighter than the rest or possibly even damage them.

  2. Is it ok to power LEDs with a higher voltage than the sum of forward voltages as long as the current is constant?

  3. Is it ok to connect LEDs in parallel like in the schematic?

  4. If this won't work, what are the other options I have to accomplish the goals I have?

  5. And if this works (which I don't think it would) is the voltage and current (20.4V 12.6A) I'm planning on providing correspond with the need of the circuit?

The schematic I was talking about

** Edit:

Is it possible to use something like this:

enter image description here

These are readily available in my country. There is an output voltage range of DC180-240V written on the device does that mean I can use a string of LEDs with a forward voltage within that range? Are these devices safe? Are they efficient? And most importantly will I be able to use the existing PWM dimming system I have? (Of course with a MOSFET of greater voltage rating)

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    \$\begingroup\$ short answer: no ... more current will go to the path with the lowest voltage drop \$\endgroup\$
    – Pete W
    Feb 5 at 21:16
  • \$\begingroup\$ You need to add a small resistance to leds with lower Vf. \$\endgroup\$ Feb 5 at 21:20
  • \$\begingroup\$ ISHRAQ, For something like the goals you have, you may want to set up different, highly efficient, switching voltage regulators to set different voltage rails -- one for each distinct color system. Then, apply a current regulator for each color system, where each can be set to some maximum current value that you choose. This will set your 100% brightness level for each color system. Having used highly efficient switchers prior to the settable current sources will help minimize losses. Then for relative color system brightness adjustments use PWM for each, as desired. \$\endgroup\$ Feb 5 at 21:52
  • \$\begingroup\$ ISHRAQ, Note that plants have photosynthetic cycles that have been measured (some studies made by NASA scientists trying to get the most out of growing plants in orbit or in space) to require varying wavelengths at varying moments during each full cycle (I forget the names they discussed) together with a short relaxation time on the order of a few microseconds in order to allow a reset back to ready to occur. If I were going to all this trouble, I'd probably go back and re-study those papers to see if I could incorporate some of their knowledge in what I tried. \$\endgroup\$ Feb 5 at 21:53
  • \$\begingroup\$ The constant current driver CAN drive different colour LEDs in a series string IF you do want the same current. IF you are never changing what is on and off. You mention PWM only casually at the end of the question - it should feature clearly in the main description. Are you wishing to change the balance of what colours are on and by how much? What does your PWM do? || Parallel strings of the same colour is somewhat OK. THe more in each series string the more likely they are to be close to balanced. || If you tell us fully and clearly what you wish to achieve we can help better. \$\endgroup\$
    – Russell McMahon
    Feb 6 at 9:21

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This isn't going to work. With the schematic you posted, only the yellow LEDs will light, all others will remain dark. The yellow LEDs will also get horribly overloaded and burn out.

What matters isn't just the forward voltage of the individual LEDs, but rather the forward voltage of an entire series string.

As an additional problem, no two LEDs are equal - they all have slightly different forward voltage, even if they're the same color and from the same batch. As a result, you can't connect them in parallel without some kind of balancing scheme. You could, for example, add small dropper resistors in series with each LED string to address this. The resistors should drop about 10% of the overall string's forward voltage as a rough guideline.

To address the problem with the individual colors, you will have to give each color its own programmable constant-current buck converter. The problem here is that even if you get proper current balance, that balance gets messed up as soon as you turn off one of the colors via your PWM MOSFETs. That's because the constant current will redistribute "around" the LEDs that have been turned off, making all the other LEDs brighter. In the extreme case, if you turn off all LED colors except for one, that one LED color will be hit with the full 12A from your buck converter and burn out.

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    \$\begingroup\$ I don't know how I missed the part about turning off some of the colours and the others getting the full current through them! What a shame. Thank you very much for your help. \$\endgroup\$
    – ISHRAQ
    Feb 6 at 5:03

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