How to power up 50 LEDs? (DIY 'fireflies' project)

I want to install lots of tiny LEDs lights in my garden (at least 50).
I DON'T want high voltage cables in my garden so I am thinking in getting a high power, low voltage transformer to power up these LEDs.

For example, I have two 24V/1.25A (30W) power supplies from Canon printers. Let's say this is a small scale test. Later I could use a PC power supply for 50+ LEDs.

The LED I will use is 1W but I want to run it at about 3.1V/0.1A because at this current it doesn't require a heatsink (it runs at under 37Celsisus - I hope in summer it won't get much hotter). So, I can have 12.5 branches, each branch will have 8 LEDs in series, so 2.4W per branch.

Is this a good design? Any alternative idea is welcome as long as it doesn't involve carrying more than 25V per underground cables.

Answer

It is not the best design. I will suggest you to go with a fixed current rather than a fixed voltage regulator. This is the design any big led installation uses (offices, street lights, huge facilities with kw of light power).

This is done becouse of several reasons:

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  As a LED heats up, its current consumprion changes (usualy increase) and this reaction makes you difficult to limit (or control) the heat.

  
  


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  LED diodes are di per se designed for a current supply and therefore if you control it it with a voltage ( that will let current to freely change according to diode temperature, design and outage ) will make them change color, deteriorate faster and eventually burn.

  
  



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  Voltage controlling few LED in series if one will burn, and there shortcut will distribute a higher current on the remaining diodes and make all of them burn faster.

  
  

If you want a reliable solution your configuration of 8 diodes in series and then branches in parallel is a good solution. But you will need to drive a fixed current in each branch, and not connect all your branches in parallel and controll them with one current driver.

Current drivers are cheap, dont worry about their price as I can assure you thah 1W LED diodes will be the greatest part of your expenses.