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

Question

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 1

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:

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

• 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.

• 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.

Answer 2

In each branch you will need a current control resistor so use 7 leds instead of 8 which gives you a 21.7V drop. Your resistor needs to be (24-21.7)/0.1 =33ohms. You can have up to 1.25/0.1 =12 branches on each supply.

Answer 3

Sounds like a good plan to me, however actual fireflies are subtle - this will be a lot more in your face- the lights at 300mW will cast shadows easily.

You might want to consider adding a DC PWM dimmer (easy to make, much cheaper to buy) and experimenting first before you commit to the current level. You may well be able to run all 50 off of a single 24V supply- for example 100mA per string would allow you to run a dozen strings (1.2A) so 84/supply. If 50mA was enough you could run 168 LEDs.

Running the LEDs at reduced current will extend their lifetime. I have used some super bright green LEDs in yard accent lighting and they have dropped to about 1/2 brightness over a few years of timed nighttime use.

Whether you use a printer supply or a PC supply, keep in mind that they are not designed for outdoor use and may become dangerous if exposed to moisture (so keep the supply indoors in a dry warm place). Definitely make sure the output is connected to earth ground with a 3-pin plug so that it cannot put mains voltage wrt earth on the output even if it fails. I am more at ease with running the Canon supplies near full rating and depending on their safety than I would be with a random eBay Chinese supply.

Answer 4

Most minimal LED installations (for example some commercial freezer lighting or cosmetic shelf lighting) use constant current power supplies. In this way, you are free to install as many LEDs in series as the jobs requires. No other components needed. The voltage will change depending on the load. However, the current should always be the same. Here is an example circuit from a Maxim Integrated Semiconductor Constant Current document:

Note, this is from an applications white paper. More parts are show because of the intended audience. If you were to buy a COTS LED Constant Current regulator all you should have to do is add the LEDs.

The power supplies you have appear to be constant voltage and can not be used in this way.

Outdoors is a bad environment for electronics. Guessing, the most likely mode of failure for long life LEDs will be power distribution. To get around this consider sealing a battery, light sensor, LED and processor (yes processor) in a water tight container. The idea is to only light at night, perhaps only flash as if a lighting bug and stop after about 6 hours past dusk. Minimal embedded processor can easily do this and can be less expensive than the battery powering them. Hence the suggestion.

Answer 5

While the number of solutions to providing power to LED's is almost endless, you should also look at the bigger picture as well and decide what is 'Safe' to implement in your garden. If you have small children or animals then this could become a critical requirement (apart from feral animals that might chew on your wiring).
The regulations for landscape lighting vary between jurisdictions/states but in general there will be a maximum RMS voltage specification, and typically an isolation specification. Where I live it's 30 V RMS @ 25 A in any cable, an isolation transformer and non-grounded supply (so I don't need Earth leakage detection).

Running DC voltages round a garden (where you expect years of service) is not a great idea IMO, you will have trouble over time as connections corrode. Providing AC will at least slow the corrosion problem over most of the wiring but means you may need to provide a rectification solution at each use point.

My preference is to run a 24 V RMS center tapped transformer (3 core direct burial cable). I can then run 12 V incandescent, and 24 V incandescent without diodes or regulators. Then I can half wave (1 diode) or full wave (2 diodes) rectify the 12 V for other lighting types such as LEDs. I can even full wave (4 diode bridge) the 12 or 24 V if that works out better.

For running your LEDs, you could run strings of 4 (4 *3.1 V) with a series resistor from 12 V or a string of 9 from 24 V. No complicated constant current drivers or regulators.