LED for deck light

Hi,
I posted this in the LED forum, but it seems like it may be better suited for the modified forum. I apologized for the multiple postings. Please read on.

Newbie to LED's here....and need some advice.
I ordered some LED's with resistors and pigtails installed for 12 volts and intend to use them on my deck lighting. I am hooking them to a 12 volt ac transformer and have a bridge rectifier installed between the transformer and LED's so I can use the transformer for the AC lights and the new LED's. I intend to mount the LED's in a custom mount made from the same composite material that the deck is made from and route the wires inside the railing posts (which are wood). I initially looked at the Microstars by Suncor & and didn't see much point in buying a single LED light for $17 when I can set up the whole deck for less if I make my own.
My question to you guys is about the mounting : Do I have to enclose the wiring and LED in a housing like the stainless steel tube the Suncor lights come in or is the shrink tube protecting the wires and resistor enough ? They will be outdoors, but protected inside the post from direct water contact. If they have to be sealed up, what kind of silicone or epoxy should encapsulate the LED's in the mount ?
Thanks for your insight.
Jeffyr
Suncor

jeffyr,

I designed the Microstar intially for use in the marine environment where salt air and UV would be taxing on most materials. In your application, you certainly don't need the stainless housing. As to what materials you use for conformal coating or potting, you have many options. Epoxy will provide a much better protection to the solder joints than silicone will. Consider usng tinned wire instead of bare copper. Keep in mind that water flows down hill and place your splices and wires, when possible, in a maner that will see the water flow away from the LED's, resistors and solder joints.

Although more expensive, for longevity as well as greater protection against spikes in the input voltage, you might consider wiring three LED's in series and resistored for a lower drive current as opposd to a single LED. You can net more light per point source at less current and with more efficiency and less demand on the LED's.
Have fun! /ubbthreads/images/graemlins/grin.gif

Ahhh.....Mr. Mcleish.....I emailed you when I came across your web site for the Microstar last week (email bounced back I think). Nice little light...and the clusters too. I got the info from your site for the bridge rectifier. Seeing these lights you designed has turned me on to the many possibilities of accent lighting using LED's rather than tungsten bulbs. I think my order for the LED's should have arrived today....maybe time to play this weekend.

Thanks for your input. My original idea was to mount the LED directly in the railing post, which is wood, and route the wires up inside where they will get exposure to humidity, but never rain. Is there still a need to encapsulate ? or will wire nuts be fine for this application ? Having a wire splice inside a wood post seems wrong. which is why I was considering a meatl sleeve of some kind.

BTW: I came across some of your photos on a link on your site--the whale breaching is a wild photo.

Thanks.

Jeff

BTW : Where do you get the parts like the lock line and small metal parts ? do you fab your own ?

Jeff,

A lot of the stuff is available from my neighbor's web site,
berkeleypoint.com . I don't know which metal parts you are referring to. If it's the housings, I designed them and then outsourced the production of them. I have had good success with some of the splice connectors that are filled with silicone grease. I have some installations in Hawaii for instance where moisture has yet to cause them to fail. I think most failures I have seen are due to imersion with inadequate protection of connections as well as probable spikes in unregulated power supplies. These failures have been infrequent enough to not cause real concern but often enough to suggest that access to the system for replacement *must* be designed in or you can count on Murphy to visit!

Too much voltage followed by too much current WILL kill LED's but not always immediately. Non catestrophic damage I have been told is very common and can result in a LED just going dim one day for no apparent reason. /ubbthreads/images/graemlins/frown.gif

- Don

Won't the resistor limit the current so the LED doesn't get too much ?

The resistor drops the voltage but higher Vin will give you a higher voltage to the LED and accompanying higher current. You need to pick the resistor value based on "worse case". I suspect that some of the failures I have seen have resulted from reality being worse than designed "worse case". You have stated that you plan to use an A/C power supply. These are typically not regulated. I am not trying to scare you off here but wanted you to be aware of possible failure modes. You would also be wise to fuse individual runs if possible to protect the runs as well as the rest of the system should a short occur somewhere. This sounds unlikely in your rails but if you decide to branch out into the foliage or garden, this becomes more of an issue.

Would a constant current device work well in this application? Doug Owen has shown me a few that he's whipped up. Some are very simple (using just the LM317 and a single resister) and others are more complicated... like the one he created for the Milky Candle.

Either way, this gives me some new ideas for use of my many LEDs.... I've got too many flashlights and need some new applications!

Constant current is ideal but you are then talking about a specific system that is not friendly to modification or growth after design. If you go for a parallel array, you want to make sure that the LED's are from the same bin and have the same Vf. You then need to set the current level based on the number of LED's in the group. If you go series, then you end up needing high voltage pretty darn quick. If there is an inexpensive and unobtrusive constant current driver for say 3, 6 or 9 LED's that can work off of "typical" 12v, I'd love to hear about it!

Ok, I'll share my experience a little.

I wired up the outside of my house and deck with the rectangle Malibu deck lights modified with 1W Luxeons; 16 ceiling lamps, plus walk way lamps and others. As you are discussing, the power supply was the biggest problem. Series or parallel was the first question. But since I wanted to stay with low voltage, I went with parallel. First thing I did was trash the stock Malibu power supply and buy a surplus switching supply.

Just so happens that the ceiling lights were divided evenly, eight upstairs, and eight down stairs. So I used two power supplies (8V 6.5A Allectronics CAT# PS-865 $9.50) and LM350T configured for current regulation set to just under 3 amps (~360ma/luxeon). And each Luxeon had a one ohm resistor in series to help ballance the load, and provide an easy way to monitor current. Also, by only working the power supplies to 50% capacity everything runs nice and cool. A 3 amp fuse in each circuit upstairs and down stairs completes the safety features.

Maybe not the best solution, but has been reliably running for a year or so without problems. The LM350T gives the circuit some adjustability and the one ohm resistor in series with each Luxeon allows me to check and set the average current to the Luxeons.

Some day I'll actually borrow someone's camera and get some photos posted of this set up....

Way to go, Lambda!

Doug Owen