UNE BONNE LED MARINE NE PEUT ETRE BON MARCHÉ

je soumets à la sagesse collective de la liste un topo qu'un specialiste a ecrit lors d'un debat sur la liste Beneteau US

sorry c'est en anglais, mais un anglais simple, mais ca presente l'avantage de quelqau'un qui a l'experience pratique et une bonne connaissance theorique, largemnet au dessus de ce que je peux comprendre.

Date: Mon, Apr 14 2008 8:45 am
From: Force Majeure

David (Grove),

A little marine LED 101:

I will say first that this can be a long discussion getting into the
details of what separates a $2 LED from a $25 LED, but I will stick to
the basics, and keep it as brief as I can. Contact me if you need
more info.

LEDs that say 12VDC are NOT constant current (these are designed to be
driven by a LED driver that plugs into the wall)
LEDs that give a small range say 11V-14VDC typically are NOT constant-
current
LEDs that give a range of 8V-30VDC (or similar) ARE constant current
LEDs with any sort of axial leaded beige fat resistor are NOT Constant
Current
LEDs with any sort of shiny red axial leaded resistors are NOT
Constant Current

Why Constant Current is neccessary on boats (bear with me):

LEDs work on current not voltage. A traditional light bulb can fry by over voltage, LEDs fry by over current.
Current causes heat
Heat is DEATH to LEDs
Higher power LEDs used for illumination have 10X the forward current than indicator style LEDs
If we want maximum light from the LEDs, we want to drive them at their design max forward current, not more (death), and not less (dim).
As a general rule, if you overdrive an LED, you will get VERY BRIGHT light, but for a VERY SHORT time. The key is to give them the maximum current while still being able to manage the heat either thru thermal efficiency of the LED package (SMDs) or thru heat sinks and the like.
So if we are making cheapo clusters, and we size the resistor for 12V, and the boat's charging system is putting out 14.5V (or up to 30V during load dump events), we actually are overdriving the LEDs by more than the 20% you would think (due to the voltage drop of the LED itself). We now have the LEDs over current and over temperature, and the resistor is just adding to the problem by causing more heat. BTW, it is possible for LEDs to catch fire if overdriven too much.
For every 10 degrees C increase in LED junction temperature we halve the life of the LEDs
Okay, so just put a resistor designed for the 14V. That should solve it right? Well that is precisely what nearly everyone making cheapo clusters does. The problem is that those LEDs were built for your kid's riceburner tailights, and your kids aren't trying to light up the galley. The problem is that when the battery charger is done with its duties, and you are now back at 12.5 volts or whatever, your galley lights are now proportionately LESS bright than they were at 14.5V. So you paid for 70 lumens, and you're getting 50 or so.
While AutoZone doesn't know any better, Honda, Toyota, Ford, GM, etc do. They would never use resistor driven LEDs.
Marinebeam, Sensibulb, Bebi, FirstStar, Hella, Lopolight, AquaSignal, and IMTRA, all use constant-current designs, and all are much more expensive than the autostore bulbs. But they are all focused on the marine market, and know that resistor bulbs have no place on boats.
Constant Current bulbs are much brighter and last longer than resistor bulbs. I bought some $17 Ebay bulbs that lasted approximately 3 minutes at 13.5V.

SMD vs Parallel pin (thru-hole) LEDs:

For ultrabright LEDs there are two schools of thought, and both are
good solutions. Because LED with two parallel leads have to disipate
their heat via the legs, they are not super-efficient at transfering
heat. Also because they are enclosed in a clear plastic dome, they
can't disipate much heat to the surrounding air. So these LEDs
usually have lower forward currents, meaning that they can't be driven
as hard. So for these type LEDs to produce a lot of light, we use
more of them. With 21 of these LEDs, we can approximate a 10W
incandescent bulb in output without overdriving them. The problem is
we start encroaching on the space available to fit our fixtures. One
positive of the thru-hole LED is the plastic dome. It allows the
light to be directed like a lens, so 30 degrees thru 90 degrees is
possible. This makes it easy to make spots or floods without external
lensing. Thru-hole LED clusters are also more expensive to produce,
however, because they don't lend themselves to automation, and take
more labor to make.

Surface Mount Device (SMD) LEDs form factor allows them to disipate
the heat directly thru the substate, and the constituent materials and
manufacturing processes allow higher thermal efficiencies. The SMDs
we use generate less than half of the heat at the junction compare to
standard LEDs at the same current. They also don't have domes, so
they disipate heat into the air more efficiently. These LEDs can be
driven much harder and much brighter, so we need fewer of them. Our 4-
LED SMD G4 cluster with the heat sink puts out 20% more light than the
21-LED, and fits every fixture due to its small form factor. It is
the equivalent of a 15W halogen.

Other things to know about quality LEDs:

They should be CE rated, which means they don't produce electrical and RF interference and don't put out harmful UV (the cheapo bulbs will)
They should be RoHS compliant (lead-free)
They should have polarity protection (diode bridge). Many cheapo's don't, which means you must switch your wiring to get them to work.
They should use "binned" LEDs. These are LEDs that are individually sorted by the mfr for forward current and color. They are more expensive by far, but are necessary for consistancy and long life.
Don't go by looks alone. I know of 6 companies that are making identical copies of our 6LED bulbs, but all are resistor bulbs.
Don't be afraid to ask if the bulbs are constant-current. If the company's website doesn't make a big deal about the fact that their bulbs are constant-currrent, then they aren't.

The big problem is that the cheapo bulbs have raised their prices to
match constant current designs because most people don't know the
difference, so buyer beware.

Jeff