A LED (Light Emiting Diode) are basically a semiconductor that emits light.It is used in a varity of applications like telephones, pagers, toys, etc. If driver curcuit is designed properly, a LED will last about 100 000 hour.
LEDs used in battery powered applications is normally selected from the highest brightness per power consumed. Use of LEDs in an automotive application demands other requirements: vertical/horisontal opening angle, color and wide temperature range (-40 to 90 Celsius). And it all has to be bright enough according to current regulations under DOT or ECE. LED brake and signal lighting offer a safety advantage over regular filament burning lamps since their brake lamps can achieve full brightness much quicker than their filament counterparts, and the LED turn signals demand more attention by ignorant drivers since they have a very 'crisp' on-off blink to them, rather than the filament's 'lazy' flutter-like flash that has been standard since taillights were implimented.
Lets look at specs for a few typical turn light bulbs. These are good
old filament ones:
Amber 1157: 24 cd x 12.57 => 302 lm
Amber 3357: 30 cd x 12.57 => 377 lm
White PY21W: 38 cd x 12.57 => 477 lm
Filament bulbs emit light more or less in all directions with same
intensity.
The aftermarked LED bulbs scam
There is several bulbs available for automotive tail, corner and city
lights. However, most of them suffers from serious design flaws:
Lets pick one of the brightest amber LEDs on the marked with a easy
obtainable 30 degree opening angle. It would have around 5000mcd => 5 *
.00024 * 30 x 30 = 1.08 lm.
(Formula: Lumens = Cd * .00024 * view angle^2)
Theoretically, the light output from 300 to 450 pieces of 5mm LEDs should give the desired brightness. As a contrast, a typical "drop in" LED bulb replacement for a 1157 (or similar) bulb contains 15-24 pieces 5mm LEDs. The conclusion is simply that light output is way too low. Some manufacturers try to compensate that by selecting
LEDs with low opening angle to increase straight ahead output. It may help somewhat,
but vehicle signal lighting must be visible for anyone on that side of the vehicle, not
just a skinny sector! You are seriously harming your driving safety with these cheesey "drop in" bulbs.
As a theoretical sidenote, a few hundred 5mm LEDs for a few dollars
each easily goes beyond most car nuts budget for LEDs. Can you imagine
the time it takes to solder them all together??? Also they would need to
be aimed. 60% should typically be aimed straight ahead, and the remaining should
be aimed evenly out to cover at least 120 degree viewing angle.
The second, often overlooked factor is that they only come with series resistors to decrease current to around 15-30mA, depending on LED brand/model. Most cars have voltage spikes on the 12Volt system created by solenoids, electrical motors, and relays. These spikes will decrease life of LEDs substansially, if only series resistors are used.
The third factor is that LED bulbs can overheat. Max current for LEDs are given at room temperature. In an automotive application current has to be reduced 10-60% depending on temperature and LED type. Whether or not aftermarked LED bulbs will withstand the heat on a hot summer day can be questionable. A dark car can get very hot. If you plan using LEDs on the hood or inside headlamps, there is even more heat to consider.
Example: diagram that shows max current vs. temprature of the 5mm "AND 157HAP LED":
Other brands and model will be similar, verify the datasheet of your
LEDs.
Retrofitting multiple LEDs into any lamp that was ment for a single filament bulb will always get some weird beam patterns.
There is 3 main designs out there:
Thermal management is chapter for itself. LEDs create heat, and the more current you send through them, the closer you mount them to each other, the bigger the amount and the warmer the environment, the more important it gets to consider it. Its the life that is going to be affected. A serious supplier of LEDs will have a document describing all factors of this. Even plain jane 5mm LEDs driven at 20mA gets noticably warmer when 6 or more placed close to each other.
Normally no. That would be assuming that system voltage on a vehicle is always 12V. Nominal voltage is 12V. If you have a voltmeter in your car for one year, you will see that it varies from 10-15 volts, depending on temperature outside and the load of the electrical system. If you connect a oscilloscope, you will see that there are peak pulses in the range of +/- 300 Volts on top of it from time to time. One series resistor is not able to continously create constant 20mA (or 40, 100mA or whatever you have decided to run). The suffering component is the LED, which will degrade 5-100 times faster. If your LED is rated for 50000 hours, it will only last 500-10000 hours, so how can you tell? That many hours on a car is a lot of miles, but it is still a design flaw. When designing any driver for a car, it must have the voltage range of 9-16V, with peaks +/-300 Volt. Both SAE and DIN organisations has documents that discusses voltage in vehicles in more detail.
Example #1: Spike eliminating driver curcuit for 2 LEDs. Price of these
parts should not be more than a few dollars:
Example #2: Spike eliminating driver for multiple LEDs. The cost of
driver is probably less than the LEDs themself:
The serial regulator (7805) should be the 1Amp version. Examples of
full part# is LM7805C. Also LM341 can be used, but note that this only
0.5Amps. Many other 5V/1Amp serial regulators from other manufacturers
can be used as well. For the pin config (data sheet) on LM7805C, goto www.national.com
and type in part #. This part can be ordered from Digikey.com
if you do not have any electronic parts store nearby.
The resistor (R) must be calculated from this formula: R=5-U(diode)/I.
If your LED has a voltage drop of 3.6Volts at 0.022Amps, R=5-3.6/0.022
=> R=63.63ohm => Choose R=58 ohms. Wattage of the resistor can be
either 1/8Watts or 1/4Watts. Any larger wattage has too big physical measurements
and gets clumsy.
Example #3: Spike eliminating driver for multiple LEDs
The data sheet is at http://www.national.com/ds/LM/LM1117.pdf and this
example is taken pretty much straight from there. The simplest version
to use is the one called LM1117T-ADJ, which comes with 3 easily solderable
terminals. This has a price of $1,16 from digikey.com, and it becomes much
cheaper in quantities. The low dropout voltage of only 1.2V means the output
can be as high as 10.8V with a 12.0V input. Note that if you have underdrive
pulleys or during starting, you might not always have a minimum voltage
of 12V, and the LEDs go dimmer. As the drawing shows, this is perfect for
putting 3 LEDs in series assuming a 3.6V drop per diode. The regulator
can supply up to 800mA current at 75F room temperature, so assuming 25mA
per LED, this driver can support 32 strings of 3 LEDs each without any
additional resistors needed. Quite a large array. The bypass caps are tantalum,
as suggested in the datasheet. Also, the regulator is supposed to withstand
spikes up to 20V on its inputs, so I haven't included any protection diodes
on the circuit. The LumiLeds appnote suggests that spikes could be much
higher than that, and so a protection diode may still be a good idea.
Thanks to Howard
Lumileds has a very good in detail discussion of LED drivers. Go into library and look for "AB1149-1: Using SuperFlux LEDs In Automobile Signal Lamps".
The simple series regualtor described above does the work, but can kill the efficiency. In worst case, a LED setup may draw more wattage than incadscent bulb. This is typically what happends when you start using linear drivers on Luxeons. A much more efficient driver is a switched driver. Basic functionality is simple. It works like a pulsating unit. Imagine turning power on, power off, power on, etc, for a period of time. The result is that you have used half the current over this period of time. Speed this up so that it switches a few hundred times per second, and you cannot see light is flashing, and only half the current is running. Imagine now that you increase time on versus time off and current is further decreased. All in all, pretty neat technology. You might have read that there is two main types of switched converters out there. First we have the "buck" type which is ment for LED clusters needing less than 12V, then there is "boost" type that is ment for LED clusters needing more than 12V. There is no reason to pick the one style over the other, either one will do. In most cases you can choose yourself. If you have 6 LEDs in your cluster and each needs about 3 volts, and u want to put them all series, then you need about 18V, so select a boost style. Another way of accomplishing same light output is to arrange two chains of 3 LEDs in series. 3 x 3V is 9V, which is less than 12V, so you need a buck style.
What is on the marked:
You may wonder why I mention dimmers here. Dimmers are most commonly used for dimming household lighting. In automotive lighting, dimmers can be very useful. In situations where you want to have more than one brightness level from a LED array, a dimmer is ideal. One example of this is brake and tail light function. A brake light function is several times as tail light function. In detail, with incandescent bulb, wattage would be about 21-37W for brake light function and 4-7W for tail light function. From a solutions standpoint there a are three ways to approach this:
What PWMs are on the marked:
Certainly! They are the brightest LEDs currently in the world, has good quality and used by OEM.
The "Star" come in lots of types, but the most important is:
"Superflux": around 0.1W and 1.7 - 4.5 lumens. Single package. Does not exist in white, but 3 LEDS, 1 red, 1 green and 1 blue LED together will produce white light.
For retrofitting automotive tail, turn and marker lamps it makes alot of sense to steer away from the 5W versions. Light output is too concentrated to make intensity even. One 5W lumiled would make an intense bright spot on lense. Even the 1W version might create spots on the lens. The "superflux" leds is probably the best choice. They are used in some OEM applications. A "side" emitter may be a good choice as well, but u will be limited to using one. It is a side emitter, right?
LEDs are just a light source and needs optical components to keep light where you want it. If too much light is bleeding off in unwanted directions, more LEDs are needed, cost goes up, power consumption goes up, heat sink gets big and fabrication time increases. OEM normally uses a single parabolic reflector around each LED. This is cheap for them to make because of volume, but creating or reproducing them in the garage is not economically feasable. 5mm diameter LEDs comes with a built in lens that will narrow down the beam. A low angle beam LED can relativly easy be made into a wider angle by simply making shape of the cone flatter. An oval beam can also be made by shaving in one direction. A low powered hand sander with around 150 grit will do this easily. Be careful not to overheat LEDs. If you get finger burns, the LEDs have been burned too. Finsh with one or two layers of clear coat to bring back original gloss to lens surface.
For the well known Luxeon Star, there are some optical components available:
Grote™ LED 4" universal truck modules,
(red: P/N 53252-3) provides legal candela at all the required angles. To emphasize: These
are not cheesy, illegal 'twist-in' taillight bulbs that the 'bulb peddlers'
sell. These are the real thing. Because of the 4 inches diameter, they
will fill up the area of the old reflector on many cars. If original reflector is
still not covered, use 2 beside each otherl. Which means that it will not be a small
"dot" illumination on the glass, but the entire tail light glass will be
lid up. And they will be brighter than the stock filament pattern that
was there. They will typically be available where truck parts are sold.
A quick search on yahoo.com in July 2003 showed that these can be bought over the
internet for $18 each from westfleet.com. That price should set a quick stopper
for home soldering modules and not to mention "bulb peddlers".
Another good quality truck lights are
AL Lighttect
This is actually a normal respons from turn signal curcuit. The curcuit is load sensitive and this is the way to tell the driver that one or more bulb is out. LEDs draw less current than filament bulbs, and it "sounds" like a bulb is out. There are 2 solutions to this:
(Sorted in alphabethical order)
Copyright 2001 - 2005 Automotive Lighting FAQ