help : Sperflux leds burning out in my heahlight

[dineane]

HI .
I have 12 superflux leds in my car headlight in paralel with 12 ,180 ohms at 14 volts.
Each led has a resistor on them . The superflux led is a 7500k at 3.2 volts and 80ma 1watt for 12&14volt and 0.5watt or 9 volt .Now I had then in the headlight for the summer and they are great , and now that it is colder (45 degrees) the leds are starting to burn out. Now I realized that the leds might not be getting a steady 14 volts from the car ,the voltage keeps going up and down from 10 to 12 to 14 to 13 (get what Im saying). Do I need a voltage regulator to keep the voltage steady and if so what type and at what voltage is good 9v or 12v , and will I have to put 1 on each led or can i just use 1 for all.............

I know there are good tech out there please I need some information

THANKS

 

[ben7]

You will need a voltage regulator or a led driver

 

[dineane]

What type and how many voltage regulator ? led driver ?

 

[ben7]

You could use a 7812 12volt voltage regulator.

A led driver drives the led at a set current, and keeps it at a set current when the input voltage drops or increases.

 

[dineane]

so if I go with the 12volt voltage regulator can i put 1 for all 12 leds or 1 for each led ?

 

[ben7]

One voltage regulator should be enough for all the leds (6leds on each vreg.)

Total current for twelve leds will be 920mA, so a 1.5A regulator will handle 12 leds each without being very stressed.

 

[o07wray]

make sure you use a to-220 case 7812.. and you might think about a heat sink too. cool winter will keep it from over heating.. but summers under hood temps will somke em'..

 

[dineane]

what is a 220 case? were can i get one ?

 

[dineane]

Ok found it.But what if the voltage goes under 12 volta what will happen? Is a 9 volt better?

 

[MikeMl]

Your problem is NOT voltage regulation. The highest voltage your battery/alternator ever gets to is ~14.4V. Adding a regulator that drops a regulated voltage of 14.4V to a regulated voltage of 12.0V is going to do what??

The root cause of your failures is almost certainly inadequate heat-sinking of the LEDs. How are they mounted? What are they mounted on? Where are the current limiting resistor mounted? Inside the headlight?

Driving each LED with its own current limiting resistor is a poor design. Much better to run three LEDs in series, and then use one current regulator for the string of three.

 

[audioguru]

A Philips Luxeon SuperFlux LED has an absolute max current of 70mA only when it is cooled to 25 degrees C somehow.
Your LEDs have no cooling so they will melt.

You also don't have any current-limiting so again your LEDs will melt. LEDs work on current, not voltage like light bulbs.

I use Philips SuperFlux LEDs as nightlights at "only" 53mA and they are extremely hot.

 

[dineane]

the led can range from 5 to 9 to 12 to 14 volts .So I thought the led isn't getting a consistent voltage cause a car voltage goes up and down.Yes the voltage is 14 v at its highest but a car voltage goes up and down . Remember every time you turn on something in a car the voltage goes down the back up .

The leds are in the headlight . They are mounted in a plastic cap that fits nice snug .The resisters are 8 inches from the led raped in heat shrink.Like i said before the led is perfect in the summer but always fail in the winter .

 

[dineane]

What do you mean current limiting ????????

 

[MikeMl]

the led can range from 5 to 9 to 12 to 14 volts .So I thought the led isn't getting a consistent voltage cause a car voltage goes up and down.Yes the voltage is 14 v at its highest but a car voltage goes up and down . Remember every time you turn on something in a car the voltage goes down the back up .

You obviously have never measured the voltage at your car battery. When parked, the voltage will be 12 to 12.6V. With the engine running, even at idle, the voltage will be 13.5 to 14.5V. A few minutes after a start, the alternator will bring the battery voltage to 14.25V +- 0.1V. This will vary a little with ambient temperature, the hotter it is, the higher the battery voltage.

If you designed your current limiting resistors for an input voltage of 14.4V, what do you suppose would happen if the engine was stopped, and the battery voltage sagged to 12V? Answer: nothing, except the LEDs would be a little dimmer, they would run cooler, as would the current limiting resistors, and they would be less likely to burn out...

The leds are in the headlight . They are mounted in a plastic cap that fits nice snug

There is your problem!!! The LEDs must be mounted on a copper heatsink. Download the data sheet or application notes from the LED maker's web site. They will specify how the LEDs are to be mounted. The LEDs produce heat (lots of it) when lit. Having them bottled up in a closed container is very bad! Not having them mounted on a large metal substrate with many square inches of area per LED is bad.

.

The resisters are 8 inches from the led raped in heat shrink....

The resistors are dissipating even more power than the LEDs. If you start with 14.4V and the LEDs each have a forward voltage of 3.2V, then the resistor must drop 14.4-3.2 = 11.2V Since the current through each LED and each resistor is supposed to be 0.09A, the resistor should be 11.2/0.09 = 125Ω. The resistors are dissipating 11.2*0.09 = 1.008W, so they should really be 2W resistors. What are you using?
Wrapping a 2W resistor in heatshrink will reduce its ability to dissipate heat, and will likely start a fire...

Using one resistor per one LED is very wasteful. I would connect three LEDs in series with one current limiter (resistor or more complex current limiting circuit)

 

[ben7]

I think he understands now!

 

[dineane]

I understand ............
The led is failing cause of the cap on the back which causes the led to heat up (cause it breaths from the rear) . So if the rear was open, the led wouldn't get so hot right.
And what can I use to dissipate the heat from the led better?

 

[alex69]

Ok. Let me know if I understand:
You have led look like fig.1? Right?
If it's so, you need a led (or only the heatsink) like fig.2 so you can mount it on a big heatsink with screw and conductive past.
Second thing (not less important) you must use a driver (small, functional and cheap) for leds. So you have tension and current stable.(remember a heatsink also for it).
Alex

 

[audioguru]

No.
Philips Luxeon does not make a white SuperFlux 4-pins LED but make all the other colours. the max allowed current when cooled properly are 70mA for red and 50mA for blue.

Chinese "superflux" LEDs are probably white. Who knows what is their max allowed current? Who knows what is their voltage range?

SuperFlux LEDs are cooled through conduction of their 4 legs to a large copper part of a pcb. Philips datasheet spec's the amount of copper area required on the pcb to cool down to cetain temperatures at various currents.

For readers who didn't bother to look-up SuperFlux LEDs, the 4 pins are in a 0.2" square, here is a photo:

 

[dineane]

here are the leds I'm using:
**broken link removed**

 

[dineane]

I will post the data sheet soon