dual bright leds

Good answer. I have a question however.

Lets assume the voltage will never rise above 14 as if the regulator in the vehicle went bad and sky rocketed the voltage, there would be alot more to worry about then burned out LED's.

But that aside, if resistors are being used, and the LEDS would have no reason to draw more current then required for them to operate, would there really be a risk of them blowing?

I understand why a regulator should be used in this case as it would not be a good thing to have the tail/brake lights fluctuating in brightness. But if the LED's are rated for 12v, and resistor values were used that under any but the most extreme circumstance would prevent over 12v from reaching them, is this really a concern?

Just curious, not looking to dabate, I am no LED expert but do use them often and never really recall seeing one blow out.

It's always good practice to control the system in every aspect against what 'might' happen.  It's not uncommon for the battery voltage to rise and fall above 'normal' (13.8vdc - 14.4vdc) during normal operation. 

LEDs that are rated at 12vdc generally have resistors built in to them.  These resistors should provide enough protection against voltage changes (I would imagine they actually size them for 16 or so volts to be on the safe side).

However, when buying bulk LEDs (like I assume this guy is going to do) they don't come with any resistors at all.  When sizing the resistor you need to know your MAX voltage and the maximum current you want to run through them.  If you calculate your max voltage to be 16vdc you won't have to worry about 15vdc spikes.  If you calculate your voltage around 12vdc you are asking for trouble.

The other side of this is the guy doesn't know how bright he wants to run the LEDs.  He might buy 12kmcd LEDs but only run them at 6kmcd.  That will provide some headroom as well. 

I guess it all goes back to my post earlier saying that the electrical environment found in a car is absolutely horrible, and when designing electronics for cars you need to take that in to account and build in as much protection as possible/feasable for your application.

It's not the voltage that will hurt them, it's the extra current that comes along with it.

Ok I see that point. Ohms law. If the voltage increases, so will current. If the voltage doubles, so will current.

Bulk LEDS without resistors are probably rated at 2volts so I understand the concern.

Thanks again.

OK, with 1 set of 6 seried LEDs let's say you want to run 10mA to each LED.  You would want to set your resistor up to limit current to 60mA.  You are using 14.8VDC as your source voltage. 

R=V/I

R=14.8/0.060

R= 246.667 ohms

The closeset 'standard' resistor is 240 ohms

Now let's say you're driving down the road and shut your A/C off.  The quick load change shoots your battery voltage to 16.0vdc.  That same resistor is now going to flow 67mA, overdriving your LEDs by 10% a piece. 

Depending on how hard you are driving your LEDs this may be ok, but it will cause the LEDs light output to flucuate.

Now, the same system, with a 7805 from Radio shack (1A 5vdc voltage regulator) would give you a 5vdc system voltage that is going to be constant under all conditions (as long as battery voltage is between 7-30ish vdc).  You would want the same current, 60mA and would calculate your new resistor to be:

R=V/I

R=5/0.060

R = 83.3 ohms

82 ohms is a standard value, so that is what you would use on each 'leg'.

The 7805 is capable of running up to 1A of current, three legs would be about 180mA, well within it's ability, but when using it with a load over 100mA you should use a heat sink.

You are left with the same amount of power (watts) going to the LEDs, but now you are using a regulated power supply that will provide constant voltage/current to the load.