L e d s tail lights on my 64

Dan:

There is no question that the quickest way to destroy quality is putting a Harvard MBA in charge of engineering. However even a good manager will soon find out that managing engineers is like herding cats.

Given that there is no source locally for the stanley lamp, wht exactly makes the sylvania lamp "junk" ? Short life?

Sadly, most of the aftermarket LED lamps are garbage. That first link is typical, and a fine example of doing it wrong as far as LED's go. The second link looks like better equipment - I see an actual heat sink. I don't plan on spending 90 bucks for one to take apart & see if they used an actual driver circuit.

The problem is very basic. LED's are low voltage devices, with a narrow operating voltage range. The standard cheap answer is a series resistor. That does not work well at all in an automotive environment. With a fixed voltage coming in to the resistor, you get a fixed voltage out to the diode junction. Great in a computer or home stereo. In an automotive environment, the voltage can swing from 15v on a cold start to 11.5v on a very bad day. So what value do they pick for the resistor? Well, one that will give 10k hours of life @ 15v of course. That means that light output suffers under normal conditions, and really sucks when you need it most.

What is missing is a driving circuit. If the LED needs 3.3 to 3.5v @ 20mA, and you use a resistor of 680 Ohms, you get 17mA @ 3.4v from a 15v feed. Great. The LED will last 30k hours. But when the system voltage drops to 13v with that resistor, the junction current drops, and you give up some light. You should really have a 480 ohm resistor at 13v But that would provide too much current to the LED at 14v

You also piss away quite a bit of power in that resistor. In the above example @ 15v, the LED dissapates 58mW, but the resistor dissapates 198mW - as heat.

A proper driving circuit can provide a stable 3.4v with an input range from 9 to 18v Moving the driver outside of the housing leaves any heat it generates outside of the housing. A 3W LED can be very bright - around 120+ lumens. Incandescent lamps just can't match that. Even a 10W incandescent driven at the 1000 hour life rate is not as bright. With more sophisticated driving, that LED can be driven to make it appear even brighter.

Should be a fun project.

B.