Wilsons Sandbox

[color=#000000]Seems to me that a single, heftier voltage regulator that provides a fixed voltage source in the 6-10 volt range could be a useful package for under dash mounting, set up to provide constant voltage for modules of a specified number of series-connected LED's at each one of a dozen terminals on the box . If the voltage is clamped to 7.8 volts, for example (to drive half a dozen LED's @ 1.3 volts each in series), the LED's could simply be used in bundled packages of six (or whatever, for appropriate ratings) wired in parallel (perhaps two bundles for each brakelight, one for each taillight, etc.). We ought to be able to avoid individual fixed voltage sources for each location on the car, as well as the jumble of resistors, and simplify maintenance in the bargain. There's got to be someone on the list with regulator or zener diode technology in his resume who can design a box for the rest of us to build so that we can treat grouped LED's like so many Lego blocks. How may T-ABC listers does it take to replace a taillight? All of us. Austin [/color] [color=#0000ff]Naahhh, we can do it. I've got a deal going with Messrs. Volta, Ohm, Ampere, Maxwell, Gauss and Faraday. With them on our side, we can't miss. Using a fixed voltage to power LED's would, as you suggest, make things simpler all around, except for one problem. All LED's are not born equal. Just as some people in our touchy-feely PC society, some LED's are more equal than others. The problem comes from several variables, two of which really matter. First, various types of LED's have significantly different internal impedance, and second, have slightly dissimilar forward (junction) voltage. This means that minor variations in voltage source can cause wide variations in junction current. Consequently, even with a fixed, regulated source, it would still be necessary to use a series resistor, although, it would not have to absorb more than something like 10% of the source voltage. Hence, the truly proper, and easiest care and feeding of LED's is with a current source. [/color] An analogy for this goes like this. Imagine you have to suspend an object above the ground, But the only available suspension point moves up and down. So if you suspend the object by a cable to that point, it will too will go up and down. If you tie a second cable from the object to the ground in order to keep the object in place, then the tension on the cables will be limited only by the force the suspension point is capable of exerting. The cure is to replace the cable between the object and the suspension point with a spring, so as to keep the tension on the cable below the object more constant. As the spring absorbs the variation in the position of the suspension point, the current source absorbs the variation in BOTH the available voltage AND the small variations in LED characteristics. A current source for this kind of application is a very simple device that uses less than a dollar worth of parts, exclusive of the LED's. A sloppy schematic is attached. You can comfortably wire this up on a piece of project board about one square inch in area and mount it inside the tail light assembly or under the dash. To package it, you can get fancy and mount it in a metal box with terminals, or you can put it in a little match box with wires hanging out and encapsulate it with Bondo or Epoxy and drill a mounting hole anywhere that won't hit anything but the encapsulating material. Need I say, test it before you pot it? The transistor is any half amp rated, AF (audio frequency) PNP with a minimum Beta (gain) of 50 at a collector current of 100 Milliamperes. It should be well under a buck. The diodes are small signal silicon such as 1N914 and generally go for 5 bucks for a bag of 100. Resistors are also near free at 3 bucks per 100. LED's can run anywhere from 20 cents to a several bucks each, depending on color and output. You can even get white light output LED's if you don't care about your money or have an unreasoned hatred of tungsten lamps. Check out the Electronic Goldmine in Scottsdale AZ. They usually have bright red jumbo's for around a buck. Cheers, Stan TC2569