Zeners conduct if the back bias exceeds their zener voltage. Here is a zener diode, a Schottky diode, and a zener diode circuit. They make good voltage regulators.


Keep in mind that when the engine is running we now have close to 14 volts not 12v. a 12v zener (11.4v) leaves us 2.6v over limit to conduct @ 140ma almost two watts going straight to ground.
Could we add a regular diode or two to drop the excess voltage before the zener?

 

The whole point of the circuit is to regulate the voltage from the car so the LEDs don't vary in intensity. This circuit stands as is, does what it is supposed to.

It does this by creating a super simple voltage regulator in front of the diodes. The circuit is saturated if the car voltage is 12V, but as the voltage goes up it prevents this increase from reaching the LED circuits.

I don't know much as I would like about cars, but I know my way around diodes and transistors. The problem with this design is tolerances. The zener has a ±5%, better is needed. That is to say, the fixed voltage of a particular diode can be too far off to be usable, if it is on target then it is fine.

 

When a vehicle roles off the assembly line and reaches the dealership there are already at least 100 or more TSB related faults. Some are electrical safety faults that turn into fire hazards and recalls. Are engineers perfect?
I was addressing a different protential broblem area before the current even has a chance to reach your transistor. I seen a posibility of slowly cooking the wire from the brake\tail switch to the lights. What you see is just making the lights work efficiently. but hey, You're the expert here. That's ok, we all know that engineers and technicians don't get along very well. Never did and probably never will - It's just our two different natures. Nonetheless, I am here to be humble so I can learn. Being a tech for all my life has taught me to always be humble, open minded, and respectful towards others thoughts and opinions. So, until the day I am made perfect, that will be my attitude.

I understand the prupose of the zener and how you want to regulate the voltage (they do make regulators) but as the voltage increases your circuit prevents this increase to the LED circuits, so where do you want this increase to go? Straight to ground? A diode in series before the zener will decrease the voltage before the zener by 1.5v thus decreasing the excess current to ground. Adjust your branch circuits for 14 volts instead of 12 so that your saturation is at 14v. Other than that, I think the circuit is awsome and will do just fine.

 

I started to get angry with that last post, there seemed to be a bit of excess sarcasm, but I tweaked this reply after I cooled down. The transistor drops the excess voltage by adjusting its internal resistance. It will output 11.4V, regardless what the input voltage is. Worst case (assuming 13.7V) the transistor will drop around 2.3V. If we use the schematic I drew it will pull around 0.12A (hopefully worst case). This works to around ¼W (2.3V X 0.12A), which is within the transistors rating. There are higher wattage transistors out there if it isn't good enough. Where does the extra voltage go? The transistor dissipates it as heat. If it doesn't have to drop the voltage (12V input) it doesn't generate heat.

I have been an electronics tech for over 30 years, this is pretty basic stuff for electronics. I have described the circuits deficiencies (the diode tolerance is major), but none match what you seem to be stuck on. This pretty basic for a solid state voltage regulator.

If you are interested in learning how it works I'll explain in further detail. Look up a common collector for a key word search, or read up on transistors in the AAC book.

 

Well Im not an expert but still a suggession..Get Diffused Lens type Leds rather than the convex lens type.. The Former scatters the light for a brighter view whch would do more under the tail cover... And when I wired my car for a blue led rail instead for Neons, I couldnt get that type of leds, so i filed off the lens and connected them parallel to an lm317 set at the correct voltage.. LEDs are Still good with no probs at all and no volatge drop according to battery volatge.. Just a suggession..

 

The LM317 has the same problem as the 78XX series, it drops at least 2V reguardless. If you're trying to squeeze every volt out then that is excessive. It is a good part, but you have to know the limitations of the components. If you plan on using a LM317 then the max out for a car is around 10VDC. It will still get hot when the voltage goes to max, just like the transistor.

Defused LEDs are good for this application, no arguement there.

 

Bill, I already knew (as I had mentioned) by reviewing the threads posted that you have your act together. I also knew that there is no way that the wiring would have any danger with the watts it was producing. However, it is sometimes a good thing to get someone fired up after being cool for too long. My boss as a way of doing the same to me - he knows exactly what buttons to push! I get so mad it makes me shift into high gear and psychologically\scientifically causes my brain to think in a totaly different mode. Somehow I become smarter and sharper. pressure produces diamonds! It was awesome to read your reply with such great detail and explanation of the circuit. Sometimes a good technician will become too relaxed and looses his sharpness or better to say, he doesnt apply it as he should. They taught us this psychological effect when I was in school. It works! Even on you!
It is ok to be angry, we have that emotion for a good reason. The important thing is that neither of us ever got out of controlling it. No cussing or swearing, no fighting, etc. Just a tad smarter than we were before.
Thanks Bill, keep up the good work, I look forward to reading more of you posts, you're just a genius and I'm picking your brain. No hard feelings here, lets be friends! C-ya!

 

Taint no problem on this side. I do make mistakes, and know it. When I get caught I admit to it.

LEDs are a favorite subject, I lika da blinky lights. I also like drawing schematics, sometimes when I'm too tired to do it right. If you have anything you need to work through just ask (on another thread). No genius, just a tech slogging through.