6V LED Question, (tail lights on F-100)

I have a buddy that has this F-100. We are trying to get these tail lights he has to work. They are a 6V system and he uses a voltage clipper to cut it down from the 12V to 6V. Question is that when brake is applied a full 6V is give to the LED's and they work fine. Well it has you using the same lights for the tail lights as well. I drew a simple picture to help me illustrate our issue.

So there is three rows going vertical in each tail light. This is an example of one row and is obviously bent instead of straight.

The question is this. When brake is applied and all 6V are sent to the lights all 6 light up fine. There is a resistor in the line (when brake is hit it bypasses the resistor via relay) for the tail lights to cut some of the voltage off and lower the available voltage. But instead of all 6 lights dimming, 2 of them just don't light up, and the other 4 stay bright. Why would it just drop 2 off and leave 4 bright? Yes that is only 4 LED's and is a hair dimmer than with the 6 but that's not what we are going for. We want all 6 dim for tail lights and all 6 bright for brake light. I am lost as to why it would drop two off when they are wired in series?

So for example using the picture all 6 LED's are bright when full 6V is given. If a resistor is put in line to say drop it down to 4V for dimmer tail lights why would LED #'s 5 & 6 quit working and leave 1-4 bright? Excuse me but don't remember if it was 5-6 dropping off or 1-2 dropping off. But you get the idea, wired in series how do 2 just drop off instead of all six just dimming? Any suggestions?

 

[asafley]

Hey Turbo,
Interesting project. Now I am no expert in LEDs but I do know some basic principles. Wiring in Series, does pose a problem in your case. LED in series do not do well when it comes to dimming. In your case, the LEDs in front are nice and bright because they are getting around 4 volts from supply voltage(Taillight situation), but that voltage will start to drain because each LED is like an in-line resistor for the next LED down the line. So the last two do not have enough voltage to light bright enough for you to see but are still able to complete the circuit. If you are looking to dim the light via voltage you should wire the LEDs in parallel. You still only need one resistor but each and every LED will get 4 volts or 6 volts and also they won't act like a resistor down the line compared to wiring in series. Therefore, they will dim for the taillights but will be bright for brakes also they will all be equally bright. I hope that makes sense.

Good LED resistor calculator

http://ledcalc.com/#calc

 

[turboexplorer]

That's a lot of what I was thinking. Doing a volt drop across each LED would be 1V with 6V source. So actual voltage available would decrease 1V after each LED. That would leave only 2V for the second to last and 1V for the last. But what I thought was weird was the first 4 look equally bright with no visible difference and it left a 0V drop across the last 2 LED's that was the weird part that is throwing us off.

May have to go into the tail light and wire the whole thing in parallel. Trying this 6 volt system because the truck has a generator not an alternator so it has varying voltage with engine RPM. Mainly low voltage at idle. So he may switch it over to an alternator and would solve half the issues.

Also any suggestions on how to power the lights differently then a voltage clipper? It gets way to hot even trying to run more than one row of LED's and running three is more wire and more money for clippers. Any suggestions?

 

[albi1cnobi1]

LED's are solid state diodes. You are on the right track, each one has a voltage drop across it but being solid state you won't notice the difference between each one. Only when it gets to the cut off voltage do you see it. They should be run in parallel not series to achieve what you're trying to do.
Sorry didn't read the whole post. What you can do is use an inexpensive 7805 5v regulator. As long as the input is above 5v you will have constant 5v out. You'll just have to lower the resistor value to compensate for the lower voltage. If you're not familiar with this I can draw you up a quick diagram if you would like.

 

[turboexplorer]

That would be awesome if you could!

 

[albi1cnobi1]

No problem. I'll do it when I get home from work and post it up for you.

 

[albi1cnobi1]

Here it is, a little crude. I did it quickly with paint, but you get the idea. You should be able to run around 8 LED's on 1 7805 but you'll have to experiment with the resistor value. You could probably run more but you're limited by the low input voltage.

Any questions let me know.

 

[turboexplorer]

Awesome! So the 7805 is a transistor, a PNP one. And on the V + IN side I can put in 12V and it cuts it down to five out? Or do I need to cut it down to 5V before it goes into the transistor?

 

[turboexplorer]

I answered my own question. It can handle up to 36V of input but the difference between the input V and the output V is turned into heat. so If we use a small aluminum heat sink we should be able to stick several LM7805's on there and they should be fine.

Can we use the LM7806's that regulate it to +6V?

 

[albi1cnobi1]

Sure. You just need to adjust the resistor value according to your input voltage. LED's normally have 3-4v input, so the farther you go above that what you gain in brightness you lose in longevity. (By input voltage I mean to the LED's not the regulator)

 

[turboexplorer]

Is there a good calculator for trying to find a ballpark resistor to use when they are wired in parallel?

Maybe:
http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator#parallel

I am lost on the LED resistance, because there is no set value of the resistance for an LED. Can you figure it by using OHM's Law and just by knowing the the source voltage and the amperage draw of each LED? Guess the better question is how are the LED's rated? Or does it depend on the LED's?

 

[albi1cnobi1]

A good ball park is 30 ma(milli amp) each. So you can use this in the ohms law calculations. They're all different but most of the decent ones are about that.

 

[turboexplorer]

What If I where to run a +9V regulator could I run 12 LED's per regulator?

The tail light has 36 total LED's in it and has three vertical lines at 12 LED's each. So If I could run each vertical on one regulator that would cut down a ton on wiring also make it easier to work the stop and turn lights as well. A +9V regulator would allow more available power to run the LED's? Right

So for the tail lights (DIM) setting would I want to run like 20 ma through them and then run the 30ma for stop and turn?

 

[albi1cnobi1]

You could use a 7809 but if I remember correctly you said your supply voltage drops as low as 6v because the truck uses a generator. If the input voltage dips below 9v the output will dip as well. I have an idea for a slightly different circuit to cover the stop/turn duties. I'll work on that today and post it up for you to look over. It will still be pretty simple and inexpensive.

 

[albi1cnobi1]

This should solve the stop/turn/tail light issue. However the low input voltage might still be an issue.