No resistors in series with the groups?
I suggest 3 LEDs in series, and 180 Ω in series, or 2 in series and 360 Ω. Have as many strings in parallel as you want, but each string has its own resistor and no LEDs should ever be directly in parallel with another.
The forward voltage for LED is not the voltage that you should run them at. The current is what you should run them at, and the forward voltage is what you should allow for.
If you alter the supply voltage a little bit, the current changes a lot, so the variations of voltage between the engine running and not are enough to go from the LEDs being off to being burned out, with 4 LEDs and no resistor. With 3 LEDs in series you will get quite a bit of variation, and with 2 LEDs the current won't change significantly.
I've recently converted some car interior lights to white LEDs. They have the additional complication of being dimmed rather than switching off. For the first, I used two LEDs in each string and a simple resistor, but of course the LEDs turn off abruptly as the voltage drops below 6 - 7 V.
On the second, I used a boost converter to increase the supply voltage and I ran 5 LEDs in series. 5 is enough so that they won't turn on from the battery voltage alone. The boost converter is arranged to control the current rather than the voltage. I also made that current proportional to the supply voltage, so that the light dims as the voltage is reduced by the other electronics in the car.
The other advantages of a boost converter are that the current can be accurately controlled, so that it doesn't change with supply voltage, and the efficiency is better as there is little or not voltage drop in resistors. The circuits are more complicated.
Measure your cheap LEDs and you will see that only a few might have a forward voltage of 3.5V. Datasheets for LEDs always have a range of voltages because they cannot make them the same.
The ones that burnt out had less forward voltage than 3.5V.
If you want maximum brightness then you apply too much current but then they won't last very long (1minute? 5 minutes?).
Thanks very much. I don't want to add extra weight in the car so i won't be running a controller. So the best thing to do is to take measurements of current along the leds when the engine is running, so i can find a resistor value which is stable at all times. Also is there a specific current that gives maximum luminosity(and probably lower lifetime) other than the recommend 20mA? Also can you explain me the effect of the led flickering?
The boost controller for a few LEDs at 20 mA only weighs a few grams. You can also have a constant current circuit which would weigh even less.
The resistance of a resistor is generally very stable. Don't run resistors beyond half their maximum power as they get very hot at maximum power. The voltage will vary when the engine starts and that will vary the current a lot more.
For example, if you go with 3 LEDs and 180 Ω, when the engine stops and the voltage is lower, say 12 V, the LED voltage will be much the same, at about 3 * 3.5 V = 10.5 V. That only leaves 1.5 V across the resistor so the current will only be 8.33 mA, while it is 19.44 mA. There is less current variation with 2 LEDs, and virtually none with a constant current circuit or a boost controller.
The light from LEDs is virtually linearly related to the current. So at 10 mA you get just about half the light that you get at 20 mA. You probably get slightly more than half the light at 10 mA. You can run them at any current you want. It is a trade off between brightness and life. If you never go above 15 mA they will probably outlast the car. If you go to 25 mA they may not last for hours.
As Audioguru points out, there is a range of voltages, and if they are low and you use a resistor there will be more current, which is why a larger value resistor would lead to more reliability. It is another reason to use constant current circuits.
I wouldn't worry about the flickering or other funny effects. You put the LEDs in a circuit where there was too much current some of the time, and they died. The fact that they didn't die suddenly isn't relevant. If you treat them correctly they will work for years.
You must put current control in each series string of LEDs. It makes no difference where in the string you put the current control. If you want to use SM resistors, that's fine, and one might take the power, but you should go to 1206 size or larger.
If you are using smaller ones it would be better to use a few to spread out the power. The resistors can be in series or parallel, but you need to adjust the resistance either way. You can have the resistors anywhere in the string.
Those two regulators are buck regulators, and they are both designed for higher current systems than you are using. The first is adjustable between 40 mA and 600 mA, and the second is fixed at 1A
Hello again,
The resistor selection in theory is always:
R=(Vs-vLED)/I
where
R is the series resistance,
Vs is the source voltage,
vLED is the total voltage of all LEDs in the string,
I is the required forward current.
That's the theory, but because of the way many voltage sources work (they vary a little bit) the way we have to apply this is to follow up the calculation with a quick sensitivity analysis. The idea is to figure out how sensitive the current and the light intensity are to a change in source voltage.
The critical point for the current is when Vs goes to its highest level, and the critical intensity point is when Vs goes to its lowest level. If you can stand the decrease in light when Vs goes to its lowest level then you are ok with the intensity, and if your LEDs can handle the current when Vs goes to its highest level then your LEDs will be ok with the highest expected current level.
If you want to do a thermal sensitivity check too then all the better, but combining this with the voltage sensitivity works well. Figure the LEDs forward voltage each changing by -2mv/degreeC, so worst case is with Vs high and temperature T high also. If they survive that then they last for quite a while. Worst case light output is with temperature low and Vs low also...if you can stand the decrease in light output then this design is a go.
The problem with some LED products including traffic lights is the right information was not conveyed soon enough for some designs. The outcome of this was usually early LED failures. The manufacturers seemed to act like the LEDs could never fail and designers did not pick up on this until it was too late.
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