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white leds problem(flickering/die:S)

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Hello again,


Well strictly speaking i wasnt talking about spikes really, i was talking about the normal fluctuation that occurs in an app like you would find in an automobile. Spikes are also of concern though, and a capacitor is often a simple protective solution or a zener and capacitor. The cap and/or zener is used with a series resistor to dampen the spike and protect the LED(s).

For the normal fluctuation though it could go from around 11.5v to 14.5v so you should analyze your circuit for at least those two extremes.
If you can afford the power and you dont have too many LEDs then a single resistor for each LED is a good idea. If you dont want to use as much power then you can start to string them in series. One resistor per series string works pretty well. You can put two LEDs in series or possibly even three, although the more overhead voltage you have the better the regulation between extremes will be (more constant light output with change in source voltage).

I wrote an article for the Washington Post a while back on a similar application where the voltage can change significantly (photovoltaic) and it also addresses the power dissipation with higher power LEDs. It's on the web somewhere too but im not sure where anymore.
 
Hello again,


Well strictly speaking i wasnt talking about spikes really, i was talking about the normal fluctuation that occurs in an app like you would find in an automobile. Spikes are also of concern though, and a capacitor is often a simple protective solution or a zener and capacitor. The cap and/or zener is used with a series resistor to dampen the spike and protect the LED(s).

For the normal fluctuation though it could go from around 11.5v to 14.5v so you should analyze your circuit for at least those two extremes.
If you can afford the power and you dont have too many LEDs then a single resistor for each LED is a good idea. If you dont want to use as much power then you can start to string them in series. One resistor per series string works pretty well. You can put two LEDs in series or possibly even three, although the more overhead voltage you have the better the regulation between extremes will be (more constant light output with change in source voltage).

I wrote an article for the Washington Post a while back on a similar application where the voltage can change significantly (photovoltaic) and it also addresses the power dissipation with higher power LEDs. It's on the web somewhere too but im not sure where anymore.

Thanks for the clarification. Mostly the industry uses resistors only as i can see, and a driver if there are many leds...for my simple project i'll stick with resistors only, or resistors with lower resistance than the required one and a led driver to regulate the current.
 
Hi again,

Oh yes a regulator is always nice :) Many people love the LM317 as it is so simple to set up as a current regulator given enough overhead voltage. Still a capacitor or two and you should be good to go.
 
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Hi again,

Oh yes a regulator is always nice :) Many people love the LM317 as it is so simple to set up as a current regulator given enough overhead voltage. Still a capacitor or two and you should be good to go.

i've searched and is seem very popular! **broken link removed**
i just adjust it a the circuit above? It seems to regulate voltage only and not current..what i want is for every row of leds to be passed by a 20mA current...though it can be achived by setting the correct voltage, if i have instability on the resistance of leds then it would allow more current to pass from them and burn them...how to i solve this?
 
Hello again,

Yes in automobile applications temperature often deserves reasonable attention so LED characteristic voltage (and thus dynamic resistance) would be a concern.
To set the LM317 up as a current regulator, you leave R2 out and take the load from the ADJ terminal rather than Vout. The output current is then equal to 1.25/R1. For 12.5ma output for example resistor R1 would be made equal to 100 ohms, and the load (LEDs) would connect to the ADJ terminal and ground (where R2 is now). The limits in current are something like 10ma minimum and 1 amp maximum, but the maximum also depends on the power dissipation in the LM317. If the LM317 gets too hot it will cut out and the LEDs will go out, so either proper heat-sinking or low current is the key. Another concern is proper ventilation for the LM317...it has to at least have free air ventilation and shouldnt be all boxed in.
 
Last edited:
Hello again,

Yes in automobile applications temperature often deserves reasonable attention so LED characteristic voltage (and thus dynamic resistance) would be a concern.
To set the LM317 up as a current regulator, you leave R2 out and take the load from the ADJ terminal rather than Vout. The output current is then equal to 1.25/R1. For 12.5ma output for example resistor R1 would be made equal to 100 ohms, and the load (LEDs) would connect to the ADJ terminal and ground (where R2 is now). The limits in current are something like 10ma minimum and 1 amp maximum, but the maximum also depends on the power dissipation in the LM317. If the LM317 gets too hot it will cut out and the LEDs will go out, so either proper heat-sinking or low current is the key. Another concern is proper ventilation for the LM317...it has to at least have free air ventilation and shouldnt be all boxed in.

It seems that is better to go with a driver module this way which is adjustable and standalone...then i will not have to worry so much...i could also add a variable resistor in R1 and fine tune it...if i run 20 rows per 3 leds i will need 0.4A to run them so it will probably over-heating since it has to be covered to be waterproofed:s
 
Hello again,

Oh ok if you think that would be easier, and you are probably right :)
When something has to be enclosed completely sometimes the heat sink is mounted such that the fins are on the outside of the box and a gasket can be used between the heat sink metal and the case metal with a hole for the part mounted on the other side of the heat sink (inside the box). Just thought i would tell you in case you are feeling adventurous.
 
Hello again,

Oh ok if you think that would be easier, and you are probably right :)
When something has to be enclosed completely sometimes the heat sink is mounted such that the fins are on the outside of the box and a gasket can be used between the heat sink metal and the case metal with a hole for the part mounted on the other side of the heat sink (inside the box). Just thought i would tell you in case you are feeling adventurous.

yeah i understand...i'm a mechanical engineer...don't worry i just always find solution...but we are talking about a simple bumper light....plastic housing fitted to the bumper...it can be only air-cooled...but i have another question...the led drivers i saw seem to have capacitors to stabilize the voltage...do the use this LM317 also?
 
Hello again,

Oh well there are a lot of drivers out there, many different types, so you'd have to show me a web site where they have the one you are looking at.
There are so many types mostly either switching or linear.
 
Hello again,

That looks like it would work if your load was around 700ma, but im not sure if you can adjust it or not. Dont they make some 350ma ones too?
A way around this of course would be to load the regulator down with more LEDs or resistors, but resistors would waste power too.

The cap is for smoothing the output voltage or making the input look like a lower impedance. Usually use a cap for both is used. The inductor is used for energy conversion which makes that device a true power converter. It's a switching regulator so the output pulses up and down a little and the output cap smooths it out. The input draws a somewhat pulsed current so the input cap helps to smooth that out so the regulator doesnt get a pulsed input (input lines act like large impedances sometimes so a cap helps there too).
 
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Yeah i should thought that...the cheap regulators seem to be having a fixed current range, so i must choose specific with the application..but i think i'll go with resistors(smd's are too hard to weld on wire/leds?) and put a general variable resistor or something...isn't there a single component which can dampens voltage spikes?
 
Hello again,

Well normally an electrolytic cap would be used and that usually takes care of it as long as there is some impedance to work against.
 
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