Bad LED: Rapidly Oscillating?

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theshadow27

Gawd
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Sep 7, 2004
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I have 2 sets of 6 high brightness LEDs wired up as interior lights in my car. Each light is arranged in 2 strings of 3 with a resistor at the end. I know ohm’s law and I am sure the resistor is correct.

Everything was working fine for about 2 weeks until today when I noticed a strobe effect on one side. When I pulled the assembly out and began to test it, it turned out that one of the LEDs was acting like an oscillator, which was causing the other LEDs on its string to flash with it.

I replaced the bad LED but I am wondering if this is my fault or not. Did I do something wrong? Or is it just a bad LED didn’t last very long (I got them off of ebay).

One note: every car bulb-to-LED project I’ve seen has a diode across the leads. Is this really necessary? I didn’t include one because I figured LEDs are already diodes…

Thanks for your help in advance… JSD
 
the diode used might be a zener diode. a zener diode in reverse bias will bring the voltage down.
however even in that case it would be unnecessary if you use the correct resistor.
I dont see much of a reason to use one.
 
theshadow27 said:
One note: every car bulb-to-LED project I’ve seen has a diode across the leads. Is this really necessary? I didn’t include one because I figured LEDs are already diodes…
Click to expand...

diode.png


It's probably reverse-biased to prevent it from frying if you put it in backwards.
 
fat-tony said:
It's probably reverse-biased to prevent it from frying if you put it in backwards.
Click to expand...

Yeah, thats it... but isnt an LED already a diode? why does it need another diode??
 
Because those diodes serve two purposes. One, they prevent backflow. Two, they prevent overvolt. Yes, virginia, not only do they control the direction of electricity, but also how much! (And now you know why I like the new Fairchild Optos with integrated resistors.)
 
theshadow27 said:
Yeah, thats it... but isnt an LED already a diode? why does it need another diode??
Click to expand...

http://www.vishay.com/docs/83222/83222.pdf

From this datasheet (which I assume to be a typical white LED), we see the maximum reverse voltage rating. If you were to exceed that, things would go poof. In the reversed-voltage case with the parallel protection diode, the forward voltage of the general-purpose diode is much smaller than the max reverse voltage rating of the LED. Since those general-purpose diodes only cost around 7 cents, it makes sense for manufacturers to install them to keep customers from returning a pile of burnt out ones when the car's electrical system sucks.

I *think* that you could use 4 general-purpose diodes and rig up a system that would light the diode no matter which polarity were given to it, but I'm not positive. I'm at school and don't have circuit layout software with me right now. Maybe I'll whip up the design when I get home. On the back of this piece of paper though I think it would work.
 
First off well done, most people go to a lot of effort to get an LED to oscillate. I'd put it down to a bad led. They don't normally do it even when abused.

The extra diode would be back emf prevention, the same as a reverse connected diode across a relay. When and inductor (relay, aircon compressor, etc) is turned off it can produce a large reverse polarity voltage spike. The diode acts like a piece of wire and shorts out the spike. I don't think it would be zenner because you don't want to keep the voltage across an led constant. You want it to vary to keep the current constant.
 
Okay, it happened again on the other side. so I ripped out my LEDs out of the car and hooked it up to my test bench supply -- but i could not get it to flash! it may be an over voltage thing, so I am going to try a 12.1v zener as well as the reverse diode thing.

any other suggestions?
 
Noisy power? You could try putting a cap across it to see if that helps out.
 
It turns out the LEDs that I bought have an internal thermal shutdown. If you put too much current through them and they over heat (for example when the car's electrical is at 14v instead of 12v) they rapidly overheat, shut down, cool down, and turn back on again.

this feature is not only saves burnt out LEDs, but also is a good indication that you need a bigger resistor!
 
here is a picture of the new 3x3 LED arrays in place
front.jpg


EDIT:
I also added the old, 3x2 array to the back dome lights
back.jpg
 
Did you use that block of wood as a sort of "template" to hold the leds while you were soldering them? If so, what a fantastic idea!

:D
 
fat-tony said:
Did you use that block of wood as a sort of "template" to hold the leds while you were soldering them? If so, what a fantastic idea!

:D
Click to expand...
yeah wow good eyes! I drilled a 4x4 grid of holes, but only ended up using 3x3. it worked perfectly, that is how I got them so perfect :cool: It helps 10000x over doing it by twisting the leads and trying to hold the LEDs at the same time while you are soldering.

does anyone think I should move this to a new thread with a different title? I think it is cool but does anyone else... :(
 
I think it's sweet. Personally, I would have opted for a mix of Red, Green, and Blue to get something closer to a full spectrum, but what you made is very nice. I really like the idea of using the block of wood to hold the LEDs. I'll have to do that the next time I'm putting together a project.
 
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