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Old 09-26-2010, 04:10 PM
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Technical Electronics Layout Question

I am working on an LED project for my tail lights. I got a bunch of LED 3rd brake lights from the junk yard and I am going to adapt them into my standard tail lights.

When I pulled the lights apart to get to the circuit board inside, I noticed a couple of things. Each board has 16 LEDs. The board also has resistors, which would step the voltage down to work with the LEDs. The board also has a capacitor and some diodes. What would these be for? Capacitors store energy, but I don't understand why one would be needed for this application since the lights turn off immediately after the voltage is removed. I also don't understand what the diodes would be for since there are 16 light emitting diodes already there.

I haven't yet started to adapt them into my original tail lights, but i imagine that I will have to alter them somewhat.

Here is what they look like.

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Old 09-26-2010, 04:31 PM
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actually those are mini halogens from the looks of them and I cant answer your other questions. But they do look like mini halogens
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Old 09-26-2010, 04:34 PM
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Also if they were LEDs I believe the circuit board where they mounted would have a D instead of an L printed on it D1 D2 etc, diode 1 diode 2 etc. I see L 1 , L2
like light 1 light 2 etc.
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Old 09-26-2010, 04:34 PM
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diodes are there to prevent fraying the whole thing if it was hooked up backwards, as to the capacitors the only reason I can think of is to provide filtering of the DC but that does not really make any sense to me .....with all of the diodes there any AC voltage would be rectified any way.....
my thoughts.........
Kenny
also those look like surface mount super brights LEDS......
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Old 09-26-2010, 04:51 PM
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Its possible that the diodes and capacitor provides "surge suppression" to protect the LEDs. Inductive loads like relays, motors, solenoids, and starters can produce high voltages at the moment they are de-energized. As the magnetic field in them collapses, a relatively high voltage can be present across their terminals. I'm guessing that those additional components may provide protection from those stray voltages. If I'm wrong, I'm sure someone will chime in here.
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Old 09-26-2010, 05:10 PM
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sounds to me like you are on the right track. Momma allways said" if it sounds like it makes sense then its probably true" .
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Old 09-26-2010, 05:55 PM
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Quote:
Originally Posted by Oldguy48
Its possible that the diodes and capacitor provides "surge suppression" to protect the LEDs. Inductive loads like relays, motors, solenoids, and starters can produce high voltages at the moment they are de-energized. As the magnetic field in them collapses, a relatively high voltage can be present across their terminals. I'm guessing that those additional components may provide protection from those stray voltages. If I'm wrong, I'm sure someone will chime in here.
What Oldguy says.....makes sense ......as mag field collapses can also produce oscillations that "bounce"across direct current paths....I seam to remember something along those lines, been too long since I worked with that stuff .....dang CRS
Kenny
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Old 09-26-2010, 07:34 PM
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So whatever is there needs to be there when I'm finished. Hopefully I don't need to alter much anyway, but my plan is to have two of these things per side. One will be for running lights and the other will be for brake/turn lights. I'm still not sure what I'm going to need to keep them from flashing too fast with the turn signals on. Hopefully nothing more than a couple of resistors.
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Old 09-26-2010, 07:41 PM
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Led

Hey PreTuner, Its likely that the board is set up in a three series-parallel circuit. This way if one LED goes out, you don't lose the entire light. Its probably the center six LED's in series and then the outside left and right in series of five. The diodes providing reverse bias protection and some voltage drop(the LED's are low voltage, low current) with the remaining voltage drop necessary done with the resistors. The center six LED circuit resistor would be of a slightly different value the the left/right five LED circuit resistor. The capacitor is for filtering the circuit from transient voltage fluctuations. If you would flip the board over and take more good photos, then we could probably blow it up and look at the circuit tracing and draw a schematic to verify. Hope this helps. olnolan
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Old 09-26-2010, 07:57 PM
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That sounds good. Here is the photo:

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Old 09-26-2010, 09:29 PM
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Sorry

Man, I looked at it until I strained my eyes and got a headache. Sorry, I can't figure it out from the photo to draw a schematic. If I had it in my hand it would be alot easier. Perhaps others here with better vision could do it. olnolan
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Old 09-27-2010, 07:16 AM
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Some diodes have a cutoff voltage and will not pass current past a certain set point.

Vince
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Old 09-27-2010, 07:51 AM
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Circuit

Ok, I looked at it again on coffee instead of beer. I think the resistors are in series to provide the voltage drop(r1+r2+r3), the LED's are grouped 4 in series times 4 parallel for a total of 16. Capacitor for filter and diodes for reverse bias protection. Wish DocVette was still with us, he made electronics circuits like childs play. olnolan
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Old 09-27-2010, 10:17 AM
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My 2 cents: I'm betting the cap is nano or picofarad? My guess is it's only a debounce cap. When the brake switch or relay (blinker) goes on it actually bounces many times in a split second which can cause a flicker or unwanted "on and offs" at the led. Saves the LEDs from too much wear. Cap has to be small for speed, a big cap won't be able to filter the bounce fast enough

diodes are for spike protection. Led lights are self protected against reverse hookups because they're diodes. They won't turn on if they're backwards but won't fry.

Parallel circuits allow 1 row to break and the rest still function. If the designers weren't so cheap they could have put a cap across each led which would allow 1 individual led to burn out (the cap would become a short) and all the LEDs would still function minus the burnt one

what car were these from? How long are they

Just my guesses
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Old 09-27-2010, 08:53 PM
406cu.in. of tire smokin' fun
 

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Great info, thanks guys.

These lights are actually from a Kia (Spectra I think). There were 4 totalled at the junk yard so I grabbed the lights since I wanted them to match. They are about 9" long.
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