LED circuit - Page 1
Forum section: electronics / repair

author: mikeRmike
author_rank: Newbie
timestamp: December 02, 2018, 09:42:16 pm
content: I have a LED bulb circuit that keeps failing after 3 to 4 months. It is for an aftermarket headlight assembly. The bulbs are for the halo ring. The circuit board powers 4 leds total. 2 leds in series for 1 halo ring and 2 other leds for a other ring. The wiring instructions instruct you to wire up the white wire to 12V and the black wire to ground. The led bulbs flicker or turn off completely. Very randomly. If some one could describe this circuit board and how it works. I have drawn it out several times and I can't wrap my head around how it works. Thanks. I've includes some photos.

author: floobydust
author_rank: Super Contributor
timestamp: December 02, 2018, 10:00:52 pm
content: It's a basic bridge rectifier and dropping resistor arrangement. D2-D4 are the bridge rectifier, so input polarity does not matter. D1 might be a TVS surge protector.R1, R2 and R3 (not populated) are series current-limiting resistors.So the halo could see 14V- the LEDs drop - (2*0.7V)/(43R+62R) for current. Assuming white LED's, that are 3.2V each, that's only 11mA- not too much really unless my math is wrong.To troubleshoot, it's most likely LED's. Heat can make them flicker and go open circuit, so look for any one LED going open-circuit in the string.If you are getting a ground-fault where one end of the LED halo shorts to chassis ground, hard to make happen.

author: tsman
author_rank: Frequent Contributor
timestamp: December 02, 2018, 10:02:45 pm
content: The flickering and dying is a classic symptom of overheating and/or overdriven LEDs. Not a huge amount you can do about it except buy a different brand/model.

author: mikeRmike
author_rank: Newbie
timestamp: December 02, 2018, 10:19:11 pm
content: Could I just de-solder the leds and solder in replacements. They are white. Which ones should I purchase. Thank you so much for your time.

author: floobydust
author_rank: Super Contributor
timestamp: December 02, 2018, 10:29:54 pm
content: If you post a pic of the LED's we can tell you what size they are, if they can be replaced.You can test each one with a multimeter, or run them on the bench off a battery charger or something.They also might be getting cooked by the headlight, if it's really hot for them. Or they are low quality LED's.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: December 02, 2018, 11:47:42 pm
content: Quote from: floobydust on December 02, 2018, 10:29:54 pmIf they are surface mount leds the first spec is the dimemsions in mm. A 3030 led is shorthand for 3.0 x 3.0 mm. Then there is the shade of white in degrees Kelvin. The higher the number the bluer the white. Then there is voltage and and current rating. Basic white leds are around 3 volts but some are actually two leds in seres and require 6 volts. Then we come to the wattage. Brighter leds are higher wattage and this would detirmine the value of the series resistor. That is a lot to take in. If you can get it down to the input voltage and the series resistor value and assume it is a 3 volt led, you can figure out what wattage led was used, for replacement purposes.Sent from my SM-G900V using Tapatalk

author: mikeRmike
author_rank: Newbie
timestamp: December 04, 2018, 07:27:45 am
content: Thanks. Here is a pic.

author: floobydust
author_rank: Super Contributor
timestamp: December 06, 2018, 05:33:32 pm
content: Those look like ordinary 3mm/T-1 size LED's. You can confirm measuring their diameter. There are larger 5mm, and 10mm etc. parts but the pic looks like 3mm. Very easy to find on eBay or Digikey for literally pennies each. You can replace them if you can solder.The only flaw I can see is if the two halos are wired in parallel.The circuit board then outputs about 60mA so each pair of LED's is hopefully sharing and getting 30mA.If one halo goes out, the remaining one gets over driven at 60mA. LED's like 20mA for long life.There really should be a sharing resistor somewhere for each halo, and if they have been failing I would add those and back off current down from 30mA to 20mA.If the two halos are wired in series, they would be dim and cut out with low battery voltage. I'm not sure how they are wired.

author: royarms
author_rank: Newbie
timestamp: December 08, 2018, 07:37:12 pm
content: Replacing the 43 ohm resistor with a 100 ohm resistor would get you close to 20ma draw per LED. I'm guessing the circuit was designed for 12v which would yield 45ma per string, or 22.5ma per LED. When you put it in a vehicle it is running at 14.7V while the vehicle is running, which is around 70ma per string or 35ma per led.(Vbatt-(3*2 drop of led + 0.6*2 drop of bridge rectifier diodes))/(62+43ohm Resistor total)Or use a 12v regulator like a 7812 to drop it to 12V while the vehicle is running to get constant brightness vehicle off or on.

author: Gyro
author_rank: Super Contributor
timestamp: December 08, 2018, 08:28:20 pm
content: The other thing I notice is that the LEDs are soldered with 90' lead bends very close to the packages. Use fine nosed pliers (between the package and the bend) when you form the new ones. Any mechanical stress transferred from the leads to the LED package will decrease its reliability, particularly in damp conditions.Also, don't linger too long with the soldering iron.

author: mikeRmike
author_rank: Newbie
timestamp: December 09, 2018, 07:56:12 pm
content: Thank you so much for your time and input. It is really appreciated!

author: mikeRmike
author_rank: Newbie
timestamp: December 09, 2018, 07:58:22 pm
content: Thank you so so very much. I really appreciated you taking time to help me with this issue. You rock!!