Led Burnout WHY!!!

I might be new to this forum and I I am hoping there are a lot of high educated and great people out there that can help me a little bit!

I wanted to ...and then I did make a lighted SMV. BUT with delayed malfuctioning of leds after the customer had the unit for a few weeks I was getting discouraged. I am using a total of 24- 20 milliamp, T1 L.E.D. s , there are three-.5x12'' boards in the smv unit, with total of 8 leds on them,Four in series and the other four likewise ,then the series of four are tied together in parallel at hte end of each board to connect the other two boards , and the other two boards with the two sets of four are likewise. Then all three boards are tied together in parallel with one board (which is where the 12.8 VDC is hooked up) having a 1/2 watt 27 ohm resistor. Now! (Problem IS) about a week later or month , it varies, there would be one or two leds sometimes very dim or most times burned out ! I checked the volts across each led and it is about 1.9 VDC and while the board with the series that has burned out led has significant high voltage across leds that are in the same series as the bad one AND the bad one has quite a bit lower voltage across or barely enough to light it WHILE the others in series are significantly higher .
What I would like you guys to do is give me Ideas or a schmatic with what type of resistor you would use and soforth ,,and what would be the pro. way to run this type of setup. I would like or prefer one resistor to run all 24 leds but if that is not proper, then give me some info or tips !! HeyThanks !! Greatly appreciated!

What is a SMV? what is the forward voltage of the LEDs? The more resistors you use the less bad LEDs will effect the rest. You should probly have a resister for every 4 in series. Andy

I am sorry it is An S.M.V. emblem for( slow-moving-vehicles) like farm equipment! Gets your attention !
Leds are rated at 2.0 vdc across but I run them at 1.9vdc or 1.8 ....I had used a 56ohm resistor 1/2 watt for whole curcuit!

4pyros pointed out, you should have a resistor for every 4-series string. Or at least one resistor for every 2 strings.

Assuming the LED strings have 1.9 x 4 = 7.6v across them, the resistor per string should be about 260Ω. Or 130Ω per two parallel stings.

Now the real problem isn't so much that you have all the LEDs in parallel strings(although this does makes the issue worse).

The issue is likely that this vehicle is gas powered and thus uses an alternator to charge the battery and power the lighting. The LEDs are at full rated power at 12.8v, but the alternator will put out at least 13.8v to charge the battery and this voltage can easily ring way higher than that.

With 24 LEDs, I think that there are 6 strings of 4 LEDs from your description.

At 27 ohms the total current is 190 mA, so a bit over 30 mA per string which is too much for 20 mA LEDs.

If you have a 56 ohms resistor, the total current is 93 mA, which averages 15 mA per string.

However, on a vehicle, the voltage is likely to be 14.4 or so when the engine is running, so you end up with 120 mA, or 20 mA per string.

The real problem is that there is nothing to ensure even distribution of the current between the 6 strings. You should have a resistor in each string, so that small variations in LED voltage will only cause small variations in current, rather than one low LED voltage causing its string to take much more than its fair share of current.

You should put a resistor of about 390 ohms in series with each string. The LEDs should not be paralleled anywhere. The strings, each of which consists of 4 LEDs and a 390 ohm resistor, can be paralleled.

OK !! Thanks I got this thought of using a volt regulator , like a LM7812 or curcuit that keeps voltage consistant and then fine tune the resistance to top performance while using 1 resistor every 2 parallel string series of four .
Also what is the maxium amount of leds you can put in series till it really starts causing weird problems ? Especially if you have the volts there to do it !

You need at least 14 V input to get 12 V from an LM7812.

If you are going to use active devices, you might as well use current control circuits so that you have direct control of the current.

You should really have one resistor for each string of 4 LEDs. If you post some pictures we may be able to help more.

There is no limit to the number of LEDs that you can put in series as long as you have voltage to drive it. If you get beyond 30 or so LEDs, the voltages get to where electrical safety becomes an issue as the voltages are big enough to hurt people. Also, if you have that many LEDs in one application, one broken wire will stop the lot, so you might want to go for a few strings.

This topic is about a similar problem. https://www.electro-tech-online.com/threads/led-ground-effects-for-car.11494/

The circuit in this topic is a classic current control circuit. You can miss out R3, R4 and Q3 as you can turn the whole circuit on and off by just connecting or removing the power. https://www.electro-tech-online.com/threads/is-this-circuit-too-good-to-be-true.106156/

Right on!! that high voltage might cause problems and one bad led,, you are left with none . Thanks ! I will try out with more resistors and see if that takes care of the problem. I might consider an led driver curcuit . WHAT driver would you recommend using ? There are many differant types oy led drivers out there ,But what is your best pick and what has had the best results in running led curcuits.

how are the leds arranged? what are we trying to do here?

you can get LED arrays at/through the tractor dealers, for tail light arrays and others, and 12volt leds(for interior console) w/wo switches

how are the leds arranged? what are we trying to do here?

Click to expand...

**broken link removed**

Leds are in two strings of 4 in series tied to gether in parallel on a board which is .5x12'' ,behind the red reflector part of the smv emblem . Each side which has a total of three... have 8 leds... and each side is tied to gether in parallel at the ends of each board . The bottom board which is behind the bottom reflector has one 1 watt resistor 56ohm which evidently is causing burned out leds... I though with the right calculations I would be able to get away with one large resistor ....less components .... or maybe not ?
This lighted SMV works great and it gets your attention ...and plus it looks awesome!! I want to get these on the market sometime after we get all the bugs out of it .
I would have pics attached but my program did not want to convert the file from DXF to JPEG. Sorry! If you want i can scan the printed document.

Hi DZ12488,
As your putting led's in series strings a simple yet handy step will help ensure the led's will share even current. Put your DMM onto diode test and test the forward voltage of each led. Match the led's the best you can to be within 0.x0 volts. For example what do you think would happen if a string of 4 leds had a 0.2 volt forward voltage swing between them ???
I made a 16 led light for my magnifier and matched the led's so all 16 were within 0.1 forward voltage, used 2 resistors, one fet and one transistor and got an even 9mA thru each led. Man is the light nice and bright too @ 9mA. By putting in a 50% safety margin the led's will still be bright enough and it will leave some headroom if the voltage does spike.

Cheers Bryan

Use one current limit resistor per every 4 LEDs in series set to about 15ma at 13.8 volts and you should be good. Resistors are cheep dont skimp. Andy

Thanks Bryan ! I appreciate all your tips of trade . Ill have to give that a try and also as Andy said dont skimp on resistors !Thanks again!!