5mm. blue led resistor help.

[rpm49]

hello, I need help with what kind of resistors I need. I'm trying to string together multiple led's and I have no clue what to do :/ The led's specs are 3.0-3.4V/8000mcd/24mA max. they are 5mm wide angle blue leds. thanks!!!!

 

[ronv]

We need to know the voltage you are supplying to run the LEDs.

 

[rpm49]

okay ill find that out and get back to you.

 

[dr pepper]

You might need to series parallel.
If you wire leds in series then multiply the voltage per led to get the supply volatge, 2 leds 6v, 3 leds 9v etc.
If your supply is 12v then run 3 leds in series, and then through a 120 ohm resistor, if you want more than 3 then multiply this arrangement, so 6 leds would be 3 in series with a 120 ohm, and then another 3 in series with another 120 ohm and so on.

 

[kubeek]

choose R greater or equal to (Vsupply-3.0v)/0.024A

 

[Mike odom]

it's best to use the 3.4V figure per LED drop, since that's worse case and LEDs could drop that much...

 

[Mickster]

I think we need to know the OP's actual requirements for the application first, given the quote:

"I'm trying to string together multiple led's and I have no clue what to do :/ "

An illuminated wearable suit, powered by a 240V extension cable, is quite different from a case-mod or accent lighting powered by a 12v wall wart.

 

[kubeek]

it's best to use the 3.4V figure per LED drop, since that's worse case and LEDs could drop that much...

Actually no. If you use the 3.4V figure and calculate the resistor to get exactly the maximum allowed current, then if you happen to get diodes that have Vf only 3.0V the current through those will be larger than the maximum current you aimed for. And that is not very good for long term reliability.
For example with a 6V supply the current would jump from 24mA to 27.7mA.

 

[Mike odom]

that's why you don't design for the exactly maximum current... but if you only use 3v, then when you do your calculations and get LEDs with 3.4V drop, you don't have enough current, and can overstack and not have enough voltage to even turn them on....

Another reason you don't design for exact maximum current, not at 25°C, is what about temperature drift? you should only design for 80% of max current, using max voltage varying under temperature deviations. That gives an 8% cushion for temperature variation and 12% cushion for voltage variation. If a LED has a max current rating of 24mA, and you design to put 24mA through it, it's not going to have long term reliability, or any other part, for that matter.
Take a 35V capacitor. If you put it in a 35V circuit, and the capacitor is rated at 2000 hrs, it will last 2000 hrs, maybe, depending on voltage spikes. But if you use a 50V 2000 Hr capacitor, it will last 36000 hours (double the life per 10% reduction in circuit voltage per voltage rating). This is why the FAA had to replace all the radar power supplies a few years back... the caps were drying up and failing prematurely, because some whiz bang designer decided it was ok to use 35V capacitors in a 28V circuit, even though the design specs called for 50V capacitors...

So to reiterate... never, NEVER design a circuit to a part's MAXIMUM anything...

 

[tvtech]

I see this stuff Mike everyday.

Chinese Chassis sets......latest is the last that will be dumped here ever while I am alive......These have "coffee tin" heat sinks. And useless thin neck tubes.

And PC Boards copper that peel's off like a sticker if a Soldering Iron is brought anywhere near them....So even if you take pride in your work...and have literally repaired thousands of sets in your life...you are stuffed.

You end up looking like a pig.

So, to come back to your post above.....Chinese MIN everything to save money.

1. Smallest heat sink to do the job.
2. Cheapest caps to do the job......
3. Cheapest and rubbish PCB for the job.

TV is designed and built to last around 1 to 8 Months out of Warranty......and then I sit with this **** these customers bring to me for repair

Look at you and want help....with a stupid look on their face....

Chinese made the money....one time and bailed...and the tvtech must deal with the Crap they sold.

Aaaah, Over it.

Regards,
tvtech

 

[Mike odom]

ya, and I suspect this is also the problem with pc motherboards we are seeing with caps going bad... didn't happen in the 90's...

 

[Mike odom]

this is why on the LED data sheet, they may list 24mA as max, but if you look at all the test spec, and probably the mcd spec, the mA they use is way less, like around 20 or 10mA....

and the tv problem hit here about the turn of the century, not just crap parts, but crap boards, and I mean some kind of pcb material that grew dendrites (no, not tin plated, or at least, I think that was before the pure tin plating problem, here at least) and shorted things out. I had one client that was a tv repair type, and I remember him complaining about, wait for it, chinese boards... (apologies to our chinese viewers)...

 

[tvtech]

and the tv problem hit here about the turn of the century, not just crap parts, but crap boards, and I mean some kind of pcb material that grew dendrites (no, not tin plated, or at least, I think that was before the pure tin plating problem, here at least) and shorted things out. I had one client that was a tv repair type, and I remember him complaining about, wait for it, chinese boards... (apologies to our chinese viewers)...

Never mind LOL

tvtech

 

[dr pepper]

I think partially to blame for blown caps is higher switching speeds, both clock speeds and smpsu freq's, putting more strain on these components with sharper switching edges and the like.
Plus ultra cheap imports.

 

[Mike odom]

well, for smpsu you are supposed to use a low esr capacitor like the FC or better the FK series (both Pansonic) at the very least, but it wouldn't surprise me if they scrimped on those too...