store bought LED string...

[blk03MitsuES]

i remember seen somebody in here buying some led strings cheap and using it for the mod he/she was doing....
I got a string of 40 orange leds from halloween for 2 bucks. Taking a closer look at the string got me a little confused. first i tried looking for a little AC to DC converter but all i saw was a resistor. so i googled for about half an hour or so. noticed there were "ac led circuits" and such but they failed to mentioned if the LEDs were ac or dc. so do leds use either/or ac/dc? then i took a closer look at the resistor which is bigger than what i've seen before and i noticed something was odd. from the plug it goes straight to the end of the string to the led in the end then to every other led making it back to the resistor. then from the resistor to one led then to the plug. i though resistors had to be first, before the current/power went to the leds. so i thought well screw it i'll just take it a part and make a few sets of leds to use on a future mod and just look for the resistor. then i noticed i only had the voltage and watts but not the amps to plug into the led calculator...

"only replace with 1.9 Volt 0.038 Watt...."

anybody want to clear things up or just point me where to read? thanks!

[nevermind1534]

You can run regular LEDs off of AC power, but they will flash on and off rapidly. I don't own any of these, so I couldn't tell you how yours work.

[Xpirate]

I doubt that the LEDs will flash on AC power because it switches polarity at 60 Hz. So if it does flash, your eye probably will not even notice it.

[nevermind1534]

true. You may notice it if it's in your peripheral vision, though. And, unlike regular incandescent bulbs, they would be off half of the time, where your regular bulb won't care about polarity.

[blk03MitsuES]

ok so the DC/AC is out of the way.... how do i find out the mA for the leds? can i base it out of the ".038 watts"

Watts = Volts x Amps

or in my case Amps= watts/volts

i already tried dividing .038 by both 12 and 110 and i get some long crazy number so i think i'm doing something wrong....

then i went back to the led calculator and it said

If you have good specs from your supplier, you'll want to enter the typical forward current in milliamps here. For 3mm and 5mm LEDs, this is usually 20 mA or close to it. A few special, high-power LEDs exist, but they always come with specs. So if you need to guess, use 20 here.

so... if V = I*R

so if i use 6 of the 1.9V leds then i'm left with .6V=20*R which R=.03 but then the led calculator said R=33?!?!?!? so before coming back here and making a fool out of myself cus i might be misunderstanding some stuff i went back to my old thread and found...

....The best way to calculate what resistor you need (I'd always put one in) is to use ohm's law:
V = I*R

For 12v your V = 12v, now we can adjust this by subtracting the forward activation voltage off of 12v. Assuming a very conservative 1v, you could place 12 diodes is series, or do something reasonable and use 4, that would drop your voltage to 8v. Thus ohm's law becomes:
8v = I * R

Now for I we will set that equal to the Imax of the diode, in this case 30mA which is .03A. Thus the equality becomes:

8v = .03A * R

Solving for R yields 267 Ohms of resistance.

so going back to .6v=.02A* R i still get 30ohms for R and not 33. what am i doing wrong?

so i took MTEKK's advise and use 1 instead of 1.9 so 6v=.02a*r i get 300 led calculator gets 330 ohms

so should i use a 33 or 330 ohm resistor?

so if the 33 ohm is right, then radioshack has a pack of 5 for a buck which i dont think it's that bad BUT it says "1/2W" while the led calculator said the "wizard thinks 1/4W resistors are fine for your application" so using 1/2w instead of 1/4 going to have any negative consequences?

if i go with the 330 ohm one the radioshack does have them at 1/4w... I knew i should have paid more attention to my electronics teacher

[crazybillybob]

Lets see if we can Clear the water.

First we'll Look your watt=IxV Issue, I=P/V or I=0.038/12 => I= .3mA To low to light even a standard LED. By playing with the Math These LEDs were most likely Wire to a 3V or 2V source (This assuming that they were all wire in parallel Ie. Plug-Resistor -LED-Plug)

Mekk's Statement is correct, as the least Voltage the LEDs would Drop is 1V ea.

First lets find the Resistor for 6 LEDS and see how that works. so supply is 12V Forward current is 20mA voltage drop is 1V, You have 12-6=6 So Ohm's law says R=E/I that means R=6/.02 or R=300 A 300 Ohm Resistor isn't common (Some thing the LED Calc takes into account) and most Resistors (at Radio shack) are 5% which mean they can be high or lower then rated be 5% so the 330 is a good choice.
This is the conservative method, your LEDs Should light and have a long life, but not be quite as bright (some times you can't tell the difference in brightness between this method and the over current method).

Over current method, Will shorten the life of the LED!
You start by assuming the LEDS drop more Voltage. In this case we'll use the 1.9V number (1.5V would be more industry standard but we like challenges here at TBCS )
(voltage Dropped =Vd) Vd=6x1.9 Vd=11.4. Voltage supplied =12 so 12-11.4=.6 so in our Ohm's law formula R=E/I or R=.6/.02 R=30 (we match you had this far). Now based on experience 33 ohms is a little low, Don't get me wrong, You'll have some Bright LEDS for about 12 secs then there dead. If you have some extras and want to test it Go for it, for me Trying things is the best way to learn! In this case I'd go with a 100 or a 150 Ohm Resistor.


If your tight on parts Go with the 330.
If you must have a brighter LED or want to test it out a 100 ohm or a 150 ohm should work great for about 100 to 1000 hours (most LEDS are rated at 10,000 hours so the over current really does shorten the life of the LED).


As for the 1/2W versus the 1/4W... It's always best to go bigger. They can handle more heat so if your calculations are off or if something is out of tolerance more then expected, or there is a voltage spike your covered.
For a hand full of LEDs the 1/4W is the best beat, but if you can't get there is no issue with picking up a bigger size.

Now that I've muddy the water even more I'll get off my soap box
If you have any questions about what I've posted please let me know...I'll see if we can straighten them out.


CrazyBillyBob

[xmastree]

"only replace with 1.9 Volt 0.038 Watt...."

They're probably not LEDs.

LEDs shouldn't ever need replacing so why mention the spec?

I'll bet they're just normal filament bulbs.

[blk03MitsuES]

They're probably not LEDs.

LEDs shouldn't ever need replacing so why mention the spec?

I'll bet they're just normal filament bulbs.

if they're the average filament bulbs then i'd have the right to sue them for saying LED in about 8 different places between the box and little sheet of paper

can't find the thread right but on it you made it clear about the placement of the resistor. so thanks for your knowledge even thou it was in a different thread!

edit*

by the way, the instructions that mentioned how to replace the LEDs and what not also stated that you shouldn't use the lights on a daily basis for periods longer than 3 months....

[xmastree]

if they're the average filament bulbs then i'd have the right to sue them for saying LED in about 8 different places between the box and little sheet of paper

Fair enough. I've been fooled before, that's what made me think of it.

[blk03MitsuES]

....
First lets find the Resistor for 6 LEDS and see how that works. so supply is 12V Forward current is 20mA voltage drop is 1V, You have 12-6=6 So Ohm's law says R=E/I that means R=6/.02 or R=300 A 300 Ohm Resistor isn't common (Some thing the LED Calc takes into account) and most Resistors (at Radio shack) are 5% which mean they can be high or lower then rated be 5% so the 330 is a good choice.
This is the conservative method, your LEDs Should light and have a long life, but not be quite as bright (some times you can't tell the difference in brightness between this method and the over current method).

Now that I've muddy the water even more I'll get off my soap box
If you have any questions about what I've posted please let me know...I'll see if we can straighten them out.


CrazyBillyBob

i'm gonna go with the 330ohm way. i'm not trying to make sunshine inside the pc, just lighting it up a bit, so if dimmer is gonna give me longer life then thats good enough for me. Thanks to you too for making things clear