Need help with a LED resistor...

[macrospect]

To kee pthis short I bought a lot of 50 super bright blue LED's off of ebay. I was going to tie them into my 12V line in my computer, but dont know what resitor to get. Actually I know the average voltage of the LED's are 3.5V. The problem is I have no idea how to read the chart that was posted on the listing to determine the millaamps of each led. I dont know what the average is or how to read the symbols on the chart on the listings page.

I would greatly appreciate it if someone told me the mA's on these leds? I think I can find the resistor from there using this calculator.


HERES THE LISTING

HERES THE MANUFACTURERS PAGE FOR THE LEDS THEY MAKE.
(Its a PDF)
(I have the first one part number BUBC333)

PS. I know I can only tie 3 LED's together on a single 12V wire

DaRkF0g

 

[Brisbylan]

The optimum settings

are 3.5v
20mA peak current

so
using ohms law

V / I = R
12 / .02 = 600Ohms

If i were wiring that up

i'd go -
negative - LED - LED -LED - 600ohm resistor - Posiive 12v
/\ - LED - LED -LED - 600ohm resistor - Posiive 12v
/\ - LED - LED -LED - 600ohm resistor - Posiive 12v

Do a few rows of LEDs and tie them to the same ground llok at radio shack for pcb bradboards they'll really clean it up. hell if you wanted to get really into it instead of using the resistors get a potentiometer it varies the resistance so you could make them brighter or dimmer.

 

[macrospect]

Thank you very much Brisbylan.

According to that calculator (im not saying ohms law is wrong) I would need a 425 ohm resistor. I used that calculator before for other led lights and it worked fine. When I put in a 12V supply voltage, with the LEDs themselves drawing 3.5V/20mA, it came up with a 425 Ohm resistor. However the next standard 5% resistor is 470 ohm (yellow violet brown). It looks like I need to go over to the electronics store to pickup a (yellow violet brown) resistor.

Thanks for your help!

 

[macrospect]

The optimum settings

are 3.5v
20mA peak current

so
using ohms law

V / I = R
12 / .02 = 600Ohms

If i were wiring that up

i'd go -
negative - LED - LED -LED - 600ohm resistor - Posiive 12v
/\ - LED - LED -LED - 600ohm resistor - Posiive 12v
/\ - LED - LED -LED - 600ohm resistor - Posiive 12v

Do a few rows of LEDs and tie them to the same ground llok at radio shack for pcb bradboards they'll really clean it up. hell if you wanted to get really into it instead of using the resistors get a potentiometer it varies the resistance so you could make them brighter or dimmer.

Arent you supposed to have a resistor in between of each LED light?

I found some 470 OHM resistors on ebay for a great price. However they vary in wattage. I have no clue how many watts I need for a led (to be more specific the ones I mentioned above) The seller offers 1/4 or 1/2 watt resistors. Which should I go with, or neither?

Heres the listing for the 1/4 watt 470 ohm one

 

[Brisbylan]

I was just using the source voltage as an example

Also if you wire these up in parallel you can put more then 3 on a line

Because in a parallel circuit voltage is the same accross each branch.

So if you have 5 branches each line will get 12v then you cabn have a resistor and 3 leds on each branch cause each branch will act as a series circuit.

No in series current remains the same through out the circuit so if you put more resistors in ull drop the current down more

 

[macrospect]

So if you have 5 branches each line will get 12v then you cabn have a resistor and 3 leds on each branch cause each branch will act as a series circuit.

Gotcha. Im still learning how leds work, but that makes complete sense. Now to figure out if I should get the 1/4 or 1/2 watt resistor.

 

[zoobaby]

Ok you need 20mA running through the LED.

12V - 3.5V = 8.5V drop across resistor. V=I*R. V=8.5V I=.02A that makes R = 425 Ohms.

The above is for a 12V -> Resistor -> LED -> GND setup.

If you want three LED's on the rail you need a Resistor equal to 75 Ohms and you set it up like:

12V -> R -> LED -> LED -> LED -> GND

However, this setup may cause some of the LEDs to go bad after a bit. Safe bet is:
R = 25 OHM
12V -> R -> LED -> R -> LED ->R -> LED -> GND

EDIT:
Or put them in Parallel and go
12 -> R=425 ->LED -> GND.
R=425 ->LED -> GND
R=425 ->LED -> GND

In this configuration you can put more than 3 LEDS on the 12V rail.

 

[macrospect]

I put some leds in a fan a while ago, annd used 3 leds. It works fine but heres how I had it setup. I believed I used the same 470 ohm resistors if I remeber correctly. Why would you use a 25 ohm resistor?

12V->R->LED->R->LED->R->LED->Ground

 

[macrospect]

EDIT:
Or put them in Parallel and go
12 -> R=425 ->LED -> GND.
R=425 ->LED -> GND
R=425 ->LED -> GND

In this configuration you can put more than 3 LEDS on the 12V rail.

How many max could I put on a single 12V wire if I used that configuration?
(Off to bed for now getting late ill pickup this thread tomorrow )

Thanks for all the support!

 

[Brisbylan]

Ok you need 20mA running through the LED.

12V - 3.5V = 8.5V drop across resistor. V=I*R. V=8.5V I=.02A that makes R = 425 Ohms.

The above is for a 12V -> Resistor -> LED -> GND setup.

If you want three LED's on the rail you need a Resistor equal to 75 Ohms and you set it up like:

12V -> R -> LED -> LED -> LED -> GND

However, this setup may cause some of the LEDs to go bad after a bit. Safe bet is:
R = 25 OHM
12V -> R -> LED -> R -> LED ->R -> LED -> GND

EDIT:
Or put them in Parallel and go
12 -> R=425 ->LED -> GND.
R=425 ->LED -> GND
R=425 ->LED -> GND

In this configuration you can put more than 3 LEDS on the 12V rail.

just remeber to add up the current of each branch to get the total and make sure you dont go over the power supplys max on the 12v rail

also why only 1 resistor on each path in parallel voltage is the same on each branch thats on parallel to each other.

So in that setup theyd all get 12v, with a 3.5 volt drop off 8.5 volts more ta go.

 

[Brisbylan]

How many max could I put on a single 12V wire if I used that configuration?

depends on your power supply, there should be ratings on the side of it, look for max current on 12v rail

 

[Teancum]

Putting
12V -> R -> LED -> LED -> LED -> GND
will NOT make one of the LEDs burn out. It's perfectly safe. If you're doing it this way, you need to pick your resistor this way:

(12V - 3.5V - 3.5V - 3.5V)/.02 = 60 Ohms

There's absolutely no benefit to putting a smaller resistor between all the LEDs over just using one larger one, except that the smaller resistors will each be dissipating as much power. However, we're talking about so little power here anyway that it won't matter. So save yourself the extra solder joints.

You can put as many
-> R -> LED -> LED -> LED ->
strings in parallel with eachother, as long as each one is tied to 12V on one end and GND on the other. Considering each string of three will only draw 20mA, all 50 LEDs put together will only draw about 350mA.

Ooh, let me see--<runs to check spare PSU> that means that this little 170W ATX PSU I ripped out of an old Mac could power almost 1000 LEDs just off the 12V line.