LED Resistor HELP!

woox · Post by **woox** » Sun Jun 29, 2008 8:29 pm

Hey,

I am trying to light 2 3MM leds from a USB power source, these are the led stats:

3500 MCD, 3mm
MCD 2500-3500
Vf(3.0-3.4)

I have tons of 300 OHM resistors lying around, I am wondering is this resistor safe to use with LED being powered by a USB?

Post by **bicostp** » Sun Jun 29, 2008 8:33 pm

Here's a pretty handy tool for figuring this out:
http://ledcalc.com/

Looks like a 120 ohm resistor should work.

woox · Post by **woox** » Sun Jun 29, 2008 8:40 pm

I have a 300OHM resistor would that be ok to use?

Post by **bicostp** » Sun Jun 29, 2008 9:24 pm

If you don't want the LEDs to get power, yes.

(300 ohms would probably knock it down to less than 0.1 volts.)

nitro2k01 · Post by **nitro2k01** » Mon Jun 30, 2008 6:02 am

If you have two 300 ohm resistors, you could connect them in parallel, which is the equivalent of a 150 ohm resistor. That's almost right at least. The LED won't shine with maximum power, but it will shine.

timmeh87 · Post by **timmeh87** » Mon Jun 30, 2008 8:46 am

Bic: I dont see why 300 ohms wouldn't work, the led might look a little dim or something. Dont know until you try. You are making the fatal assumption that the LED current would remain the same for a higher resistance, but it wont. If the voltage was 0.1v, the current would be 0mA, the voltage across the resistor would be 0v, and the LED would get all 5v. Of course, in that case the current would increase, decreasing the voltage until it reached some balance point., etc, etc. You couldnt really calculate the actual current without the I-V curve for the LED. When we do our calculations, we have one point form this curve provided in the form of a voltage and a current. Since we are using a slightly different voltage with a larger resistor, the current is no longer a valid number.

Otherwise, the parallel thing will work.

woox · Post by **woox** » Mon Jun 30, 2008 8:32 pm

nitro2k01 wrote:If you have two 300 ohm resistors, you could connect them in parallel, which is the equivalent of a 150 ohm resistor. That's almost right at least. The LED won't shine with maximum power, but it will shine.

Can you show me how I can do this with 2 leds? (A pic or something would be nice!)

thnx

Post by **bicostp** » Mon Jun 30, 2008 9:24 pm

Well, I'm mainly going by LEDcalc, assuming each LED draws about 20mA. At a supply voltage of 5 volts, with a voltage drop of 3 volts per LED, you only need 120 ohms of resistance, with each LED paired with a resistor, and each pair in parallel. According to the tools I've seen, in order to use a single 300 ohm resistor,you would need to put the LEDs in series with one and have a supply voltage of around 12 volts.

Plus most LEDs I've seen won't light up at all if you're not giving them power much lower than they're designed for.

But yeah, two 300 ohm resistors in parallel should equal to about 150 ohms, which would probably make the LEDs light up, albeit dimmer than they can be.

superdeformed · Post by **superdeformed** » Mon Jun 30, 2008 11:05 pm

A single 300 ohm resistor should work just fine, it will just be about half as bright as it would be with a 120 ohm resistor. The general current and voltage specs for LEDs are fine if you're using them at those levels, but as timmeh87 said if you really want to know what's going on you need to look at the datasheet and the curves for forward current, voltage, and relative intensity. With 300 ohms, you would probably still get about a 3V drop across the LED and 5-6mA of current going through it.

woox · Post by **woox** » Wed Jul 09, 2008 7:56 pm

Hi can anyone confirm if this is the proper conenction to make for 2 LED's?

Valium · Post by **Valium** » Wed Jul 09, 2008 7:57 pm

woox wrote:Hi can anyone confirm if this is the proper conenction to make for 2 LED's?

Perfect

Rekarp · Post by **Rekarp** » Wed Jul 09, 2008 8:06 pm

This is my favorite led calculator site

http://led.linear1.org/led.wiz

Even gives you the optimized layout and circuit diagram.

Electric Rain · Post by **Electric Rain** » Thu Jul 10, 2008 3:37 am

Rekarp wrote:This is my favorite led calculator site

http://led.linear1.org/led.wiz

Even gives you the optimized layout and circuit diagram.

Ah yes... I found that site as well a long, long, long time ago... Great calculator.

timmeh87 · Post by **timmeh87** » Thu Jul 10, 2008 5:10 pm

Technically if you put two LEDS in parallel like that, only the one with the lower voltage will light up. Two identical LEDs usually work, but in general each parallel string of LEDS should have its own resistor.

woox · Post by **woox** » Sat Jul 12, 2008 6:52 pm

timmeh87 wrote:Technically if you put two LEDS in parallel like that, only the one with the lower voltage will light up. Two identical LEDs usually work, but in general each parallel string of LEDS should have its own resistor.

they are same leds