LED question

vskid · Post by **vskid** » Fri May 26, 2006 4:30 pm

Hey, sorry for the double post, but, the resistor calculator said I needed a 1 ohm resistor. So would this be the same for whenever you use input voltage that is the same as the LED's voltage, just so that it won't suicide?

timmeh87 · Post by **timmeh87** » Fri May 26, 2006 5:43 pm

uh, could you be more clear?

vskid · Post by **vskid** » Fri May 26, 2006 5:53 pm

My LED is 3.3v. I will run it off of 3.3v. Needs 1 ohm resistor. Is this just to prevent suicide?

Clear enough?

timmeh87 · Post by **timmeh87** » Fri May 26, 2006 6:07 pm

yeah. same reason you always need a resistor.

thats an interesting scenario though, where your voltages are the same... the math breaks down, but common sense says that you still need a resistor. that calculator always says that you need a 1ohm resistor if the voltages are the same...

so yeah. i dunno. just do what it says. it knows what its talking about.

Post by **Gamelver** » Fri May 26, 2006 7:26 pm

well, it KIND of knows what it's talking about

...it rounds the resistor up to the next highest standard ohm value...and 1 ohm happens to be the next highest standard after 0

.

Makes sense that you don't need a resistor, though. Using the good ol' resistor LED equation (or just ohm's law

) you get R= (3.3v-3.3v)/I

0/anything = 0, so R must be...non-existant

!!

vskid · Post by **vskid** » Fri May 26, 2006 9:40 pm

So should I use a resistor?

Post by **Gamelver** » Sat May 27, 2006 9:12 am

not if your supply voltage is the same as the forward voltage of the LED

...

vskid · Post by **vskid** » Sat May 27, 2006 10:19 am

What about the LED drawing too much current, though?

timmeh87 · Post by **timmeh87** » Sat May 27, 2006 4:22 pm

if i remember, next time im at school ill test it with a digital power supply.

i think you should still have a small resistor.

codeman · Post by **codeman** » Sun May 28, 2006 7:47 pm

if the LED is rated for 3.5v and you run it off of 3.3v there's still a possibility it could die. If the voltage spikes briefly to 3.6v or something, it will cause a problem. A tenth volt increase causes an order of magnitude increase in current through a diode as a rule of thumb. So if you're running 10mA through it at 3.5v without a resistor, 3.6v would give 100mA or so...probably enough to kill it.

vskid · Post by **vskid** » Mon May 29, 2006 11:33 am

timmeh87 wrote:if i remember, next time im at school ill test it with a digital power supply.

i think you should still have a small resistor.

So if I have several LEDs in parallel, wouldn't there need to be a small resistor in series with each of them.

timmeh87 · Post by **timmeh87** » Mon May 29, 2006 7:31 pm

i recon you could use just one

vskid · Post by **vskid** » Tue May 30, 2006 3:41 pm

timmeh87 wrote:i recon you could use just one

So just have one resistor and have all the LEDs going off it in parallel?

vskid · Post by **vskid** » Wed Jun 07, 2006 11:08 am

I tried one LED with a 1 ohm resistor and it got warm in a few minutes, so I tried adding another 1 ohm resistor (1+1=2 ohms) and it got about as warm in 15 minutes. I was running it off of a lm317 outputing about 3.3v. Should I add a bigger resistor, or is it okay for the LED to get warm?

timmeh87 · Post by **timmeh87** » Wed Jun 07, 2006 11:41 am

its ok for it get warm.

the thingy im making gets too hot to touch if you leave the leds on for too long; they seem to be doing just fine.