LED voltage/resistor calculation

[ik632]

Ok, I think I wired it wrong now that I read a little. Please check this out:

5v 30ma LED
RadioShack® 5mm LED (Blue)
Model: 276-311 | Catalog #: 276-311

Anyway, I have it wired direct to the 5v on the power regulator and I'm thinking know I need a resistor in there to keep it from burning out.

Some of the reviews said that I need to put in about a 68 ohm 1/8 watt resistor.

So, do I need to stick a resistor in there or am I good? It's been years since I took e-sci.

[ik632]

Cool, that helps. The only thing is the LED I purchased doesn't say what the voltage drop is. I guess I'll just get a mixed pack of resistors and give them a whirl. It definitely won't hurt to have too high a resistance (it just won't light up) but too low is bad.

[ridepate]

Originally Posted by team3six

go to the very bottom of my page.
* Electronics 101 - team3sixRC

Bookmarked!!! Thanks!

[SMR 510RR]

If you are powering a LED with a forward voltage rating of 5v with 5v then you dont need a resistor.

[ik632]

Originally Posted by SMR 510RR

If you are powering a LED with a forward voltage rating of 5v with 5v then you dont need a resistor.

I thought the resistor was to limit current though. It's a 5-6v LED though and is nice and bright. I just don't want it burning out prematurely. Or, wasting too much juice and killing my battery.

[nocfed]

for a 5 volt source and up to 30ma you want a resistor that is 166 ohms or greater. The higher the value of the resistor the lower the brightness. if you go lower resistance you will run brighter but probably blow the led very early.

calculatios:
5 volts / .03 A= 166 ohm
a 180 ohm is a standard value(value they make resistors at).

using 180 will give you a current through the led of 5v/180= 27.7 ma

now using a 180 ohm resistor you have to make sure the wattage (resistors are rated up to wattage they can handle)
5V^2 /180 ohm= .138 W
so a standard quarter watt resistor will work fine for you.

For the voltage drop, the issue would be if your led's had a higher drop then your voltage drop. in the case of blue led's there no more then 3.7 volts. The resistor in this case is acting as a current limiter as an led will draw as much current as it can get until the junction overheats.

[ik632]

Thanks nocfed, that's exactly what I was looking for. I ended up picking up some 68 ohm resistors at lunch so I can run two in series and get 136 ohm which will equate to 36.7 ma which should be within tolerance but still nice and bright.

[SMR 510RR]

Thats interesting because if you use this calculator it says that you would need a 1 ohm resistor so i figure there is no need...

LED calculator for single LEDs