resistors burning up my leds

I bought some L.E.D.s on fleebay with resiters. The green leds came with a resister dropping voltage to 3v. BUT I have some that are free flowing. I have hooked up three or four resisters now and I am getting readings of 11.75v on the other end.

Do they burn out? I thought when a resister goes nothing gets through. Are these cheap or am I missing something. I get a 465ohm reading with them.

I hooked one up and it was 3.15v removed my burned out led replaced it with another one, checked again and I am getting 11.74v again. Whats up with these??

Resistors resist current, not voltage.  Working with series resistors is completely different then working with a resistor in series with an LED, because LEDs are discreet components, not resistive.

If you have a single resistor connected to a battery you will read battery voltage on the other side of it, however, that voltage will be current limited.  If you are using a 470 ohm resistor hooked up to 12vdc you are limiting the current to the LED to around 25mA.  Depending on the LED, that may be too much current.  If you can find out what the max Forward Current is you can determine the proper resistor value for your application.  If in doubt wire two or three of the supplied resistors in series, that will limit the current even more.

so how do you limit voltage? the led is rated at 3-3.6v

it is being used as an idiot light on a motorcycle with 12-13.8v output range

You don't worry about the voltage, just limit the current.

Discreet components work a little bit different the resistive components.  If you limit the current the LED will take care of the rest and will work with no issues.

Emitted Colour : Green

Size: 5mm

Lens Color : Water Clear

Forward Voltage (V) : 3.0~3.6

Forward Current (mA):20

View Angle: About 25 degree.

Luminous Intensity: 15000mcd

Free Resistors:

A 465ohm resistor is going to drop you to 25.8mA at 12 volts.  This is already 6mA too high.  Also remember that your standard automotive battery is actually 12.6 volts, and when running, produces over 13 volts.

I'd suggest a 510 ohm resistor (500 should do the trick) for a single LED.

Jeff

Thanks Jeff,

I think I am going to go with a 1k resister cuz these are super bright leds and I want to dull them down a bit. My bike system charges at 13.4v

That sounds like a good idea? what is a pot?