# LED's and resistor's...

Hello, iv been working with LED's and have been following the basic rule: When you plug in a LED, you have to have a resistor connected in series. I'm using 1K resistor's and the 5mm LED I have connected barely light's up. When I try without the resistor it lights up as it was intended. I'v heard that the current can damage the board if there is no resistor, is that so and if i'm running only 5 volt's though it will it damage my board? Thanks!

Going without will catch up with you, sooner or later.

Rule of thumb -- When an LED [red, green, or yellow LED] is on, there is approx 2V across it; the remaining voltage, 3V (because 5V - 2V = 3V), is across the resistor. If it's a red, green, or yellow LED, that 1K resistor is resulting 3mA. 3V / 0.02A = 150?. 3V / 220? = 14mA. Do yourself a favour: Buy a dozen 220? resistors.

This is why you need resistors with LEDs:- http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

Got an multimeter? To expand on runaway_pancake's rule of thumb:

Measure the voltage across the LED when it's on (Vforward), then calculate the resistance needed for 20mA which is usually the continuous-on limit for LEDs.

Arduino usually puts out 5V to turn an LED on, or you can connect the LED-resistor to 5V and have the Aruino Low output turn the LED on. In either case: (5V - Vforward)/20mA (i.e. 0.02) = resistance

Also, this is the display (LCDs & others) section. You should use the LED one