# A question on resistors

#### Roen Hayden

Joined Sep 10, 2007
22
What is the formula for figuring out how much resistance some needs will give an example

Lets say one has a LED which is rated for 1.5 volts and it is powered by 9 volt battier but it blows every time how does one figure how much resistant is needed to keep the led from blowing.

#### Pootworm

Joined May 18, 2007
29
This is assuming you're wiring your LED directly to the battery.

If an LED is rated for 1.5V, you should assume a 1.5V drop across it. You'll need a resistor in series to limit the amount of current you're pulling - let's call that R. The amount of voltage dropped across R is 9V-1.5V (from the LED), so 7.5V. Dividing 7.5V by the amount of current you want to send through your LED gives you the value of the series resistor R. I usually run 10mA through mine, so 7.5V/10mA = 750 ohms.

So, connecting a 750 ohm resistor in series with your LED to a 9v battery should light you up.

#### thingmaker3

Joined May 16, 2005
5,084