A question on resistors

Thread Starter

Roen Hayden

Joined Sep 10, 2007
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.


Joined May 18, 2007
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.