9 Volts is applied to circuit. There is a LED which runs on 3 Volts. I want to drop 6 Volts. how much resistance (\$R_1\$) would I need for that?

Circuit image

  • \$\begingroup\$ That all depends on how much current the LED requires (or how much you want to give it). \$\endgroup\$ – Majenko Dec 29 '14 at 15:07

The short answer is: Any resistor will drop 6V.

The LED has a fixed drop of 3V. All the rest of the voltage has to be dropped across the resistor, regardless of its resistance*.

The purpose of that resistor is not to drop the voltage, but to limit the current.

You need to specify an amount of current the LED needs, and then use simple Ohm's law to calculate the resistor to limit the current to that amount:

$$ R=\frac{V}{I} = \frac{9-3}{I} $$

When you know what I should be you can find the resistor.

*assuming enough current flows to allow the LED to conduct


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