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How to Determine a Resistor for an LED

A light emitting diode (LED), needs to have a resistor hooked up in series with it to operate safely. Because it is a semiconductor, an LED has a nonlinear relationship between voltage and current. As such, it has no resistance itself, since resistance can only be defined when there is a linear relationship between voltage and current. In an LED, a slight increase in voltage leads to a huge, exponential increase in current. So a resistor prevents you from overloading the LED and frying the circuit.

Instructions

- 1
  Write the formula R = (VS - VL) / I, where R is the value of the resistor you will use, VS is the supply voltage, VL is the LED voltage and I is the LED current.
- 2
  Find all the values required in the formula. For example, a circuit supplied by a 9-volt battery would have VS = 9. The manufacturer gives the LED values, but if these are unavailable, you can find them online. For a red LED, the voltage is usually 2.0 V. For all LEDs, the current is usually 20 milliamps (mA).
- 3
  Plug the numbers into the formula and calculate the result. It is unlikely you will have a resistor of that exact value, so get as close as you can, rounding up to the next highest resistor you have.

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