A current limiting resistor is used in series with a Light Emitting Diode (LED). In this post we will explain why we need this resistor.
Table of Contents
LED Current and Voltage Specifications
To understand why we need a resistor, let’s take a look at the current and voltage specifications of a surface mount LED.
The current specifications are shown in the picture below
The max forward current is rated at 30 mA. But how much current is really required for the LED to light up? To answer that consider the following plot of the luminous intensity vs the forward current.
Consider the value of 10 mA. At this current, the intensity is about a quarter of the value at 30 mA.
Below is a plot of the forward voltage as a function of current
At a forward current value of 10 mA, the diode forward voltage is approximately 2.1 V.
We can use the calculator below to see how the forward current varies with resistor value. The supply voltage is +5 V.
What is the Maximum Voltage that an LED can handle?
As can be seen in the picture above when the current increases, so also does the voltage. However the forward voltage increases at a much slower rate and is tending toward a fixed value. For a specific value of current, the data sheet specifies a max Voltage value.
In this particular case, for a forward current of 20 mA, the max forward voltage is 2.6V. Note that this voltage is a result of the current draw which in turn is determined by the series resistor. A voltage is never explicitly to be applied at the diode terminals.
The reverse voltage however has a fixed maximum value – in this case it is +5V. Applying a voltage in excess of this will damage the LED.
LED Current Formula
Using Ohm’s law, the current through the diode is given by
I = (V-Vf)/R
where
- V is the Source Voltage
- Vf is the Forward Voltage
- R is the Current Limiting Resistor
LED Current Calculator
Example Calculations
At a resistor value of 290 ohm, the forward current is 10 mA.
If the value of the current limiting resistor is reduced to 10 ohm, the forward current increases to 290 mA. That exceeds the max current spec of the LED and will result in damage to the diode. It’s obvious that in the absence of the resistor, or R=0 the current increases to ∞.
In actual fact, the voltage source cannot provide an infinite amount of current and therefore the current will be limited.
According to the specifications, the maximum current that the LED can withstand is 30 mA. From the calculator above, when the supply is +5 V, the minimum series resistance is 100 ohm.
Summary
Every LED has a specification for the maximum amount of forward current it can withstand without damage. A series resistor is used to limit the amount of current going into the LED in compliance with this number. As an example, for the LED used in this post, the max current is 30 mA. The minimum resistance value is 100 ohm for a +5V supply. Without this resistor, the diode would be damaged.
Related Calculators
- How to Calculate the Current through a Resistor
- Total Series Resistance Calculator
- 24V LED Resistor Calculator
- 5V LED Resistor Calculator
- LED Voltage Calculator
