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LEDs & Resistors

LEDs & Resistors

 

Does a Resistor Need To Be Fitted In Series With An LED?

Powering a Light Emitted Diode (LED) might appear simple, but actually an LED is a semi-conductor device which means it has peculiar characteristics which set it aside from standard filament bulbs.

We often get asked whether a resistor is needed to be fitted in series when powering an LED.  The answer is not straight forward as it depends on a number of factors:

  1. The voltage you want to use to power the LED
  2. The LEDs forward bias voltage and current characteristics

Theoretically, an LED can be powered without a resistor, but the voltage applied to the LED needs to be precisely between the LEDs forward bias voltage range.  This is typically 1.8v – 2.2v or 3.2v – 3.8v depending on the specification of the LED.

If you apply a voltage below this range, no current will be drawn through the LED and it will not light up.  Above this voltage range, two much current will be drawn through the LED and it will pop.

Therefore, without a resistor fitted, you need to apply a voltage somewhere between the minimum and maximum forward bias voltage.  Lower voltages in the range will draw less current through the LED (less brightness), higher voltages in the range will draw more current through the LED (more brightness).  The graph below shows a typical voltage vs current curve for an LED, you can see what different voltages mean for the LED in terms of the amount of current drawn which directly affects the LED brightness.

 

 

So in summary, you can power an LED without a resistor, but to get the correct current drawn through the LED for your desired brightness and durability, you have to accurately control the voltage applied to the LED within the forward voltage range – this is not easy as most batteries or DC power supplies will be fixed at 1.5v, 3v, 5v, 12v etc.

A simple solution to this issue is to fit a resistor in series with the LED.  A resistor has a linear voltage vs current relationship, so you can ensure the desired amount of current is always flowing through the LED to achieve your desired brightness and durability.

This means we can use a higher supply voltage without worrying about damaging the LED as the resistor will dissipate the excess power that the LED doesn’t need.

The resistance needed depends on the supply voltage, nominal forward bias voltage and nominal forward current.

So, if we had the following:

Supply Voltage = 12v

LED with forward Voltage = 1.8v – 2.2v (Nominal = 2v)

LED with forward Current = 20mA = 0.02A

We can calculate the required resistance value as follows:

Resistance          = V / I (Basic Ohms Law)

= (12v – 2v) / 0.02A

= 10 / 0.02

= 500ohm

= Rounded to nearest common value = 470ohm

So, to ensure stable LED operation we would ALWAYS recommend fitting a series resistor. This will ensure you get the correct amount of current drawn through the LED to achieve optimum LED brightness, and you don’t risk damaging the LED by over-driving it with excess voltage.