Classic Indicator LED

Calculate Series Resistors For Classic Indicator LED And Start Playing

Probably anyone in electronics has come across the classic standard LEDs that come in two diameters: 3mm and 5mm.

As cheap indicator LED, they are designed to emit a noticeable colored light to indicate a given state. That is not much asked - which is why these LED are cheap and popular among hobbyists to play around with.

Not Very Efficient

These LED are neither very bright nor very efficient: at 20mA, they consume relatively much energy for relatively low light output. Because of their low currents, they get away with it and despite their inefficiencies do not produce much heat or need heat sinks.

What matters more is their low price and the many available colors:

Wiring: Anode and Cathode

Classic indicator LED have two “legs”. The longer leg is the anode (+).

Since most LED are transparent, you can look inside the LED. This is useful if the LED is already wired or its “legs” have been shortened or cut off otherwise.

Inside the LED head, a short metal piece is connected to the anode (+). The much wider cup-shaped counterpart is connected to the cathode (-).

Calculating Series Resistor

Due to their low power, cheap series resistors are used for current limiting.

The required resistor value can quickly be calculated based on the well known LED current (20mA) and the also well-known forward voltages (based on LED color).

Each LED color has its own forward voltage. Do not use a series resistor for a blue LED on a red LED.

Here is a quick reference sheet to find the right series resistor for a given LED color and the voltage you want it to power with:

PS> 5,9,12,24  | Get-LedResistor -Color yellow, orange, red, green, blue, white

WARNING: LED Forward Voltage was guessed from color and can be completely different. Use at own risk.
Required Resistor (Ohm) Operating Voltage (V) Led Current (mA) Led Voltage (V) Led Color
----------------------- --------------------- ---------------- --------------- ---------
                    160                     5               20             1.8 yellow   
                    155                     5               20             1.9 orange   
                    150                     5               20               2 red      
                    130                     5               20             2.4 green    
                    100                     5               20               3 blue     
                     90                     5               20             3.2 white    
                    360                     9               20             1.8 yellow   
                    355                     9               20             1.9 orange   
                    350                     9               20               2 red      
                    330                     9               20             2.4 green    
                    300                     9               20               3 blue     
                    290                     9               20             3.2 white    
                    510                    12               20             1.8 yellow   
                    505                    12               20             1.9 orange   
                    500                    12               20               2 red      
                    480                    12               20             2.4 green    
                    450                    12               20               3 blue     
                    440                    12               20             3.2 white    
                   1110                    24               20             1.8 yellow   
                   1105                    24               20             1.9 orange   
                   1100                    24               20               2 red      
                   1080                    24               20             2.4 green    
                   1050                    24               20               3 blue     
                   1040                    24               20             3.2 white     

The table shows typical values. Use at own risk. Better yet, calculate the correct series resistor value yourself using the specs of your particular LED. You can also adjust the values for the LED forward voltages in the PowerShell script I used to create the table, and calculate your own.

PowerShell Script to calculate LED resistance values

Here is the PowerShell script that was used above to calculate the LED series resistor values:

function Get-LedResistor
    $Current = 20    

    $colorToVoltage = @{
      yellow = 1.8
      orange = 1.9
      red = 2.0
      green = 2.4
      blue = 3.0
      white = 3.2
    $Color | ForEach-Object {
      $curColor = $_
      if ($PSCmdlet.ParameterSetName -eq 'guess')
        $ForwardVoltage = $colorToVoltage[$curColor]
        $curColor = $colorToVoltage.GetEnumerator() | 
        Sort-Object { [Math]::Abs($_.Value - $ForwardVoltage)  } | 
        Select-Object -First 1 -ExpandProperty Key
      $voltageDrop = $OperatingVoltage - $ForwardVoltage
      $resistance = $Current * 1000 / $voltageDrop
        'Required Resistor (Ohm)' = $resistance -as [Int]
        'Operating Voltage (V)'   = $OperatingVoltage
        'Led Current (mA)'        = $Current
        'Led Voltage (V)'         = $ForwardVoltage
        'Led Color'               = $curColor
    if ($PSCmdlet.ParameterSetName -eq 'guess')
      Write-Warning "LED Forward Voltage was guessed from color and can be completely different. Use at own risk."

Run this script inside a PowerShell console or IDE like Windows PowerShell ISE or VSCode to define the new command `Get-LedResistor’.

Next, use the command inside the same PowerShell session like below. As you will see, PowerShell commands are extremely powerful and versatile, and this one new command can calculate one individual resistor as well as a resistor table for a wide range of operating voltages:

PS> Get-LedResistor -OperatingVoltage 3.3 -Color red -Current 10

Required Resistor (Ohm) : 7692
Operating Voltage (V)   : 3.3
Led Current (mA)        : 10
Led Voltage (V)         : 2
Led Color               : red

WARNING: LED Forward Voltage was guessed from color and can be completely different. Use at own risk.

PS> Get-LedResistor -OperatingVoltage 10 -Current 15 -ForwardVoltage 2.2

Required Resistor (Ohm) : 1923
Operating Voltage (V)   : 10
Led Current (mA)        : 15
Led Voltage (V)         : 2.2
Led Color               : green

PS> 3..24 | Get-LedResistor -Current 15 -Color blue | Select-Object -Property required*, *operat*

WARNING: LED Forward Voltage was guessed from color and can be completely different. Use at own risk.
Required Resistor (Ohm) Operating Voltage (V)
----------------------- ---------------------
15000                                       4
7500                                        5
5000                                        6
3750                                        7
3000                                        8
2500                                        9
2143                                       10
1875                                       11
1667                                       12
1500                                       13
1364                                       14
1250                                       15
1154                                       16
1071                                       17
1000                                       18
938                                        19
882                                        20
833                                        21
789                                        22
750                                        23
714                                        24

Wired and Preconfectioned LED

You may come across preconfectioned LED that have wires already attached to them:

Wire color typically lets you identify anode (red) and cathode (black). If the wire color is different or you want to make sure, you now know from above how looking inside the LED head can tell you the polarity.

Wired LED often already come with a series resistor that hides inside the shrink tube (like seen in the picture above). In this case, the LED is already preconfigured for a particular operating voltage.

Indicator LED exist in many different shapes and forms and can also be square, clear or fogged. They all work the same.


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(content created Feb 17, 2024 - last updated Mar 17, 2024)