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
To use the PowerShell Cmdlet Get-LedResistor, install the PowerShell Module DoneLandTools.
PowerShell commands are extremely powerful and versatile, and Get-LedResistor 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) : 130
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) : 520
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)
----------------------- ---------------------
0 3
67 4
133 5
200 6
267 7
333 8
400 9
467 10
533 11
600 12
667 13
733 14
800 15
867 16
933 17
1000 18
1067 19
1133 20
1200 21
1267 22
1333 23
1400 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)