Overview
Every light emitting diode (LED) has a current that it can safely handle. Going beyond that maximum current, even briefly, will damage the LED. Thus, limiting the current through the LED with the use of a series resistor is a common and simple practice. Note that this method is not recommended for high current LEDs, which need a more reliable switching current regulator.
This calculator will help you determine the value of the resistor to add in series with the LEDs to limit the current. Just input the indicated values and press the "Calculate" button. As a bonus, it will also calculate the power consumed by each LED.
Equation
Where:
= Supply voltage
= LED current. The usual operating range of common 3 mm and 5 mm LEDs is 10-30 milliamps. If access to an LED's datasheet is impossible, 20 mA is a good guess.
= LED voltage drop. The voltage drop on a LED depends on the color it emits. Here is a neat table of each color and their corresponding voltage drop:
= number of LEDs in series
| Color | Voltage Drop (V) |
| red | 2 |
| green | 2.1 |
| blue | 3.6 |
| white | 3.6 |
| yellow | 2.1 |
| orange | 2.2 |
| amber | 2.1 |
| infrared | 1.7 |
| other | 2 |
Bonus: Identifying LED Terminals
An LED has a positive (anode) lead and a negative (cathode) lead. The schematic symbol of the LED is similar to the diode (as shown above) except for two arrows pointing outwards. The anode (+) is marked with a triangle and the cathode (-) is marked with a line.
The longer lead of an LED is almost always the positive (anode) lead, while the shorter one is the negative (cathode). Also, if you look inside the LED, the smaller of the metal pieces is connected to the anode while the bigger one is connected to the cathode (see diagram above).
For more info and more complicate creations see the next video and calculate here https://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator
LED Resistor Calculator Equation
Let us now proceed with important aspect of the tutorial i.e. the equation for the LED Resistor Calculator. For the sake of simplicity, let us consider a simple circuit consisting of a single LED, a single Series Resistor and a power supply.
The following image shows a simple LED circuit made up of an LED, a resistor RS and a power supply VS.
Using simple Circuit Theory, you get the following equation:
VS = RS * IR + VLED, where
VS is the power supply voltage,
RS is the value of the series resistor,
IR is the current through the series resistor,
VLED is the forward voltage or voltage drop across the LED (usually represented as VF).
Since the Series Resistor RS and the LED are in series, the current flow through them will be the same and as per our previous discussions, this current should be the Forward Current of the LED (ILED or simply IF).
Thus, we can re-write the above equation as follows:
RS = (VS – VF) / IF
This is the LED Resistor Calculator Equation. The important thing to note here is that the value of the series resistor is dependent on both the Forward Current of the LED as well as the Forward Voltage of the LED. Hence, it is essential to keep an eye on both these value of an LED from its datasheet.
Different colored and types of LEDs have different forward current and forward voltage ratings. For example, the following table gives an overview of the forward current and forward voltage value of some of the commonly used 5mm LEDs.
NOTE: The following values are specific to a manufacturer and cannot be generalized. For accurate values, you should definitely check out the datasheet provided by your manufacturer.
| LED Colour | Forward Current (IF) | Forward Voltage (VF) |
White | 30 mA | 3.6 V |
Red | 20 mA | 2 V |
Blue | 20 mA | 3.9 V |
Green | 20 mA | 2.4 V |
Yellow | 20 mA | 2 V |
| Amber | 20 mA | 2.4 V |
Orange | 50 mA | 2.1 V |
Infrared | 100 mA | 1.4 V |
If there are two LEDs connected in series, then the equation to calculate the Series Resistor will be the following:
RS = (VS – VF * 2) / IF
In fact, if there are ‘N’ number of similar LEDs connected in series, then the LED Resistor Calculator equation can be written as follows:
RS = (VS – VF * N) / IF
Example
Let us now take a look at a simple example circuit and calculate the value of the series resistor for an LED to work properly without blowing up.
From the above image, the supply voltage VS is 5V, the LED used is a 5mm White LED. From the above table, a typical 5mm White LED has the following ratings:
Forward Current IF = 30 mA and
Forward Voltage VF = 3.6 V.
Substituting these values in the above equation, we get the following:
RS = (5 V – 3.6 V) / 30 mA
RS = 46.6 Ω.
The closest value is a 47 Ω Resistor. But if you want to be on the safe side, I would suggest you to use the next big value and, in this case, it would be a 56 Ω Resistor.
Power Dissipation of the Resistor
One important characteristic of the series resistor for an LED that is often neglected or ignored is the power dissipation of the Resistor.
If the voltage drop of the LED is VF, then the voltage drop across the Resistor is VS – VF. This means that in the process of limiting the current through the LED to the value of IF, the resistor has to dissipate the remaining power, which is (VS – VF) * IF. If the power rating of the resistor doesn’t satisfy this value, then the resistor will burn and you will see the magic smoke.
If we consider the above example of 5V power supply with a 5mm White LED, we have calculated the value of the Resistor to be 47Ω. In this case, the actual power dissipated by the resistor is (5 V – 3.6 V) * 30 mA. This is equal to 42 mW. So, a ¼ W resistor would be sufficient.
This might seem a small number but this is just an example and it can get pretty hot while designing and building complex and high power LED circuits.