LED Resistor Calculator

An LED (Light Emitting Diode) resistor is a component used in a circuit with an LED to limit the amount of current flowing through the LED. LEDs are sensitive to the amount of current flowing through them. Too much current can cause them to burn out, while too little current might not light them up.

Resistors are used in LED circuits to prevent such issues. They are usually placed in series with LEDs to limit their currents.

Published: August 7, 2023.

How To Calculate LED Resistor Value

The value of the resistor can be calculated using Ohm's Law, which states that the voltage across an element in a circuit is equal to the current through it times its resistance:

U(V) = I(A) * R(Ω)

The simplest circuit consists of only three elements, a source of voltage U(V), resistor R(Ω), and LED diode:

For an LED, you would typically know the forward voltage drop (U_f, the voltage required for the LED to light up) and the desired forward current (I_f, the amount of current you want flowing through the LED for optimal performance). You would also know the supply voltage (U_s, the voltage provided by your power source).

Then, you can calculate the value of the resistor R(Ω) needed using the formula:

R (Ω) = (U_s - U_f) / I_f

Where:

• U_s is the supply voltage,
• U_f is the forward voltage of the LED, and
• I_f is the desired forward current for the LED.

It's crucial to use the correct units in this calculation (volts for U_s and U_f, and Amps for I_f). Always remember that 1A = 1000mA when converting units.

The resistor's role is to "drop" the excess voltage from the supply voltage down to the forward voltage required by the LED. This controls the current flow and keeps the LED operating within its safe parameters.

Average LED Forward Voltage Drop

The forward voltage drop (Vf) of an LED, which is the voltage required for the LED to light up, is dependent on the type of LED, and it varies widely. One of the factors that can influence this is the color of the LED. Here are some typical forward voltage values for different colors of LEDs:

Red: 1.8 to 2.2 volts Orange: 2.0 to 2.2 volts Yellow: 2.0 to 2.2 volts Green: 2.2 to 2.4 volts

Blue: 3.0 to 3.5 volts White: 3.0 to 3.5 volts Ultraviolet: 3.2 to 3.4 volts Infrared: 1.2 to 1.6 volts

Remember, these are approximate values; for example, some manufacturers offer ultraviolet LEDs with Uf up to 4.4V.

Thus, the exact forward voltage can vary based on the specific model and manufacturer of the LED, so it's always a good idea to check the specifications for your particular LED.

Average LED Forward Currents

The forward current, or the amount of current that an LED requires to light up optimally, can vary based on the specific model and manufacturer of the LED. That being said, there are some general typical values for many common types of LEDs:

• Low-power LEDs, such as those often used in electronics projects or indicator lights, typically have a forward current of around 5 to 20 milliamperes (mA).
• High-power LEDs, such as those used in lighting applications, can have forward currents from hundreds of milliamperes to several amperes (A). For example, a high-power LED might operate at 350 mA, 700 mA, or even higher.

Again, these are approximate values, and the specific requirements for any given LED can vary. It's always a good idea to check the datasheet or specifications for your particular LED to know the exact values.

Overdriving an LED (providing it with more current than it is rated for) can cause it to fail prematurely, while underdriving it (providing less current than needed) may result in sub-optimal brightness.

How To Connect LED Diode

LEDs have two connectors, a cathode, and anode.

• A cathode is near the flat side of the LED and is usually connected to ground or zero voltage - that is where I_f "leaves" the diode.
• An anode is on the other side of the LED and is connected to a power source using a current limiting resistor - that is where I_f "enters" the diode.

 LED (Light Emitting Diode) Resistor Calculator

The following LED resistor calculator can be used to calculate the proper resistor value (Note: LED forward current must be in mA and NOT Amps):

LED Resistor (Ω) R (Ω) = (Us - Uf) / If

LED Forward Current If (mA): Source Voltage Us (Ω): LED Voltage Uf (V): Resistor R(Ω):

Example:

If we take a LED with a forward current of 20mA and voltage drop of U_f1 = 2.0V and the circuit is powered by U_S = 5.0V, then:

R(Ω) = (5.0V - 2.0V) / 0.02A = 150Ω

P_R(W) = I² * R = 0.02²A * 150Ω = 0.06W = 60 mW

So, the circuit in this example requires a resistor of 150Ω; a standard resistor may dissipate 60mW easily.

Multiple LEDs in a Series Circuit

Multiple LEDs of the same forward current can be connected in the series and "share" the same resistor.

In order to calculate the required resistor R(Ω) value, we use the following formula:

R(Ω) = (U_s - U_f1 - U_f2 - U_f3) / If

Having several LEDs in a series risks losing all of them if one fails, but it also increases the efficiency of such a circuit.

Example:

If we take three LEDs with a forward current of 20mA and voltage drop of:

• U_f1 = 2.0V
• U_f2 = 2.7V
• U_f3 = 3.5V

and the circuit is powered by a 12V battery, thus:

R(Ω) = (12V - 2.0V - 2.7V - 3.5V) / 0.02 A = 190 Ω

So, in such a situation, a 190Ω resistor will do the job.

Also, the resistor will dissipate:

P_R(W) = I² * R = 0.02²A * 190Ω = 0.076W = 76 mW

which shouldn't be a problem for standard resistors.

Multiple LEDs in a Parallel Circuit

If the multiple LEDs of different currents must be connected to the same power source, then each diode must have its own resistor R(Ω).

In this case, the total current of the power source is the sum of each LED forward current.

Example:

If the source voltage is 12V, and:

• U_f1 = 2.0V, I_f1 = 20mA,
• U_f2 = 2.7V, I_f2 = 25 mA,
• U_f3 = 3.5V, I_f3 = 30 mA,

Then:

R₁(Ω) = (U_s - U_f1) / I_f1 = (12V - 2V) / 0.02A = 500Ω

R₂(Ω) = (U_s - U_f2) / I_f2 = (12V - 2.7V) / 0.025A = 372Ω

R₃(Ω) = (U_s - U_f3) / I_f3 = (12V - 3.5V) / 0.03A = 283.3Ω

I_s(mA) = I_f1(mA) + I_f2(mA) + I_f3(mA) = 20mA + 25mA + 30mA = 75mA

 Using Ohm's Law, it is easy to calculate other circuit parameters.