LEDs Connected in Series

When LEDs are connected in series, the same current flows through every LED. Their forward voltages are added together.

                 R
+V ───────────/\/\/\───►|───►|───►|─── GND
                         LED   LED   LED

The total forward voltage is:

VLED(total) = VLED1 + VLED2 + VLED3 + ...

The remaining supply voltage is dropped across the series resistor.

Series LED Formula

For identical LEDs:

VLED(total) = VLED × Number of LEDs

The resistor is then calculated using:

R = (Vsupply - VLED(total)) / ILED

The resistor must be used to limit the current to a safe value for the LEDs.

LED Series Calculator

Example — Three Red LEDs on 12 V

Suppose three red LEDs are connected in series. Each LED has an approximate forward voltage of 2 V and the desired current is 20 mA.

Supply voltage = 12 V

LED forward voltage = 2 V

Number of LEDs = 3

LED current = 20 mA

The total LED forward voltage is:

VLED(total) = 2 × 3

VLED(total) = 6 V

Therefore, the resistor must drop:

VR = 12 - 6

VR = 6 V

At 20 mA:

R = 6 / 0.020

R = 300 Ω

The theoretical resistor value is therefore 300 Ω.

Resistor Power

The resistor also dissipates power. It can be calculated using:

P = VR × I

or:

P = I² × R

For the previous example:

VR = 6 V

I = 20 mA

P = 6 × 0.020

P = 0.12 W

The resistor therefore dissipates approximately 120 mW. A resistor with an appropriate power rating should be selected with suitable safety margin.

Series LED Power Calculator

Minimum Supply Voltage

The supply voltage must be higher than the combined forward voltage of all the LEDs, otherwise there will not be enough voltage available for the current-limiting resistor.

Vsupply > VLED(total)

For example, three LEDs with 3 V forward voltage each require:

VLED(total) = 3 × 3

VLED(total) = 9 V

A 5 V supply cannot properly drive these three LEDs in series because the LED forward voltage already exceeds the available supply voltage.

Different LEDs in Series

The LEDs do not necessarily have to have identical forward voltages. For LEDs with different forward voltages, add each LED's voltage individually.

VLED(total) =
VLED1 + VLED2 + VLED3

For example:

LED 1 = 2.0 V
LED 2 = 2.2 V
LED 3 = 3.0 V

Total = 7.2 V

The resistor is then calculated from the remaining supply voltage.

Series LED Current

One of the important advantages of a series LED arrangement is that the same current flows through every LED.

ILED1 = ILED2 = ILED3

Therefore, if the circuit current is 15 mA, approximately 15 mA flows through every LED in the series string.

LED Forward Voltage

Forward voltage varies according to LED technology, color, current and temperature. Typical values should therefore be treated as approximate.

For accurate circuit design, use the forward voltage specified by the LED manufacturer at the intended operating current.

Choosing the Resistor Value

The calculator gives a theoretical resistance. In practice, a standard resistor value is normally selected.

If the calculated resistance is between two standard values, choosing the higher resistance generally results in a lower LED current.

Choosing a lower resistance increases LED current and should only be done when the resulting current remains within the LED's specifications.

LED Current and Brightness

A higher LED current generally produces greater brightness, but the maximum continuous current specified by the manufacturer must not be exceeded.

For indicator applications, LEDs can often operate at considerably lower currents than their maximum rating.

Series LEDs From a 5 V Supply

A 5 V supply is suitable for only a limited number of LEDs in series. For example, three red LEDs with approximately 2 V forward voltage each would require approximately:

2 + 2 + 2 = 6 V

This is already greater than 5 V, so three such LEDs cannot be driven from a 5 V source using a simple series resistor.

Two red LEDs would require approximately:

2 + 2 = 4 V

leaving approximately 1 V for the resistor.

Series LEDs From a 12 V Supply

A 12 V supply provides considerably more voltage headroom.

For example, four red LEDs at approximately 2 V each require:

4 × 2 = 8 V

This leaves:

12 - 8 = 4 V

for the current-limiting resistor.

At 20 mA:

R = 4 / 0.020

R = 200 Ω

Series LEDs and Power Supply Variation

When the supply voltage changes, the voltage available to the resistor also changes.

For example, a circuit designed for a nominal 12 V supply may experience a higher voltage depending on the power source.

The resistor should therefore be selected by considering the expected minimum and maximum supply voltage rather than only the nominal value.

Why LEDs Should Not Be Connected Directly to a Supply

LEDs have a steep current-versus-voltage characteristic. A small change in voltage can produce a significant change in current.

A series resistor provides negative feedback in the simple sense that the resistor voltage increases as current increases, limiting the current through the LED.

Higher current
      ↓
Higher resistor voltage
      ↓
Less voltage available to LED
      ↓
Current is limited

Series LED Strings vs Parallel LEDs

Common Mistakes

• Connecting the LEDs directly to a voltage source.
• Forgetting to add the forward voltages of series LEDs.
• Using a supply voltage lower than the total LED forward voltage.
• Exceeding the LED's maximum current.
• Using an inadequate resistor power rating.
• Ignoring supply-voltage variation.
• Assuming all LEDs have exactly the same forward voltage.
• Using the nominal LED forward voltage without considering the datasheet.

Key Points

• The same current flows through every series LED.
• Forward voltages add together in a series string.
• The supply voltage must exceed the total LED forward voltage.
• The remaining voltage is dropped by the series resistor.
• The resistor limits the LED current.
• Resistor power must be checked.
• LED forward voltage varies with operating conditions.
• For accurate designs, use the LED manufacturer's datasheet values.