Limiting Current And Picking Appropriate Series Resistors

Everything You Need To Know To Limit LED Current And Operate Them Safely

LED get destroyed immediately if you run them off normal power sources: their very low internal resistance causes a short-circuit and in a blink of a moment burn the LED and destroy it forever - almost like a fuse.

To safely operate LED, you need to limit the current supplied to the LED. There are three popular ways to do this:

  • via a Series Resistor or a Constant Voltage power supply
  • by using a Constant Current power supply
  • by daisy-chaining multiple LED

It is only relevant to supply the correct current to the LED: using a constant current power supply guarantees that.
Supplying a specific voltage to the LED (using a series resistor or a constant voltage power source) is not really providing a constant current:
LED are not perfectly identical and can vary in resistance. Factors like temperature further change their resistance. All of this affects the current they receive when supplied by a fixed voltage.

Using a LED Series Resistor

For hobbyist projects and low-powered indicator LED, a series resistor is simple, cheap and typically sufficient. The resistor and the LED form a classic voltage divider: the LED gets the voltage necessary to drive the appropriate current.

A voltage divider can only lower the input voltage. The input voltage must be higher than the forward voltage needed by the LED.

A voltage divider is a very inefficient way of reducing the voltage: excess energy is converted to heat and wasted. A series resistor should only be considered for low power indicator LED.

Calculating Resistor Value

The basic principle is a voltage divider: when you connect multiple resistors in series, the total voltage is divided across the individual resistors proportionally to their resistances.

In a traditional voltage divider, to calculate the required resistance you need to know the total voltage, the required voltage and the load resistance.

That’s a problem for LED because their resistance is typically not known (and can vary). So you need to calculate the resistance using alternate parameters. These values are needed:

  • Required LED voltage
  • Required LED current
  • The voltage you want to apply to power the LED

Here are three approaches to find out the values needed for your particular LED to then calculate the approriate resistance for the protective series resistor:

…When You Know The LED Specs

If you know the specs of your particular LED (taken from its data sheet or vendor information), you are all set. This is what you need to know:

  • Required LED voltage (its forward voltage), typically in the range of 1.5V-4.0V
  • Required LED current: for simple indicator LED typically in the range of 5-20mA

…When You Don’t Know LED Specs

If LED type and specs are unknown, there are well-known rules of thumb based on LED color and materials LED are made of:

Color Forward Voltage Material
blue 3.0-3.6 InGaN, SiC
green 2.0-3.5 GaP, AlGaInP, AlGaP
yellow 1.8-2.2 GaAsP, AlGaInP, GaP
orange 2.0-2.1 GaAsP, AlGaUInP, GaP
red 1.6-2.0 AlGaAs, GaAsP, AlGaInP, GaP

White LED are made from blue LED and share their specs.
Amber LED can either be made from orange LED (and then share their specs), or can be also made from blue LED (in which case their specs are similar to those)

Regarding current, low-power indicator LED typically need a current of 5-20mA. Other LED may use drastically higher currents: High power Cree LED for use in flashlights and emergency vehicles typically need 1-2A.

Some LED like green ones have a large voltage range. Always start with the lowest voltage, calculate the resistance for it (below), and then test-drive. Measure the current with a multimeter. If it is much lower than the expected current, you now know that the LED uses a higher forward voltage, and you can lower the resistance.

Identifying LED Specs by Testing

Probably the most interesting and precise way of identifying the specs of your LED is to test them. This exposes much of how LEDs truly work. For this you need:

  • A power supply with variable voltage in the range of 1.5-4V
  • Optionally an Ampere-Meter (multimeter or clamp) in case your power supply does not show the current

1. Preparation

Set the power supply to a definitely safe voltage like 1.5V.

Add an Ampere-Meter to one of the power supply outputs if the power supply has no current display.

Now connect the LED to the power supply. Turn on the power supply. The LED will typically not emit any light at this low initial voltage: it is below the LED forward voltage, so no current flows.

2. Raise Voltage Slowly

Now slowly raise the voltage and monitor both the current and the physical LED. You are about to witness the physics of LED:

  • Phase 1: Current Increases Slowly At first, when raising the voltage, the current will also raise, but very slowly. At some voltage, the LED starts to emit light. When you further increase the voltage, this will further increase the current. And the LED light emission will raise, too.
  • Phase 2: Current Increases Rapidly At one point you will notice that suddenly the current will start to raise much more rapidly when you raise the voltage. Now, every tiny increase in voltage will result in an ever higher current. You are now entering the danger zone and have soon passed the safe area of LED forward voltage: the LED now cannot shine much brighter and starts to increasingly convert input current into heat: the LED gets hotter without emitting much more light.
  • Phase 3: Stop You must now decide at which level the LED produces enough light for your purpose. To find the sweet spot, monitor the LED temperature: the LED can get warm but should not get hot. If it does, reduce the voltage.

LEDs perform better when operated at 50-70% of their maximum possible current. At this sweet spot, they often do not get hot, do not need extensive heat sinks, and when you compare their light yield, they are close to their maximum light output anyway.
Raising the current any further just adds problems without much benefit. Should you indeed need more light, get a higher rated LED.

3. Read Results

Now you can read the values required to calculate the resistance from your power supply and Ampere-Meter:

  • Required LED Voltage: the LED forward voltage as shown by the power supply at this point.
  • Required LED Current: the LED current as shown by the power supply or your Ampere-Meter.

Calculating LED Resistor

Let’s assume you want to power a normal red indicator LED off a car battery, and you don’t know the exact values for your *LED. Then this is the data to calculate with, taken from the rules of thumb table above:

  • Total voltage: 13.8V (controlled by you, can be any voltage above the LED voltage)
  • Required LED voltage: 1.8V (guessed for a red LED, you may start with 1.6V to be extra safe)
  • Required LED current: 10mA (guessed for a typical indicator LED, you could probably go up to 20mA)

How much voltage should the resistor remove?

Now you need the required voltage drop that the resistor should cause. It is your supply voltage - LED voltage = 13.8V - 1.8V = 12.0V

Finding Out The Required Resistance

The required resistance is finally calculated using Ohms Law: 12V / 0.01A (10mA) = 1.200 Ohm.

Round this value up to the next available resistor value you have. In this case, a 1.2KOhm resistor would work.

Using Constant Voltage

Constant voltage is the same as using a series resistor: you again supply the LED forward voltage to the LED(s). Only in this case, you are using a constant voltage power supply to provide this voltage directly instead of dropping excess voltage using a series resistor.

Constant voltage power supplies are much more efficient than voltage dividers because they do not waste excess energy through heat. They are therefore suited even for higher powered LED and LED strips.
You would in turn never use a relatively costly constant voltage power supply to drive a single LED.

Using Constant Current

The most fundamental spec of every LED is the current it needs. Using a constant current power supply is therefore the best way to drive LED.

Constant current power supplies are the most complex way of supplying power mentioned here which is why it is typically used for high power LED or LED strips, not for simple indicator LED.

Constant current power supplies automatically determine the voltage necessary to drive the requested current. They adjust the voltage automatically when resistances change, i.e. when LED get hot.
A constant current power supply can not lower the voltage arbitrarily though: it has a certain voltage range.
For example, you can get a 700mA constant current supply that has a voltage range of 30-38V.
This particular power supply would be completely unsuitable to drive a single LED because it outputs *at least 30V.
If you use LED with a forward voltage of 3.5V each, though, you can connect 10 LED in series. They now have a total forward voltage of 35V which is well in the voltage range of the power supply.
The power supply would now drive the 10 LED and provide the *constant current of 700mA to each of the LED in your string.

Daisy-Chaining Multiple LED

Each LED has its specific forward voltage, and a red LED may have a forward voltage of 2.0V.

When you connect multiple LED in series, the individual forward voltages sum up.

For example, when you connect six red LED in series, the total forward voltage is 2.0V x 6 = 12V. You can connect these six LED directly to a 12V power supply without the need of series resistors.

Be aware that this does not allow for much variation of operating voltage. If you connected the string of six LED to your car battery, the true operating voltage may be in the range of 11.8-13.8V, depending on state of charge.
This would apply a voltage of 1.97-2.3V to each individual LED. Most of the time you would get away with this. Using a series resistor, or controlling voltage or current through a power supply is protecting your LED better, though.
The more LED you daisy chain, and the higher the total forward voltage is, the less important are variations in operating voltage. That is why cheap LED flood lights designed to be operated by 110/220V often use this approach: when you connect 110 LED in series, even a variation of 10V in input voltage would change the individual LED voltage just by 0.1V.


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(content created Mar 30, 2024)