The nonlinear voltage-current characteristic of LEDs has been mentioned in several articles, so it is worth learning more about what it actually entails.
LEDs are also diodes, but they are light-emitting diodes widely used in lighting and displays. The nonlinear voltage-current characteristic is a typical property of diodes, which means that for diodes, including LEDs, their current consumption does not change in direct proportion to the applied voltage. At low voltages, only a small current flows through them, and above a certain forward voltage, the current suddenly increases. With further increases in voltage, the saturation region is reached. In this region, further increases in voltage only minimally increase the current. The saturation region of an LED depends on the manufacturing technology and the type of LED, but it is generally within a few tenths of a volt above the forward voltage. In this region, the efficiency of the LED decreases, and further voltage increases do not result in a significant increase in brightness.
Let’s take a practical approach! How might an average user encounter this property?
The forward voltage of LED strips actually depends on the manufacturing technology and the type, but generally, for example, the forward voltage of 12-volt white LED strips is within a few tenths of a volt below their operating voltage. It is important to note that 12-volt LED strips consist of series-connected LED groups, which affects the total forward voltage.
The 12-volt LED strips usually consist of groups of 3 LEDs, meaning there are three LEDs in each group. Accordingly, the forward voltage is the sum of the forward voltages of the LEDs in each group. For instance, if each LED has a forward voltage of 2.5 V, then the forward voltage per group is 7.5 volts (3 LEDs x 2.5 V). Therefore, in the case of 12-volt LED strips, the total forward voltage depends on the number of series-connected groups.
- So, if someone thinks that a 12-volt LED strip will light up a little with 6 volts or 5 volts, they will find that it will not work.
- If someone tries an RGB LED strip this way, they might notice that the red will light up a bit. Generally, red LEDs have the lowest forward voltage (among LEDs designed for the visible light spectrum).
- With an RGB LED strip, if you power a too long section, resulting in increasing voltage drop as you move away from the connection point, due to the different forward voltages of the different colors, if you set the RGB LED strip to white, you may observe that the tone of the white light gradually changes towards the end of the long strip.
The same logic applies to 24-volt LED strips, but here the summed forward voltage of the LEDs in each group is higher, as 24-volt strips contain more LEDs per group (usually 6 LEDs). Therefore, you can expect a forward voltage around 15 volts.
- This means that anyone who thinks that 12 volts will be enough for a 24-volt LED strip will be mistaken. It will not light up.
Another interesting issue that 5-volt DRGB LED strip users might encounter if they install their LED strip improperly:
- If you set the 5-volt DRGB LED strip to white, it is originally cool white, but even after just 1 meter (with a strong 16-watt per meter strip), it will turn warm white, then shift to orange, and by the end of the 2nd meter, it will be red.
LEDs are manufactured in various sizes and efficiencies, and higher efficiency LEDs emit more intense light for the same operating current.
Detailed Analysis of the Nonlinear Voltage-Current Characteristics of LEDs
LEDs, like most types of diodes, have a nonlinear voltage-current characteristic. This means that the current flowing through the LED does not increase proportionally with the voltage.
The characteristic can be divided into three main regions:
1. Pre-threshold region:
- In this region, the voltage is less than the threshold voltage (approximately 1.8-3.5V depending on the LED type).
- The current is very small.
- The LED does not light up.
2. Conduction region:
- In this region, the voltage exceeds the threshold voltage.
- The current increases exponentially with the increase in voltage.
- The LED starts to light up.
- The brightness increases gradually with the voltage.
3. Saturation region:
- In this region, the voltage continues to rise, but the current does not increase, or only increases slightly.
- The LED lights up at full brightness.
- Further increases in voltage can lead to a temperature rise and damage to the LED.
Consequences of the nonlinear characteristic:
- LEDs require a constant current source to ensure stable brightness and lifespan.
- LED driver circuits need to have a current limiting function to protect the LED.
- LEDs cannot be used directly on AC power sources.
Graphical representation of the nonlinear characteristic:
In the picture, the voltage is shown horizontally and the current vertically for different colored LEDs.
It is important to note:
- The nonlinear characteristics of LEDs can vary by manufacturer and type.
- The exact characteristic can be found in the LED’s datasheet.
The question arises: If LEDs require a constant current source to ensure stable brightness and lifespan, does the use of PWM signal to control LED brightness adversely affect the expected lifespan of the LED?
The PWM signal is a variable width pulse signal whose average power reflects the desired brightness. LEDs respond quickly to changes in brightness, so the high frequency of the PWM signal does not affect the LED’s operation. This is because carefully switching the LED drive does not cause problems for the LED. PWM control does not mean changing the power supply, i.e., changing the current or voltage, but rather it is as if the unchanged power supply is being turned on and off continuously. LEDs tolerate switching well.
So, what is PWM dimming?
PWM stands for Pulse Width Modulation. This method controls the brightness of LEDs by adjusting the width of the electrical pulse.
How does it work?
The PWM signal rapidly turns the LEDs on and off. The brightness is controlled by varying the ratio of on-time (ON) to off-time (OFF) within a given frequency. These frequencies can vary by manufacturer, and in some cases, the frequency can be adjusted to avoid flicker that may bother sensitive eyes or cameras when set to low brightness. For instance, if the signal alternates such that the on-time is longer and the off-time is shorter, the LED will be brighter. Conversely, if the on-time is shorter and the off-time is longer, the LED will be dimmer. LEDs have no thermal inertia, so they respond quickly.
However, it is important to note that the PWM control must meet the specifications of the LED. Too low a frequency can cause the LED to flicker, while too high a frequency can lead to a temperature increase. Additionally, the quality of the PWM signal is crucial. A noisy PWM signal can cause fluctuations in LED brightness.
PWM controls for adjusting LED brightness can come in different types.
- The most common are constant current PWM controllers. These controllers provide a constant current to the LED and regulate brightness by changing the pulse width of the PWM signal. This type of control is ideal for adjusting LED brightness because it ensures stable brightness and lifespan.
=> This is known as a dimmable LED driver or a dimmable current source power supply. - There are also constant voltage PWM controllers. These controllers provide a constant voltage to the LED and regulate brightness by changing the pulse width of the PWM signal. In this case, the current draw of the LEDs is set with current-limiting resistors (these resistors are mainly easy to notice on LED strips). This type of control is considered less ideal because the current-limiting resistors also consume power, which fundamentally reduces the theoretical efficiency of the LEDs.
=> These are typically LED strip dimmers, but CCT, RGB, RGBW, RGBCCT LED strip controllers operate on the same principle – they just handle 2-5 color channels to match our color, color temperature, or light flux needs.
Summary:
- LEDs require constant current to ensure stable brightness and longevity.
- Using a PWM signal to control the brightness of LEDs does not adversely affect the expected lifespan of the LEDs if the PWM control is properly designed and implemented.
- Constant current PWM controllers are ideal for adjusting the brightness of LEDs because they ensure stable brightness and longevity, but they can only be used with light sources that require a constant current power supply.
- Always use power supplies with the parameters specified by the manufacturer for LED light sources. It is crucial to know whether the LED’s power requirement is for constant current or constant voltage. If the LED requires constant current, only use a power supply that provides the specified current—no more, no less—and ensure the LED’s voltage requirements fall within the power supply’s voltage range.
- If you have an LED lamp that requires a constant voltage power supply, use only a power supply that provides the same stabilized voltage as specified by the lamp, and ensure it can supply at least the maximum expected current. If the power supply manufacturer indicates it can operate continuously at 100% load, it is sufficient for the power supply to match the LED’s maximum current draw. If this is not indicated, assume the power supply can only be loaded up to 80% in continuous operation, so the LED lamp’s maximum current should be no more than 80% of the power supply’s maximum output current.