Choosing Static or Dynamic Lighting for LEDs

Table of Contents

Introduction
Selection of Lighting Method
Dynamic Lighting Frequency
Static and Dynamic Lighting

Introduction

Two methods exist to light an LED: the static lighting method, where a steady current is fed without a break; and the dynamic lighting method, where current is continuously input in ON-OFF pulses. If the pulses are short, the human eye perceives dynamic lighting as static lighting. Figure 1 compares static and dynamic lighting, and shows the constants for real circuits using the two types of lighting methods.

Figure 1a & 1b. Static lighting and dynamic lighting drive examples.

Selection of Lighting Method

The key applications of static and dynamic lighting and the corresponding viewing angles are listed below:

Application Viewing Angle
High-Brightness LED information panel 15° to 30°
Signal applications 8° to 30°
Low-Brightness LED information panel 30° to 120°
Narrow-Direction indicator 30° to 60°
Wide-Direction indicator 60° to 120°
Automotive stop lamp 20° to 50°
Automotive dashboard narrow directionality 20° to 60°
Automotive dashboard wide directionality 60° to 120°
Automotive dashboard wide directionality 60° to 120°

Dynamic Lighting Frequency

The use of high-frequency lighting makes dynamic lighting appear continuous for a stationary observer. Although when the frequency drops below a specific value, the naked human eye can notice a flickering effect.

If square-wave lighting or sine wave lighting (now widely prevalent) is used, flickering can be observed when the lighting frequency decreases below 50 Hz and flashing can be noted when the frequency is around 40 Hz. To avoid any problems the lighting frequency selected should be of minimum 100 Hz.

When the observer is jolted, for instance if a person is walking or riding in a car or is being imaged by certain types of cameras, flickering can become apparent irrespective of high lighting frequency. The lighting frequency selection should be based on the requirements of the specific application.

Static and Dynamic Lighting

Figure 1a shows an example of static lighting used in an LED lamp served as an indicator. For instance, when a 10 mA forward current is used to illuminate an LED lamp, the forward voltage is estimated to be roughly 2 V based on the LED lamp characteristics diagram (forward voltage - forward current). With a power supply voltage (Vcc) of 5 V and a resistance R as illustrated in Figure 1, the voltage drop Vr is: Vr = Vcc - (forward voltage) = 5.0 - 2.0 = 3.0 V. If a 10 mA current flows through the resistor, the resistance is 300 W (R = 3 V/10 mA). After calculating the resistance R, the problem, if any, caused by power voltage fluctuation is observed. The closest resistance value of an LED in a LED series is selected as the value of R.

For dynamic lighting or pulse lighting, FETs, bipolar transistors, and dedicated ICs are often used. Figure 1b shows an example of pulse lighting. The transistor utilized in the circuit is a 2SA1298(Y) transistor, the power supply voltage (Vcc) is 5 V, and the LED lamp forward current is 80 mA.

The base current (Lb) utilized for the 80 mA transistor collector current(Ic) is roughly 1 mA, based on the characteristics curve in Figure 2. Since the transistor voltage Vbe is typically 0.7 V, the resistance Rb in the circuit Figure 1b is: Rb = (5 - 0.7)V/1 mA = 4.3Ω. Setting lb to 2 mA for stabilizing the circuit leads to Rb = (5 - 0.7)V/2 mA = 2.15kΩ, which is sufficient.

Figure 2. 2SA1298 Transistor characteristics

This information has been sourced, reviewed and adapted from materials provided by Marktech Optoelectronics.

For more information on this source, please visit Marktech Optoelectronics.

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