This article describes various circuit concepts that may be suitable for custom LED tail light designs. The content within was intended for educational purposes, in an attempt to provide knowledge to the reader about the circuit theory associated with automotive or motorcycle LED brake tail lights and turn signals. For the readers convenience, the context has be broken down into individual paragraphs with a sub-title describing each section.
Most automotive and motorcycle electric systems operate from a 12 volt power source. This power source becomes the first consideration for any new custom LED tail light design circuity. It is important to understand the concept of a non-regulated power source, such as those found in automotive systems. In a non-regulated power supply, the source voltage can vary based on numerous factors. The current may also vary based on the circuit load. As the current and/or voltage fluctuate, the operational characteristics of the LED tail light may also vary.
The first voltage variable results from the vehicle alternator. In order for the battery to remained charged, the alternator must provide a voltage potential greater than the voltage potential of the battery itself. Therefore, when charging occurs, the entire power system within the vehicle becomes exposed to the higher voltage level required for charging. A typical car battery may provide 12 volts DC. While the alternator is charing the battery, this voltage level may increase to 13.8 volts. In rare cases, the voltage may even increase as high as 14 volts or greater. These increased voltages need to be taken into account during the initial LED tail light design process.
The second voltage variable that can cause voltage levels to change is referred to as loading. The battery and alternator can only supply a pre-specified electrical current at a specified voltage. Exceeding this current rating can result in voltage reductions. For example, we can measure 12 volts DC across the car battery with the headlights and taillights turned off. However, when the automotive headlights and tail lights come on, they will draw approximately 10 amps from the battery, which also results in a small voltage reduction. With the headlights and taillights on, we now measure only 11.7 volts across the car battery. Automotive sound systems may cause even greater voltage reductions unless the proper precautions are taken. Such reductions in voltage will directly affect the operation of the LED brake tail lights incorporated into the automotive electrical system.
Overall battery health can also affect voltage levels. As the car battery ages, the internal plates begin to build up oxidation. This degradation leads to a greater internal resistance which in turn, can lead to a decreased voltage levels. It is important to conciser this during the design of a custom LED tail light, to ensure the LED taillight will continue to function in cases where the battery voltage level drops below typical operating values.
LEDs always require some type of current regulation in order to achieve circuit normality. Since LEDs are current driven, the typical and maximum current ratings must be taken into consideration for LED tail light designs. Although a simple resistor to limit current can achieve this, the constant current source proves more reliable. The constant current source not only provides a stable current output, but also offers the advantage of protection against voltage source variations.