Product Specifications

Overview:
The S2 BA20D is a classic single-filament, single-contact halogen bulb primarily used as a tail light/brake light on motorcycles. It is a very common and standardized signal light source in the lighting systems of older motorcycles and some small motor vehicles.

Core Features and Specifications:

1. Model & Specifications

   • S2: This is its optical and electrical designation. The S2 specifically refers to a single-filament, dual-powerbulb design. It contains one filament but achieves two brightness levels:

1. • Rated Voltage: 12V, compatible with most motorcycle electrical systems.

   • BA20D: This is its base/bayonet type. It is a single-contact, symmetrical double-pin bayonet base. This is physically the same specification as the "BA20d" base mentioned previously (different notation). The letter "D" or "d" here denotes the "double-pin" mechanical structure, and it typically has a single electrical contact.

2. Working Principle & Application

   • Working Principle: The bulb achieves power switching via two separate grounding paths in the socket. When the tail light circuit is engaged, current flows through a specific portion of the filament, producing 5W output. When the brake switch is activated, full power is supplied to the filament, producing 21W output.

   • Primary Applications:

     • Combined tail light on motorcycles/scooters (serving both tail and brake light functions).

     • Rear position lights on some vehicles.

     • Occasionally used as a turn signal on certain models (requires an amber lens or bulb).

3. Key Distinctions & Compatibility

   • Vs. Standard Single-Power Bulbs: Ordinary single-filament bulbs (e.g., P21W) have only one power rating (e.g., 21W) and cannot provide the dual-brightness function of tail/brake lights.

   • Vs. Dual-Filament Bulbs (e.g., 1157): Common automotive dual-contact brake bulbs (like 1157) use two independent filaments. The S2 is a single-filament, dual-power bulb that changes brightness through circuit design.

   • Compatibility with other BA20d base bulbs: The BA20D base is physically identical in size and pin layout to the BA20d base. It can be interchanged in the socket with other bulbs using this base (e.g., single-power P21W). However, functional compatibility is crucial: if the vehicle's circuit is designed for the S2's dual-power function, replacing it with a single-power bulb will cause the tail light or brake light function to fail.