Product Specifications

Halogen Bulb 12V HD with BA20d Base

This is a dual-filament halogen headlight bulb primarily used in early (1970s-1990s) small to medium-displacement Japanese motorcycles, scooters, and some ATVs. It is a lower-wattage, simplified version of the 1HD bulb. While sharing the same physical appearance and base, there are crucial differences in their electrical parameters.

Key Features and Specifications:

1. Model & Specifications:

   • HD: This is the bulb model. It must be clearly distinguished from 1HD. The HD is a single-contact, dual-filament bulb, whereas the 1HD (equivalent to H4) is a dual-contact, dual-filament bulb. This is the most fundamental difference.

   • Voltage: Operating voltage is 12V.

   • Wattage: Its standard wattage is Low Beam 25W / High Beam 25W (i.e., 25/25W), significantly lower than the 35/35W of the 1HD. It was designed for the very limited electrical output of small vehicles of that era.

   • BA20d: This is its base/bayonet type. It is a single-contact, symmetrical double-pin bayonet base. This is the key point where it looks identical to the 1HD but differs electrically: The BA20d base of the HD has only one center electrical contact, with its outer metal shell serving as the common ground. The two filaments are connected in parallel inside the bulb. Therefore, while an HD bulb can physically fit into a 1HD socket (and vice versa), they are electrically incompatible. Direct substitution will cause the bulb not to light or circuit malfunction.

2. Application:

   • Primary Use: Used as the combined low/high beam main headlight in motorcycles designed with a single-headlight bulb. Common in models like the Honda Cub series, early small-displacement street bikes, etc.

   • Vehicle Era: Primarily found in earlier or smaller models with simpler charging systems.

3. Core Distinction (vs. 1HD/H4):

   • Contact Count: HD = 1 electrical contact; 1HD/H4 = 2 electrical contacts.

   • Working Principle: Both filaments of the HD bulb share the same power input (via the single center contact) and are wired in parallel internally. The selection of which filament lights up is controlled by different grounding paths in the socket or an external switch. In contrast, the two filaments of a 1HD/H4 bulb have independent contacts for separate control.

   • Compatibility: Not directly interchangeable with 1HD/H4. Installing a 1HD bulb in a vehicle designed for HD will not work properly (usually only one filament will light). Conversely, installing an HD bulb in a vehicle requiring 1HD may cause both beams to light simultaneously or functional disorder.