28 April, 2022
How to Wire a Dual-Filament Light Bulb
Dual-filament light bulbs are made to provide both a brighter and a dimmer filament, typically at the same time. They are commonly used in a variety of applications, including halogen lighting (in the form of headlamps and directional lights) and LED lights.
The light bulb consists of an air-tight glass enclosure (the envelope, or bulb) with a filament of tungsten wire inside, through which an electric current is passed. Contact wires and a base with two or more conductors provide electrical connections to the filament. The emitted light is usually a warm white color, although colored bulbs are also available.
Filament materials: Tungsten, Carbon and Metal
The most common type of filament is a long, thin length of tungsten wire. It is coiled to make it small enough to fit in the envelope of a standard bulb. In a 60-watt bulb, the filament is 6.5 feet (2 meters) long and one-hundredth of an inch thick. It is supported by several wires, which remove some of the heat that would otherwise be lost. This increases the lamp’s mechanical strength but reduces its efficiency.
Tungsten, the first choice of filament material for a light bulb, is a metal that can withstand high temperatures and resist corrosion. It is also a good conductor of electricity, meaning that an electric current will flow through it without producing much heat. Tungsten is more expensive than carbon, but it produces less heat and lasts longer. It is also more durable than carbon, so it is used in a wide range of lamps.
In addition to its use as a filament, tungsten is an important component of most power transformers and other electronic components. It is an effective insulator, which helps to prevent unwanted power spikes from damaging the power supply.
A tungsten filament can be shaped in various ways to optimize the amount of light it emitted, and it can be coated with a material that improves its brightness or its lifespan. A coating of phosphorus is common, and other conductive oxides, such as aluminum, can be applied to the filament. Other coatings may include titanium, nickel, zinc, niobium, cobalt, copper and other metals.
Early light bulbs with carbon filaments used a fill gas, such as nitrogen or argon, to protect the filament from oxygen and reduce its evaporation. The fill gas also reduces convective losses, which can decrease efficiency. The resulting lamp was less efficient than a vacuum-enclosed tungsten bulb, but it lasted longer and was easier to manufacture.
The use of fill gases also stabilized the operation of early lamp filaments against minor variations in the power supply. The negative temperature coefficient of the carbon filaments lowered their resistance, so the lamp was sensitive to small changes in voltage. This is why carbon filaments were replaced by metal ones that have a positive temperature coefficient of resistance, like a metallic conductor.
Similarly, the metal filaments in early fluorescent lamps had a fill gas, such as a mixture of mercury vapor and carbon monoxide. The fill gas reduced the filament’s temperature by about a hundred degrees Celsius, which helped to increase its longevity.