A 3-way lamp, or tri-light, uses a 3-way light bulb and socket controlled by a 3-way switch to produce three light levels—low, medium, and high. Early incandescent 3-way bulbs, featuring dual filaments operating at full voltage, date back to 1902, allowing adjustable lighting. Modern alternatives include certain compact fluorescent lamps designed to replace incandescent 3-way bulbs, incorporating extra contacts and circuitry to dim light levels similarly. More recently, LED 3-way bulbs have become available, offering energy-efficient options for adjustable lighting needs.
3-way bulbs
A 3-way incandescent bulb has two filaments designed to produce different amounts of light. The two filaments can be activated separately or together, giving three different amounts of light. One common 3-way incandescent bulb is the 50/100/150 W. It has a low-power 50 W filament and a medium-power 100 W filament. When they are both energized at the same time, 150 W of power is delivered, and a high level of light is produced. Usually screw-base 3-way bulbs fit into regular Type A sockets (E26D after ISO 60061-1:2014). Larger 3-way bulbs (up to 300 W) have a larger "mogul" base (E39D). These 3-way bulbs can also come in spiral-shaped compact fluorescent lamps.4
3-way sockets
A standard screw lamp socket has only two electrical contacts. In the center of the bottom of a standard socket is the hot contact (contact one in photo), which typically looks like a small metal tongue bent over. The threaded metal shell is itself the neutral contact (contact three in photo). When a standard bulb is screwed into a standard socket, a matching contact on the bottom of the bulb presses against the metal tongue in the center of the socket, creating the live connection. The metal threads of the bulb base touch the socket shell, which creates the neutral connection, and this is how the electrical circuit is completed.
A 3-way socket has three electrical contacts. In addition to the two contacts of the standard socket, a third contact is added. This contact is positioned off-center in the bottom of the socket (contact two in photo). This extra contact matches a ring-shaped contact on the bottom of a 3-way bulb, which creates the connection for the second filament inside the bulb. A problem of these devices is that the ring contact of the socket (contact two in photo) digs into the lead seal on the lamp bulb's contact ring and this connection tends to fail early (sometimes in months), leading to intermittent flashes, popping noises, and loss of power to the low wattage filament as the lead seal on the bulb's ring alternately melts and solidifies.
The center contact of the bulb typically connects to the medium-power filament, and the ring connects to the low-power filament. Thus, if a 3-way bulb is screwed into a standard light socket that has only a center contact, only the medium-power filament operates. In the case of the 50/100/150 W bulb, putting this bulb in a regular lamp socket will result in it behaving like a normal 100 W bulb.
A key switch 3-way socket has the switch incorporated in the lamp socket and requires no external wiring between switch and socket. This would be typical in a 3-way floor-standing floor lamp. A 3-way socket that is to be wired to a separate 3-way 2-circuit switch, is called a keyless 3-way socket.5
| Lamp function | Switch one | Switch two |
|---|---|---|
| Off | Off | Off |
| Low, lamp one | On | Off |
| Medium, lamp two | Off | On |
| High, lamps one and two | On | On |
See also
- Multiway switching
- Pull switch – Switch operated with pull chain found on ceiling fans
- Rotary switch – Type of mechanical device
References
US 931807, "Incandescent Lamp" https://worldwide.espacenet.com/textdoc?DB=EPODOC&IDX=US931807 ↩
Matthews, Mike (October 28, 2013). "How a 3-Way CFL Switch Works". HomeGuides.SFGate.com. Hearst Newspapers. Retrieved March 21, 2022. https://homeguides.sfgate.com/3way-cfl-switch-works-79641.html ↩
Lewis, Bill (December 19, 2022). Campbell, Larry (ed.). "What Is a 3-Way Light Bulb and Socket?". The Spruce. Retrieved January 7, 2023. https://www.thespruce.com/3-way-light-bulbs-and-sockets-2175016 ↩
Matthews, Mike (October 28, 2013). "How a 3-Way CFL Switch Works". HomeGuides.SFGate.com. Hearst Newspapers. Retrieved March 21, 2022. https://homeguides.sfgate.com/3way-cfl-switch-works-79641.html ↩
Croft, Terrel (1987). Summers, Wilford I. (ed.). American Electricians' Handbook (Eleventh ed.). McGraw-Hill. ISBN 0-07-013932-6. 0-07-013932-6 ↩