3-way lamp
Encyclopedia
A 3-way lamp is a lamp
that uses a 3-way bulb to produces three levels of light in a low-medium-high configuration. A 3-way lamp requires a 3-way bulb and socket, and a 3-way 2-circuit switch. Unlike a lamp controlled by a dimmer
, each of the filaments operates at full voltage and the normal temperature for an incandescent bulb, so the color of the light does not change over the three steps of light available.
3-way bulbs fit into regular, medium-base Type A sockets; larger 3-way lamps (up to 300 watts) use a larger "mogul" base.
A 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 are of course touching the socket shell, which creates the neutral connection, and this is how the electrical circuit is completed.
In contrast, a 3-way socket is a socket that 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 to a special ring-shaped contact on the bottom of a 3-way bulb, which creates the connection for the second filament inside the bulb.
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 50W / 100W / 150W bulb, putting this bulb in a regular lamp socket will result in it behaving like a normal 100W bulb.
A 3-way socket with all three terminal connections exposed, that can be wired to a separate 3-way 2-circuit switch, is called a "keyless 3-way socket." This would be the typical setup in a 3-way floor-standing torchiere lamp.
or a pull-chain switch. Although it is referred to as a 3-way switch, it has four positions, off, lamp one (low), lamp two (medium), and lamps one and two (high). When properly connected to a 3-way socket containing a 3-way bulb, this switch will first power one filament, then the other filament, then both, then return to the off position. To do this, the switch must be capable of operating two different circuits. In other words, internal to the switch there are really two switches that are not connected electrically, but which are connected mechanically in such a way that they operate together as shown in this table.
It is very common that the 3-way 2-circuit switch of a 3-way lamp is incorporated directly together with the 3-way lamp socket. This is referred to as a "keyed 3-way socket". In this configuration, the switch internally routes power to the two live contacts in the bulb socket, so the wiring that connects the live contacts is not visible. These keyed sockets do not have three terminals where wires can be connected; a keyed socket only has two terminals to connect live and neutral. This would be the typical set-up in a 3-way table lamp.
A lamp with night light is often configured so that one of the two sockets is a medium-base socket, considered to be the Main Lamp (under the lamp shade), and the other socket is a candelabra-base socket that is the night light or decorative accent light. The night light is usually placed somewhere on or inside the body of the lamp. In a typical set-up, the main lamp would have a 150 W medium-base bulb, and the night light would have 7 W candelabra bulb. The operation of the switch is still the same, such that the night light comes on, then the main lamp, then both together, then it goes back to off.
Sometimes a 3-way 2-circuit switch is incorporated together with a regular socket, to be used as the main lamp socket in a lamp with a night light. In this configuration, the switched lamp one live terminal for the night light socket is exposed externally on the base of the socket, to be wired to the remote candelabra socket. Hence, this type of socket has a Hot terminal, a Neutral terminal, and a switched Hot terminal. This creates a confusing situation in which a keyed 3-way socket (which has a 3-contact socket at the top of it) has only two terminal screws, while a night-light socket (which does not have a 3-contact socket at the top) has three terminal screws on the base.
Lamp
Lamp is a light source, while LAMP is an acronym.-Lamps:Lamp may refer to one of the following:*Oil lamp, the original use of the term*Kerosene lamp, a lamp burning liquid petroleum...
that uses a 3-way bulb to produces three levels of light in a low-medium-high configuration. A 3-way lamp requires a 3-way bulb and socket, and a 3-way 2-circuit switch. Unlike a lamp controlled by a dimmer
Dimmer
Dimmers are devices used to vary the brightness of a light. By decreasing or increasing the RMS voltage and, hence, the mean power to the lamp, it is possible to vary the intensity of the light output...
, each of the filaments operates at full voltage and the normal temperature for an incandescent bulb, so the color of the light does not change over the three steps of light available.
Three-way bulb
A 3-way bulb has two filaments designed to produce different amounts of light. These two filaments can be activated individually or together, giving three different levels of light. A very typical 3-way incandescant bulb is a 50W / 100W / 150W bulb. It had a "low"-power 50W filament and a "medium"-power 100W filament. When they are both energized at the same time, 150W of power is consumed, and a "high" level of light is produced. Usually screw baseEdison screw
The Edison screw fitting is a system of connectors used for light bulbs, developed by Thomas Edison and licensed starting in 1909 under the Mazda trademark. Most have a right-hand threading, so that it goes in when turned clockwise and comes out when turned counterclockwise, like a hardware screw...
3-way bulbs fit into regular, medium-base Type A sockets; larger 3-way lamps (up to 300 watts) use a larger "mogul" base.
3-way sockets
A 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 are of course touching the socket shell, which creates the neutral connection, and this is how the electrical circuit is completed.
In contrast, a 3-way socket is a socket that 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 to a special ring-shaped contact on the bottom of a 3-way bulb, which creates the connection for the second filament inside the bulb.
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 50W / 100W / 150W bulb, putting this bulb in a regular lamp socket will result in it behaving like a normal 100W bulb.
A 3-way socket with all three terminal connections exposed, that can be wired to a separate 3-way 2-circuit switch, is called a "keyless 3-way socket." This would be the typical setup in a 3-way floor-standing torchiere lamp.
3-way 2-circuit switches
The switch used to control a 3-way lamp is usually a rotary switchRotary switch
A rotary switch is a switch operated by rotation. These are often chosen when more than 2 positions are needed, such as a three-speed fan or a CB radio with multiple frequencies of reception or "channels"....
or a pull-chain switch. Although it is referred to as a 3-way switch, it has four positions, off, lamp one (low), lamp two (medium), and lamps one and two (high). When properly connected to a 3-way socket containing a 3-way bulb, this switch will first power one filament, then the other filament, then both, then return to the off position. To do this, the switch must be capable of operating two different circuits. In other words, internal to the switch there are really two switches that are not connected electrically, but which are connected mechanically in such a way that they operate together as shown in this table.
| 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 |
It is very common that the 3-way 2-circuit switch of a 3-way lamp is incorporated directly together with the 3-way lamp socket. This is referred to as a "keyed 3-way socket". In this configuration, the switch internally routes power to the two live contacts in the bulb socket, so the wiring that connects the live contacts is not visible. These keyed sockets do not have three terminals where wires can be connected; a keyed socket only has two terminals to connect live and neutral. This would be the typical set-up in a 3-way table lamp.
Wiring a 3-way lamp
Wiring a 3-way lamp is a simple matter of connecting the 3-way switch's two switched live wires (frequently red for the low-wattage circuit and blue for the medium-wattage circuit) from the switch to the two live terminals on the 3-way socket. The lamp's power cord must be connected so that the wire from the wide blade on the power cord plug (neutral) connects to the neutral terminal on the socket, and the wire from the narrow blade on the plug (hot) connects to the black wire on the switch.Lamps with night lights
Another type of 2-circuit lamp is also fairly common. This is the lamp with night light. While related to a 3-way lamp, this lamp is different from a 3-way lamp in both its intention and the parts it uses. The main intent of a lamp with night light is not to offer three levels of light, but rather to offer only two levels: a bright working light, and a very dim night light or decorative accent. Typically it does still use the 3-way 2-circuit switch. However, instead of having one 3-way socket, it uses two regular sockets.A lamp with night light is often configured so that one of the two sockets is a medium-base socket, considered to be the Main Lamp (under the lamp shade), and the other socket is a candelabra-base socket that is the night light or decorative accent light. The night light is usually placed somewhere on or inside the body of the lamp. In a typical set-up, the main lamp would have a 150 W medium-base bulb, and the night light would have 7 W candelabra bulb. The operation of the switch is still the same, such that the night light comes on, then the main lamp, then both together, then it goes back to off.
Sometimes a 3-way 2-circuit switch is incorporated together with a regular socket, to be used as the main lamp socket in a lamp with a night light. In this configuration, the switched lamp one live terminal for the night light socket is exposed externally on the base of the socket, to be wired to the remote candelabra socket. Hence, this type of socket has a Hot terminal, a Neutral terminal, and a switched Hot terminal. This creates a confusing situation in which a keyed 3-way socket (which has a 3-contact socket at the top of it) has only two terminal screws, while a night-light socket (which does not have a 3-contact socket at the top) has three terminal screws on the base.