3-Way Light Bulbs and 3-Way Sockets

How 3-WayLight Bulbs and Sockets Work Together

the base of a 3-way light bulb
The base of a 3-way light bulb, with it's double set of contacts. (Bill Lewis)

Together, 3-way bulbs and sockets provide a convenient method for choosing one of three levels of light output – lumens -- from one light bulb. For that reason, we usually have them in bedside lamps or floor lamps near our favorite chair, or sometimes in a desk or table lamp.

These are all places where we tend to spend a fair amount of time doing a variety of activities: reading print material, for which we usually want a medium level of light; watching TV or reading or working with a backlit screen, when we prefer a lower level; or doing a task which requires viewing it clearly, such as sewing. That’s when we like to have more light.

With a 3-way bulb in a 3-way socket, we can have that. Most often, we use a bulb with a standard screw base (E26) that uses either 30/70/100 or 50/100/150 watts of power to produce the three levels of light. And some of us have floor lamps with a bowl-shaped shade that takes a larger bulb – a PS25 – that has a Mogul screw base (E39) and uses 100/200/300 watts of power to throw a lot of light up onto the wall and ceiling.

How Do They Work?

Screw base 3-way light bulbs and sockets work by supplying power first to the low-wattage element or filament – the 30W or 50W or 100W element in the three examples above, then to the middle-wattage element or filament (the 70W or 100W 0r 200W element), and finally to both at once. That’s why the highest wattage for an incandescent or CFL 3-way light bulb is the sum of the two lower wattages. LED 3-way light bulbs receive and apply power in the same way since that’s the way the sockets work, but they don’t always have wattages that add up, due to their different technology. But they’re close.

The way that’s done is to have two sets of contacts for the switched power in the socket and on the base of the bulb. Starting with the bulb, remember that the threaded outside of the base – the part that screws in – is connected to the “neutral,” or grounded conductor in the power supply. The contacts for the “hot,” or ungrounded, conductor are on the end of the base. To see those, turn a light bulb upside down.

A standard screw base light bulb will have one metal contact in the center, with an insulating material – usually molded glass – between that and the threaded metal shell. That’s the contact for the hot power.

A 3-way screw base light bulb has the same center contact as a regular light bulb. It’s in the center of the base and it’s usually pretty small. That’s where the power connects to the middle-wattage element in the bulb. There’s a ring of insulating glass around that, then a metal ring, and then a second insulating ring before you get to the threaded shell. That isolated metal ring is the second contact. That’s where the power is connected to the low-wattage element in the light bulb.

In every screw base socket, there’s a center contact to match the one on every screw base bulb. In a 3-way socket there a second contact to supply power to the ring-shaped contact on the base of the 3-way bulb. It’s a little tab, and it’s sitting to one side, midway between the center contact and the shell of the socket.

When I Turn the Switch, What Happens?

Electrical switches work by connecting and disconnecting the circuit from the panel to the load and back again. When you turn the switch control on a 3-way socket one click from “off,” the switch connects power to the socket’s tab-shaped contact, sending the power on the bulbs ring-shaped contact and its lower-wattage element. When you turn it one more click, it disconnects the tab and connects the power to the center contact. That takes it to the higher-wattage element in the bulb. Turning the switch one more time has it connect power to both the tab and the center point so that both of the elements in the bulb have power at the same time. One last click of the switch disconnects all of the power, and the light bulb goes off.

Off-On-On-Off Pattern

If you think this through, you’ll see that the switch in a 3-way socket is sending the power to the lower-wattage element in an on-off-on-off pattern and its sending power to the middle-wattage element in an off-on-on-off pattern. If you have a bulb in a 3-way socket that’s going off-on-on-off, one of three things is happening: the bulb may be a standard non-3-way bulb, the lower-wattage element in the 3-way-bulb may have burned out, or the contact for the lower-wattage element may be defective. It’s usually pretty easy to troubleshoot and fix the problem and fix or repair it, and have your favorite lamp functioning on all three “speeds” again.