3 way lamp

[tom_lax@bell]

how does a 3 way desk lamp change from low to medium and high. I have a lamp that does not change when the knob is turned. I have replaced the socket that the bulb is screwed into and I have a new 3 way bulb in the lamp. When the knob is turned, there are 2 clicks and the bulb is off and 2 clicks and the bulb is on, but at the same intensity. I know this is the same with a single stage bulb in it. How do I check a 3 way bulb and how do I check the 3 way fixture?

[mibn2cweus]

Best way to check is by substitution with a known working lamp/bulb combination. If either bulb works in a lamp then that lamp is probably OK. If one bulb works in either lamp and the other bulb does not work in the same lamp that bulb probably has a broken filament.

A 3-way bulb has two filaments. Both filaments are wired at one end to the threaded part of the base. The other end of one filament is wired to the center of the base bottom, (this allows a standard bulb to be used in a 3-way lamp socket). The other end of the other filament is wired to the ring at the bottom of the base. Turning the switch connects power to the ring, the center, or both thereby determining which filament/s receive current with the circuit completed through the threaded part of the lamp base.

If you do not have a known (tested) lamp/bulb to compare with you will need a tester to test each filament for continuity. Do not attempt to test outside the lamp with mains voltage. Use the filament/s to complete the circuit of a battery operated device such as a flashlight or buzzer. The bulb should have continuity between the threaded part of the base to the center and the ring at the bottom of the base.

[stuartsjg]

Yes, some lamps work with a twin filiament. a Low and med power one, both on together mean high power.

More commonly, if its a low voltage (ie. 12v) lamp, the base will be heavy as it contains a transformer.

If the lamp is a 12v lamp then the switch may chose a 4v for low, 7v for medium and a 12v tap for full power.

Others have a dimmer circuit which uses Triac control, you will have either a continuos dimmer or some fixed steps.

By the sounds of it your lamp is a bi-filiamnt type. I think perhaps the switch contacts have became dammaged and are messed about.

A new switch may be in order but a new lamp may be safet depening on your electrical skills.

Stuart.