I was about to buy some dimmable compact fluorescent light bulbs for a 3-way
light fixture (low, med, hi) but then found out that they are not the proper
ones b/c they work differently. I know I can't use regular CFL lights but
how do the 3-way switches on light fixtures work? I read how modern dimmer
switches work on howthingswork.com but couldn't find anything on how 3 way
switches work. I'm an ME so I know only basic ohm's law stuff. Thanks.
Single filament dimmers are electronic, using a triac and a variable
resistor, or a triac and fancy electronics for the type you can touch
anywhere on the lamp. There is a difference between these dimmers and
fluorescent dimmers. Don't know exactly what the difference is.
Generally, 3-way incandescent light fixtures work the same way as car
headlights, though with car headlights both filaments are not active at the
same time, but in different positions within the housing. A 3-way
incandescent bulb will have three contacts, the screw side contact, and two
on the tip.
Are you talking about a dimmer switch that can vary the brightness of an
incandescent bulb anywhere from zero to full brightness, or a table lamp
with 4 brightness positions (off, dim, medium, bright)? The continuous
dimmers modify the waveform and the effective voltage. CF bulbs are
damaged by them although they may have some special CF bulbs that work
with them now.
The table lamps have 4 position switches and a special 3 conductor socket.
The socket will accept regular bulbs, but if you use them, you have to
turn the switch to click twice to go on, and twice more to go off.
(ever wonder why your lamp does that? Now you know)
The proper bulbs have 2 filaments and 3 conductors on its base (the third
is a ring around the center), and turning the switch goes from:
off - A - B - A+B where A is a low wattage filament connected from the
ring to the shell (say 25 watts), and B is a higher wattage filament from
the center to the shell (say 60 watts) so there are 3 brightnesses: 25, 60
and 85 watts.
Look in the socket of that type of table lamp, and the base of the proper
bulb for such a lamp.
They do make special CF bulbs with 3 conductor bases that work properly
with such lamps.
Okay so that's totally clear. Now to throw another animal in the mix. An
N-way switch that works on any light bulbs. I have some regular halogen
bulbs with 2 conductors. I threw them in a new free-standing 2-bulb floor
fixture that plugs into the wall. First click on the rotary switch is low,
and second click is high. Another click goes to off.
I put in a compact fluorescent and clicked the first click for just a moment
and noticed the tell-tale flickering and slight hum. Right now the outlet
on the wall is connected to a wall switch so I left the CFLs in the fixture.
The second click silenced that and brought the brightness up. Is it safe to
say that the (cheap) light fixture uses a simple varisistor or a similar
setup to lower the peak voltage to the light bulbs? A late-model dimmer
would chop off the rise/fall of the sine wave, and I only have a DMM
available, and I don't have an o-scope or DAQ right now to take a look at
the actual waveform.
That must be a diode in there. I haven't actually heard of a lamp which
uses a diode for low, but it's a rather obvious design. (I've seen 'power
saver' or 'bulb life extenders' which are diodes in a disk that sits in the
socket at the base of the bulb)
I suspect CF's wouldn't like the 'low' setting but that depends entirely
on the design of its internal power circuit.
Again, probably a rectifier diode, and probably isn't good for the CF.
Its possible that the first switch position (dim) inserts a half wave
rectifier (a diode) in the circuit so that an incandescent bulb will be
supplied with a 50% duty cycle.
To check this, unplug the fixture(!) and remove the bulb. Place the
switch in the dim position (you might have to do this while its still
plugged in) and measure the resistance from the lamp socket center
terminal to the hot blade on the plug with an ohmmeter. Now, reverse the
ohmmeter leads and the resistance should change (switching from for ward
to reverse biasing the diode). If it does, then the fixture is using the