Compact Fluorescent Light Bulbs on a 3-way switch?

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.
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Phil



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This is how a three way light fixture works:
http://home.howstuffworks.com/question24.htm
You can find three way compact fluorescent bulbs at:
http://www.cetsolar.com/compactfluorescent.htm
http://www.1000bulbs.com/product.php?product3438
Found these places using Google. Don't know if they're any good.
CS
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CS wrote:

Weird. How many leads does the base have then?
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Phil



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CS wrote:

Actually that's how 3-way light bulbs work (a la hi-beam headlights). How does a 3-way light fixture (that dims regular single-filament light bulbs) work?
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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.
CS
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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.
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Michael Moroney wrote:

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.
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Phil wrote:

I'd suspect a simple diode switched in when set to low power. Easy enough to check with your DMM - use it to look for a diode between mains plug pin and lamp socket pin, on the low power setting.
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Palindrome wrote:

You mean that it cuts off either the positive or negative half of the waveform? I can see how it would work for incandescents due to the ramp-up, ramp-down... That would wreck the CFL pretty quick...
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Phil wrote:

Yup.
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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.
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Phil wrote:

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 diode trick.
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Paul Hovnanian P.E. wrote:

Yep. I get a 3.6Mohm setup one way and infinite resistance another way. I guess I can't use a CFL with this thing. Thanks so much for helping me decipher this thing.
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