Newsgroups: sci.electronics.misc,sci.electronics,sci.engr,sci.engr.lighting
Subject: Re: WHY are some CFBs incompatable with dimmers?
References: 
[email protected] (Dr F) writes:

> Hello everyone,
> 
> Many compact fluorescent bulbs (CFBs, CFLs) say that they should not
> be used with dimmer switches. Why not? 

The electronic power supply used in this kind of bulbs 
is built using switched mode power Supply. 
This type of power supply uses a full wave bridge rectifier 
followed by an energy storage capacitor, and then followed with 
the rest of the electronics. 

This type of circuit draws current only near the peak of the line voltage.
Triac-based dimmers do not like this type of load. 
The troac-based dimmers put out very sharp rising edge to the 
voltage waveform when they start to conduct, and the power supply 
circuit might not like this (very sharp rising voltage edge going 
to a rectifier and capacitor circuit, can lead to very high current 
peak taken from current, the rectifier and capacitor deigned for 
sinewave power might not like it). 

> Will the bulbs simply burn out,
> or might they cause fires and blown fuses? 

You could burn out the dimmer, or damage the lamp if it operates in short
pulses given by triac based dimmer. 

In best cases the lamp will operate on some dimemr setting well, and 
when you dim it just starts working baddly (flashing, etc.) without 
dimming nicely...

>  What causes this incompatability?

It is related to the features of the both dimmer operation and 
bubl electronics design. Maiking them such that they would work 
nicely together and dim nicely woudl make the system much more 
expensive...

> My specific question is this: I have a three-bulb ceiling fixture (3x
> 75 watt max) on a 270 degree turnpot dimmer. Can I safely put
> non-dimmer-compatible CFBs in this fixture? 

I would not recommend it. 

> What if I always keep the dimmer switch at full power, 
> and never at partial power? 

This could work possibly (quite often I think), but there is 
no guaratee that it will be problem free. And consider the situation 
when you just happen to leave the dimmer to wrong position....
I woudl not risk on this.

> Should I just replace the switch?

Replace the switch if you want to use non-dimmer-compatible CFBs.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/