What resistor is needed for dimmer and CFL combo

[fahdoos]

So here's the issue....

I have a 600watt dimmer running 3 x 15watt dimmable CFLs connected in parallel (so I believe 45watts in total). When I turn the dimmer on, the lights go from barely on to full brightness by the 2nd step, and anything more puts the dimmer into what seems to be a fault protection mode where it flashes the lights on and off every second. I was wondering how I could limit the power being distributed to the lights so that the brightest level of the dimmer would only push 45watts to the bulbs, and would give an gradual increase of brightness throughout.

Hope this makes sense, and any help is appreciated. Thanks!

 

[chuckey]

You have got the wrong sort of CFL. The dimmer works by pulsing the mains on and off rapidly, the CFLs have an oscillator in them that is running slower then the dimmers pulses so the results are unpredictable. You can get CFLs that sort out the pulsing mains and appear to dim correctly.
Frank

 

[fahdoos]

You have got the wrong sort of CFL. The dimmer works by pulsing the mains on and off rapidly, the CFLs have an oscillator in them that is running slower then the dimmers pulses so the results are unpredictable. You can get CFLs that sort out the pulsing mains and appear to dim correctly.
Frank

Well they are dimmable CFLs, and seem to work correctly, just that they're driven to full brightness very quickly. Basically, there are 7 steps on the dimmer. At step 1, the CFLs are very dim. At step 2 they're at full brightness. If I try step 3, the dimmer goes into some fault protection mode where it keeps cycling through steps 1-3. I figured this was because the dimmer is pushing too much power early on due to the lightened load of the CFLs, and just needed a resistor in there - but then again, I have no idea what I'm talking about. :wink:

 

[chuckey]

As the CFls are the dimmable types, perhaps there is something wrong with the dimmer. The power rating of a dimmer is given by the maximum current it can handle i.e. your 600W one, is 230 V at 600/230 Amps ~2.6 Amps. More current then this and the output triac overheats and lets out the magic smoke. It could be that its just plain faulty - try it with a conventional 60W bulb.
Frank

 

[fahdoos]

As the CFls are the dimmable types, perhaps there is something wrong with the dimmer. The power rating of a dimmer is given by the maximum current it can handle i.e. your 600W one, is 230 V at 600/230 Amps ~2.6 Amps. More current then this and the output triac overheats and lets out the magic smoke. It could be that its just plain faulty - try it with a conventional 60W bulb.
Frank

I tried the dimmer with a single CFL and incandescent 60W. The single CFL exhibited the same behavior, by the 3rd step the bulb was at full brightness. The incandescent, on the other hand, worked like a charm; gradually increased brightness all the way to the highest level.

I then put both the CFL and incandescent in at the same time. In this configuration, the CFL was at full brightness by the 3rd step, then as pushed past, the dimmer didn't go into the fault mode and i was able to increase the brightness of the incandescent all the way to the 7th step. From the 3rd to 7th step the CFL stayed at the same brightness.

Thoughts?

 

[chuckey]

GOT IT!! How the dimmer works is that it only allows the power to flow for a fraction of a half cycle of the mains, let me explain:- The mains is an alternating sine wave, imagine the voltage smoothly going from zero to its max value then smoothly going back through zero to the same max value, but in the negative direction, it then reverses its direction and goes positive. This process continues all the time the power is on. If you don't know what a sine wave looks like, it could be approximated by semi circles sitting on the zero volts line, a positive going one followed by a negative one then a positive one. . .
The dimmer works by holding the power "off" during the initial part of each half cylcle the letting it flow for the rest of the cycle. So if the power is held off until the last, say 10% of each cycle, the actual power getting through is low, so the bulb is dim. If the power is let through right at the start of each half cycle the power flowing is at max so the bulb is bright. What is happening is that as the dimming level is reduced (bulb getting brighter) you actually pass the maximum voltage point, the CFL has a rectifier in it to turn the incoming AC into DC and this then has the maximum voltage on it and it charges up its capacitor to maximum voltage and hence the CFL goes to maximum brightness.
How to cure? Get different CFLs or dimmer
Frank ( I'll see if I can attach a diagram)