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Flickering dimmer switch


STEVE
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I have recently installed a dimmer switch to control a circuit of 6 lights (spots) running off the mains (not low voltage lights), but when the dimmer is switched on the lights continuously flicker. It's as if the switch isn't sensitive enough.

Is there anyway of adjusting the switch so that I get no flicker? I had an idea of putting capacitors across each of the lights to smooth the voltage out but am unsure if this will work ... ???

Any ideas would be much appreciated

Thanks

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reply to steve , be sure the stated wattage for the dimmer your useing agrees with the bulb wattage. allso low quality dimmers tend to cause flickering of the lights, vibratory hum from the switch itself. i personally had no expierience in trying six spoties on the mains but , i would just whack a mains rated ac capacitoe say 1uf across the ac output from the dimmer and see what gives.

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A small capacitor across a power AC circuit won't make any difference. A larger capacitor will simply try to reduce the AC voltage and heat the capacitor and dimmer.

Maybe the Diac or Triac is faulty and passes only half-wave. Are the lights still kind of dim when the dimmer is at maximum brightness?

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Yea they are still dim when at maximum brightness, how would I test the diac & triac to see if they are working correctly?

One of the bulbs has just blown !! I'll have a look at the power rating of the switch and the wattage of the bulbs attached in the morning (on GMT here!)

thanks

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Woopsey !! I'm afraid I've told a bit of a fib ... I have just finished dismantling the dimmer and lights.

On closer inspection the 6 spots are infact low voltage halogen lights cunningly disguised as mains spots (they are hidden in behind frosted galss). They are 12V 10W bulbs (6 bulbs = 60W) the dimmer range is between 60W - 400W. I think this is the reason for the flickering its right at the lower limit of the switch. I checked the triac and diac and they seem to be working fine.

I'm now thinking that I'm going to have to put a resistor in series with the lights. I'm guessing I should try and raise the circuit power consumption to around 200W.

So what size of resistor should i use (resistance and power rating ... )?

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Hi Steve,
Your dimmer switch is certainly faulty since it can't go to full brightness. If the total power of your bulbs is within the power rating of the dimmer, then the blown bulb may have caused it to fail at the moment that the bulb blew.
Sometimes a light bulb will blow when you turn-on the power because it is cold (very low resistance, much higher when hot) and the mains AC voltage may be at its peak during that moment. So the filiament gets a huge surge of current (which dimmers should withstand) which twists it (because it was old and weak anyway) creating a short circuit, blowing the dimmer. Maybe you were just unlucky with the timing.

You could see what the dimmer is doing with a 'scope. But don't connect a 'scope to the mains without proper isolation.

There is a lightbulb in Alaska, I think, that has continued to light for about 80 years! Because it was never turned off, it had that surge of current only once, at the very beginning when it was strong. Its filiament must be very fragile today, don't bump it.

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Ah-ha,
Don't "ordinary" dimmers say on them "not for use on transformer powered low-voltage lights"? I think that "special" dimmers are made for that purpose.
The inductance of the transformer will screw-up the phase control at low settings and cause flickering.

Hi Steve,
Don't use your "mains" dimmer on the low-voltage side.
Do you realise how big a resistor must be to dissipate 200W or more? What are you going to do with its heat? That's why people use cool, efficient dimmers that are designed for their purpose.

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