Lights rating

[Luke1]

Hey just a quick question, ive got 9 x 50w halogen down lights that i would like to switch from a dimmer. How many watts is the switch goin to take? because ive got a 250w one and it smellys a little funky.

 

[davy_owen_88]

A simple calculator will tell you the answer. 9 x 50 = 450w - a little more than the 250w dimmer will handle which would explain the smell.

Davy

 

[Luke1]

got'cha.

but i thought lights in a house are installed in parallel. so you couldnt just add the wattage up?

 

[RF Lighting]

But as the lights are halogen the dimmer needs to be derated by 25%, so you would need a 1000W dimmer.

 

[Luke1]

But as the lights are halogen the dimmer needs to be derated by 25%, so you would need a 1000W dimmer.

whys that then matey, just out of curiosity. Is it becuase halogens are dirty energy zappin whores?

 

[Adam_151]

But as the lights are halogen the dimmer needs to be derated by 25%, so you would need a 1000W dimmer.

whys that then matey, just out of curiosity. Is it becuase halogens are dirty energy zappin ******?

Because the cold resistance of a TH is smaller than that of an equivelent rated normal lamp

 

[davy_owen_88]

got'cha.

I never underestimate the stupidity of some people so a seemingly sarcastic question receives a suitably sarcastic response.

When certain loads that draw high starting currents are connected to dimmers it must be derated so the inrush current doesn't knacker the dimmer.

 

[RF Lighting]

Basically there is a more severe inrush current with halogens, which can kill a dimmer if it hasn't been derated.

The lamps pull their rated current when they are running, but when they are first switched on they use more power to get the filament upto temperature.

 

[Luke1]

think i understand now.

Thanks for clearing that up for me.

 

[tim west]

A bit of preheat will get round this, you could wind up the dimmer just a bit before going to full on, it takes milliseconds to warm up the filaments and this will stop the inrush current, it's also good practice with GLS incandescent lamps as you don't want to stress the filament turning full on straight away.
Of course this will not work with some dimmers that do switch to full first before you can dim them, such as some X10 dimmers. you should still stay within the wattage rating of the dimmer though.

 

[securespark]

But as the lights are halogen the dimmer needs to be derated by 25%

Thought it was 50.

 

[bernardgreen]

A bit of preheat will get round this, you could wind up the dimmer just a bit before going to full on

But the triac is still having to switch the cold to warm filament current 100 times a second so if the triac is un-able to carry that much current it will still fail.

it's also good practice with GLS incandescent lamps as you don't want to stress the filament turning full on straight away.

That is true, significantly extends the life of bulbs that are being turned on and off continuously and it allows to use a smaller triac in the controller at the small expence of the capacitor for the pre-heat.

 

[RF Lighting]

MK and vari-lite (and some others) are soft start dimmers.

I have a soft start dimmer on my bathroom, which is not derated and we have not had any of the lamps fail in 5 years (5no. 50W 12V MR16 lamps)

 

[tim west]

But the triac is still having to switch the cold to warm filament current 100 times a second so if the triac is un-able to carry that much current it will still fail.

Sorry to disagree but that is not the case, The way a triac based dimmer works is that it in effect changes the power characteristic of the load ie it chops the sine waveform at a particular timing in the cycle,
this is set by the timing circuit which has been preset by the user pot, this has the effect of not only controlling voltage but also current therefore the available power to the load is changed.

Think of a 100 watt lamp load, the rating is 100 watts but at 230v (Sine wave) .The power rating of the lamp is changed dependant on supply or to put it another way the supply sees a different wattage than it would at 230v( sine wave), when dimming you lower the voltage available but the current doesn't go up proportionately in line with ohms law as you are also limiting the current available because you are not taking the full RMS value of the available sine wave as in raw mains but a chopped waveform of the sine wave (the triac is only conducting for a portion of the full sine wave) this is why you cannot get an accurate reading with an ordinary voltmeter when reading dimmer outputs and you have to use a True RMS voltmeter as readings can be way out between the two meters (I have read differences as much as 50v at different settings of a dimmer.
Therefore if as i suggested you don't wind up the dimmer to full straight away but just a little from off, within a short time of milliseconds the inrush problem will not exist and it is therefore possible to use halogen loads within the rated spec of a triac dimmer and not have to double rate,
I'm not disagreeing with what's been said from a user point of view but from a technical point of view with what you have said. Users as they are,will ignore advice to preheat as suggested or they may be using dimmers with memory of previous level set, these could get damaged agreed if wound up past a certain level.

 

[Adam_151]

I'm not sure what you are trying to say there Tim, but certainly running a dimmer at half mast is more stressful than turning it full on, your explanation of the phase cutting that happens with a standard dimmer is spot on, but the instantaneous currents (which matter to electronics) are going to greater,if for example you set your dimmer to loose the first third of each half cycle, your lamp is obviously going to be cooler, which means a lower resistance which means that for the part of the cycle for which the triac does conduct the currents will be greater (just ohms law)

I suppose if you just put it on say a 10th where the waveform is just about to cross zero again and the voltages involved are a long way from peak then the currents would be manageable and it'd just take the chill off the filament, like preheat does, before turning it up to full pretty quickly, but turning it up slowly and going through the range when you can actually see any visable light is going to be more stressful for it than putting it straight on (though with most dimmers going through this would be difficult to avoid!)