Dimmer switch what one for 10 mains downlighters ?

[dino89]

hi
have ten mains halogen downlighters in my front room would like to put a dimmer switch in ,so max blub for each is 50w so what rated dimmer do i need most seem rated in VA!! see one in screw fix ....Au Dimmer Switch 1G 800Va White ...what is 800 VA ? could some one tell me what i require please in watts and VA
thanks
Dean

 

[BJS_Spark]

Mains lamps have a power factor of 1 so VA=Watts. So 800VA = 800W in this case, and your total load would appear to be 500W, so fine.

You should be made aware that dimming Halogen lamps may ruin their 'Halogen cycle' as they may not get as hot as they should. This may result in reduced life of the lamps.

 

[ban-all-sheds]

1) When people talk about "watts" in a domestic context, they actually mean VA, i.e. volt-amps, i.e. the product of voltage and current. In a domestic context watts and VA are virtually the same, since domestic loads are mostly non-reactive, so the power factor is extremely close to unity. You can read all about it here http://en.wikipedia.org/wiki/Apparent_power .

2) Mains halogen lamps are not the easiest of loads to dim because when they are cold they draw a great deal of current, so the surge when you turn them on is very high. As a result dimmers need to be de-rated when used with halogen lamps. The amount of de-rating varies; a quality make such as MK Intelligent only need 20%, but some require as much as 50%, i.e. for your 500W of lighting you might need a dimmer rated at 1kW/1kVA - you need to look at the technical specs of the one you are considering to see if it will handle the load - do not go by the headline figure of 800VA.

 

[ban-all-sheds]

Mains lamps have a power factor of 1 so VA=Watts. So 800VA = 800W in this case, and your total load would appear to be 500W, so fine.

A somewhat sweeping statement (or have you checked the specs for this dimmer to see if it needs de-rating - something I've not done...?)

 

[BJS_Spark]

Au Dimmer Switch 1G 2W 800Va White

Smooth-dimming switch with neon indicator and integrated over-temperature protection fuse. Use with mains and low voltage dimmable light fittings and resistive and inductive loads. Compatible with dimmable electronic transformers and tungsten lamps without de-rating. Fits 16mm plaster box depth.

So not so sweeping!!

 

[ban-all-sheds]

Fair enough - I did say that I hadn't checked - my advice was generic, and said that the specs should be checked...

 

[dino89]

Many thanks for the answers and help really appreciated
Dean

 

[olly_k]

just on the subject of electronic dimmers, do they still chop 'on' half way through cycle or are they 'reverse phase' these days i.e. they turn on at zero cross and off where conventional would turn on???
The reason I ask is that speaking with someone lately they thought a dimmer would improve lamp life, i.e. initial surge reduced but I argued because of the way they work this isn't nescesarily true....
so what is everyones views / experiences on this?

 

[ban-all-sheds]

Both types are made: http://uk.altavista.com/web/results?tlb=1&fr=tb&kgs=0&ienc=utf8&q=+leading++trailing++edge++dimmer*
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