Dimming halogen lighting

[Andrew Simpson]

I recently installed 6 50W mains powered halagon recessed lights in my hall. I was quite suprised when switching them on how bright they were. They make my 15 20W ones in the kitchen look like candles. I thought I would put a dimmer switch in and went up to B&Q. The switch has also the outside light on it so I will need a double dimmer with the outside light set to max on the dimmer. However every double switch I pick up has a sticker on the pack stating that if I am using halogen lighting then the maximum load is 250W or

150W. Can I assume then that on the maximum setting that the halogen bulbs can only receive 250W of power or will something happen in the dimmer to stop it from overloading, or will the dimmer cook!.?

Thanks for your help.

Andrew

[Tim Mitchell]

In article , Andrew Simpson writes

The dimmer will probably cook. (I'm not sure why they are derated for mains halogens).

Go to an electrical wholesaler like City Electrical Factors or someone, and tell them what you want to do, they will have something more suitable than B&Q.

[Andrew Simpson]

I've phoned City Electrical Factors and they only do one with a max of 250W for halogen lighting. However, they do a 35W bulb so I will buy those to replace the load and one of their dimmers.

Thanks for your help.

Andrew.

[Dave Plowman]

I'm pretty certain a low voltage model doesn't have to be de-rated for mains halogens - it's the low cold resistance of the filaments that's the 'trouble' and something designed for an inductive load should also be ok here.

TLC do a range of low voltage 2 gang 300 watt dimmers. They also sell a switch only to replace one of the dimmers on that plate (for your outside light) so you'd have a spare dimmer unit. ;-)

[Andrew Gabriel]

In article , Andrew Simpson writes

Mains halogens have larger filaments which means the light can't be so accurately controlled. They are therefore spilling more over a wider area, so you see more light. It is likely that the

20W ones are narrower beam and light their target better, but don't spill light anywhere else. If the beam target is not particularly reflective, the effective lighting of the room would not be good. The 20W ones will be a good deal more efficient, so the total light output will not be as big as a 20:50 ratio. You might just be seeing a good demo of why such lighting is rarely effective as general room lighting (my hall is lit quite brightly with around 25W c> The dimmer will probably cook. (I'm not sure why they are derated for

Two things I can think of -- halogens tend to have higher inrush currents, and they often have no space for a fuselink in the lamp. The former issue can be avoided by using a dimmer which can only come on gradually.

[Tim..]

Yes, why do some dimmers have to be derated for mains halogen lights?? I can appreciate there is more of a surge when firing up the halogens as opposed to normal say 60w light bulbs, but the running load is the same.

??

Tim..

[Dave Plowman]

Pretty well all semiconductors are rated for their *peak* current handling capacity. Exceed that for a fraction of a nanosecond and they blow - which is why a wire fuse can't protect them.

Dimmers are built right down to a price, so the triac they use is rated at the minimum possible.

If and when mains halogens become the norm, I'd guess we'll see dimmers commonly rated for them. But then you'll be able to over-rate them for GLS types. ;-)

[Andrew Simpson]

Thanks Dave I will take a look at TLC's site.

Andrew.

[Rick Hughes]

If it quotes 250W that is the maximum power rating before it will blow, you can run higher and take a chance - but you can't complain if it fails.

QH bulbs cause the derating due to high surge currents on switch on.

I have 1kW X.10 switch modules and they have to be derated to 500W on QH floods .

Rick

[Andy Dingley]

I'd replace the bulbs with lower powered ones. It sounds like you never want to adjust this dimmer, just use it permanently set at a lower power. Halogens don't like this, and you'll get a greatly reduced bulb life (depending on how low you have them). You may also get the bulbs capsules blackening internally.

-- Die Gotterspammerung - Junkmail of the Gods

[Andrew Simpson]

Hi Andy,

Thats basically what I have done. I've reduced them to 35W bulbs and there is a big difference and the sunglasses can stay off now.

Thanks everyone for your advice.

Andrew.

[Dave Plowman]

I know this is the theory, but I've got three LV 50 watt above the electronics bench which are on a dimmer, and spend a lot of time well dimmed down. And have a longer life than the mains halogen in the anglepoise which isn't dimmed and is on less.

[The Natural Philosopher]

Halogens run hotter, so when cold draw more current than a conventional bulb.

Dimming means cold filaments and high peak currents, which heats up dimmers more than running full crack.

[The Natural Philosopher]

Not so at half brightness.