Dimmer De-Rating

[m4tty]

Hi

What is dimmer de-rating all about. Is it just Halogen lighting (ie, GU10's) that needs to be derated or does it include GLS lightbulbs.

Say a dimmer switch is rated at 250VA in a purely Resistive circuit it would mean 250W. So would this be able to cope with dimming 250W of GLS Lighting and 125W of GU10's as 50% derated.

Ive tried my best to make the above make sense but its not good lol.

Thanks if anyone understands what im getting at

Cheers

 

[green-hornet]

You may find this topic or should I say link helpfull.

http://www.talk.electricianforum.co.uk/showthread.php?t=6135

 

[musicspark]

Thanks green,

but I still don't get whty they need derating

 

[green-hornet]

Thanks green,but I still don't get whty they need derating

Its all about the minimum operating voltage required for the light fitting.

The very rapid on off that dimmers use to dim the light effects some lights more than others.

So if you dim 250watts with a 250watt dimmer you have the dimmer working at full capacity for the full period of time.

Its like having a 1600 cc car pull a wagon trailer, it will pull the trailer but slowly and will damage the engine eventually.

Same thing happens with the dimmer, it eventually gets so hot that its integrity breaks down, and the rapid cycles used to turn on and off become unstable.

If you dim 250watts with a 400watt dimer the dimmer has an extra reserve for when it is operating at the desired wattage.The on off cycles are fast enough to allow the 250watts being dimmed to never drop below the minimum operating current for the lamp and is more stable.