de rating dimmers for halogen lights.

[andypandy]

Why are dimmers de rated for halogen lights when they are just filament bulbs? I can understand for l.v. halogen when there is a transformer in the cct, but not for mains halogen as normal incandescent bulbs aren't derated. Also, what are the best dimmers for feeding l.v. halogen light fittings that contain toroidal transformers? The light in question has 4 x 50w 12v halogens in it

 

[securespark]

Mains may not have tx (transformer), but still same lamp type.

 

[AdamW]

Is it possibly anything to do with transient current? If you draw a graph with current on the y-axis, time on the x-axis, an ordinary incandescent bulb has a sharp current peak when you turn it on. The reason for this is that the filament has least resistance when cold, so most current flows. As the filament quickly becomes hot, the current quickly falls.

What is the behaviour of a halogen lamp? I suspect it takes longer to heat up, thus the transient currents last longer. Hence, the dimmer must be derated for halogen loads.

Can anyone in the know confirm or deny this?

 

[dmccormick]

I suspect it is because of this:
http://www.lif.co.uk/statements/25f.html

 

[ban-all-sheds]

I don't think it's that - my understanding is that the startup surge with halogen lamps is much more marked than normal incandescents.

 

[securespark]

That's more like it, Ban. Why else increase the capacity of the dimmer?

I had a customer with 1000W of floodlighting on a 5A breaker and it popped everytime both floods came on together, but when the lamps were replaced with 300's, they didn't. Simple surge, that's all.