"Pablo" wrote in message
...
The Screwfix catalogue says that Low Voltage Halogen Spot Lights use less
energy than mains voltage. How can this be? Do they defy the laws of
Physics? 50w at 12V uses the same energy as 50w at 240V.



It's true that 50w is 50w. It's also true that a 12v 50w lamp gives a lot
more visible light than a 240v 50w lamp - because the filament is much
hotter with the result that much more of the radiation occurs within the
visble part of the spectrum. For the same reason, 12v lamps give a whiter
light.

Why can they run much hotter? It's all laws of Physics/Ohm's law stuff! 50w
at 12v uses just over 4 amps - requiring an effective resistance of 3 ohms.
On the other hand, 50w at 250 volts requires only 0.2 amps, and a resistance
of about 1250 ohms.

A 3 ohm filament is short and fat - and very robust. A 1250 ohm filament is
very long and thin - and very weak. The 3 ohm filament can thus run at a
substantially higher temperature without failing.

I suspect that what Screwfix are really saying is that for equal *light*
output (as opposed to *power* input) 12v lamps use a lot less energy.

Here endeth the Physics lesson . . .

Roger