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.

"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

Pablo wrote:

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.




The more robust filaments run at higher temp than a mains one can, and
the scinece sez that hotter bodeies will emit at shorter wavelength.

97% of normal bulb is emitted in infra red - its heat. LV halogens are a
bit better - say only 95%. So more light per 50W in etc.

The actual power output at the spot may be less, but from the initial power
onput power is dissapated in the transformer windings. But by the ohms law
formulae yes you can have 50W output for 12V or 230V but there are different
resistances in the circuit.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.

Here endeth the Physics lesson . . .

Roger

What an excellent Physics lesson, Roger.
I am an electrical engineer/ sparkie with an inquiring mind, and have
often wondered why this was the case.
John

I use a lot of 20W 12v halogens. And none have blown yet. But also I
find the light is slightly warmer in colour. I use 5 on parralel wires
in a small kitchen and get a nice cosy feel.

Where as the 5 35W (35mm) lamps in another room have blown 2 lamps in
a year.

From what I ave seen with mains halogens the life is much shorter,
especially if you are where the mains voltage is high or prone to
spikes. And at £5 a throw gets expensive. Although quickly checking
the price of mains halogens seems to be dropping from a year ago.

If efficiency is really what you need go for low energy lamps. Ikea
used to do some very good pricing on these. Great for hall lights and
others that you leave on a lot and do not sit with for a long time.

On Fri, 28 Nov 2003 20:00:24 -0000, "Pablo"
wrote:

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.


Lawrence

usenet at lklyne dt co dt uk

"Lawrence" wrote in message
...
I use a lot of 20W 12v halogens. And none have blown yet. But also I
find the light is slightly warmer in colour. I use 5 on parralel wires
in a small kitchen and get a nice cosy feel.

Where as the 5 35W (35mm) lamps in another room have blown 2 lamps in
a year.

From what I ave seen with mains halogens the life is much shorter,
especially if you are where the mains voltage is high or prone to
spikes. And at £5 a throw gets expensive. Although quickly checking
the price of mains halogens seems to be dropping from a year ago.

If efficiency is really what you need go for low energy lamps. Ikea
used to do some very good pricing on these. Great for hall lights and
others that you leave on a lot and do not sit with for a long time.

On Fri, 28 Nov 2003 20:00:24 -0000, "Pablo"
wrote:

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.



£5 a throw ?.....they're only £199 for a pck of 2 in Morrisons

what did seem a good deal at my local BQ yesterday was a 5 spot 12v system
on a 3M flexi track, looks just like the one in screwfix at £82 but they had
a full shelf of them at £19.95 each

In article ,
(John Watson) writes:
Here endeth the Physics lesson . . .

Roger

What an excellent Physics lesson, Roger.
I am an electrical engineer/ sparkie with an inquiring mind, and have
often wondered why this was the case.

Personally, I don't buy that explanation, as the filament temperatures
are not different. The design problem which causes lower efficiency of
mains voltage lamps is due to the way the surface area can't be kept
optimum due to the need to change the filament resistance for different
operating voltages. There's one optimum surface area for a given power
output and lamp life, and it turns out that this optimum surface area is
driven by a voltage of around 55V for a lamp of the order 100W. As you
design lamps to operate at voltages further away from this ideal, you
have to compromise their efficiency. 240V happens to be rather more
compromised than 12V.

I described this effect in more detail in article:
http://groups.google.com/groups?selm...net.uk.sun.com

--
Andrew Gabriel

"Andrew Gabriel" wrote in message
...

I described this effect in more detail in article:

http://groups.google.com/groups?selm...net.uk.sun.com

--
Andrew Gabriel

Could you re-check the above link URL please - I can't make it work!

Roger

"Roger Mills" wrote in message
...

"Andrew Gabriel" wrote in message
...

I described this effect in more detail in article:


http://groups.google.com/groups?selm...net.uk.sun.com

--
Andrew Gabriel

Could you re-check the above link URL please - I can't make it work!

Roger



Hi Andrew,

Having now read your article which you kindly emailed to me, I think that we
are more or less in violent agreement!

We are both saying that - by and large - low voltage lamps are more
efficient than high voltage lamps with the same power consumption because
they run at a higher temperature - which is possible, or even inherent, as a
result of the different filament profile.

You explain *why* they run hotter - which I didn't in my previous post, but
you don't really explain why hotter means more efficient - which I did in
terms of the colour temperature and wavelength of the emitted radiation.

I accept that there are limits as to how low you can go in voltage because
of other factors like the resistance of the filament carriers etc. I would
be interested in your view concerning the optimun voltage for a 50w - or
even 35w - as opposed to 100w lamp. [We have LV 50w lamps in one room and
35w lamps in another - and I'm not sure there's all that much difference in
the light output!]

Roger

In article ,
Frank add WIDPIK to subject line if replying
wrote:
what did seem a good deal at my local BQ yesterday was a 5 spot 12v
system on a 3M flexi track, looks just like the one in screwfix at £82
but they had a full shelf of them at £19.95 each

In B&Q, you can often by a complete fitting - with bulbs and transformer
etc - for less than they charge for replacement bulbs...

--
*Why is it that to stop Windows 95, you have to click on "Start"?

Dave Plowman London SW 12
RIP Acorn

Andrew Gabriel wrote:

In article ,
(John Watson) writes:

Here endeth the Physics lesson . . .

Roger

What an excellent Physics lesson, Roger.
I am an electrical engineer/ sparkie with an inquiring mind, and have
often wondered why this was the case.


Personally, I don't buy that explanation, as the filament temperatures
are not different.



They are. That was the real reson for use of LV halogen bulbs in
projectors years ago - a much better colour temperature due to use of
higher filament operating temperature.

The design problem which causes lower efficiency of
mains voltage lamps is due to the way the surface area can't be kept
optimum due to the need to change the filament resistance for different
operating voltages. There's one optimum surface area for a given power
output and lamp life, and it turns out that this optimum surface area is
driven by a voltage of around 55V for a lamp of the order 100W. As you
design lamps to operate at voltages further away from this ideal, you
have to compromise their efficiency. 240V happens to be rather more
compromised than 12V.


In vacuo, the surface area will be a function of the power input and
filament temperature anyway, since almost all the heat is lost by
radiation, so it sounds like you are merely saying the same thing in a
different way.

You aren't Ivor Catt are you?



I described this effect in more detail in article:
http://groups.google.com/groups?selm...net.uk.sun.com