Low Voltage Halogen Spot Lights

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
     snipped-for-privacy@iee.org.uk (John Watson) writes:

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 auaf%24q30%241%40new-usenet.uk.sun.com
--
Andrew Gabriel

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

http://groups.google.com/groups?selm auaf%24q30%241%40new-usenet.uk.sun.com
Could you re-check the above link URL please - I can't make it work!
Roger
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

http://groups.google.com/groups?selm auaf%24q30%241%40new-usenet.uk.sun.com
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Andrew Gabriel wrote:

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.

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?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Pablo wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
wrote:

Lawrence
usenet at lklyne dt co dt uk
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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 snipped-for-privacy@argonet.co.uk London SW 12
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.