Non-motoring > A lighting/physics question..... Miscellaneous
Thread Author: Haywain Replies: 24
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New Fri 29 Apr 16 15:17 
 A lighting/physics question..... - Haywain
Physics was never my strongest subject, so I turn to others for advice ..........

We've got one of those 'mother & child' standard lamps in the living-room i.e. with a bright up-lighter on top and a reading lamp sprouting from the side. The uplighter is as bright as the rising summer sun and is normally kept under control by the dimmer switch.

Whilst hoovering dead insects from the up-lighter part, I noticed that the bulb is a R7s halogen linear job rated at 300 watts ....... not good for the electricity bill. I note that smaller wattage bulbs are available e.g. I'm sure a 120 watt would do nicely.

The question, before I go out and purchase a smaller bulb, ....... when the light is dimmed - say to 50% brightness - am I actually saving electricity, or is half of it being dissipated as waste heat by the dimmer? If the latter were the case, I may as well simply get a smaller bulb.

Thanks.
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New Fri 29 Apr 16 15:23 
 A lighting/physics question..... - Manatee
There are different sorts of dimmers but the general answer is that although the dimmer will save power, it will be more efficient to use a lower wattage bulb. The detail is largely beyond me.
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New Fri 29 Apr 16 15:30 
 A lighting/physics question..... - CGNorwich
I was listening to the Radio 5 science program that goes out in the middle of the night the other week and Dr Karl discussed this very issue.

Old fashioned dimmer switches did indeed use a resistance but modern dimmers use some complicated electronics which effectively switch the circuit on and off and so do actually reduce the power consumption. Its all pretty complicated and I never got further than physics O level but its all explained here.


home.howstuffworks.com/dimmer-switch1.htm
Last edited by: CGNorwich on Fri 29 Apr 16 at 15:31
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New Fri 29 Apr 16 15:37 
 A lighting/physics question..... - Focusless
>> electronics which effectively switch the circuit on and off

Hence the wifi interference and incompatibility with low energy bulbs?
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New Fri 29 Apr 16 15:38 
 A lighting/physics question..... - Manatee
The leading and trailing edge dimmers do reduce the power consumption compared with the purely resistive type; but apart from the efficiency of that aspect the efficiency of the bulb then becomes a factor - e.g. if you halve the power usage you will get less than half the light, would be my non-expert prognostication.
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New Fri 29 Apr 16 15:49 
 A lighting/physics question..... - sherlock47
Why not go the whole hog in a search for energy economy and get a R7 LED relacement

As an example, but others exist
www.lightbulbs-direct.com/megaman-non-dimmable-led-9w-117mm-r7s-halogen-replacement

But be careful - there are 2 lengths of R7 bulb. Also you may need to check the clerances on your reflector/bulb capsule as some can be tight.

Also some of the cheaper LEDs are nondimmable.
Last edited by: sherlock47 on Fri 29 Apr 16 at 15:51
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New Fri 29 Apr 16 15:53 
 A lighting/physics question..... - WillDeBeest
...and a lot of online retailers seem to source abroad and supply things that don't conform to UK safety standards. Be careful.
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New Fri 29 Apr 16 16:00 
 A lighting/physics question..... - Armel Coussine
>> Be careful.

Yes. Do a safety check by sticking a couple of fingers into the light fitting and turning it on.

Best to be standing on a wet metal floor if you can manage it.



:o}
Last edited by: Armel Coussine on Fri 29 Apr 16 at 16:04
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New Fri 29 Apr 16 16:13 
 A lighting/physics question..... - Haywain
"Yes. Do a safety check by sticking a couple of fingers into the light fitting and turning it on.
Best to be standing on a wet metal floor if you can manage it."

You are a wicked old leftie, AC, but you don't get rid of me that easily! ;-)

Whilst I consider myself weak at physics, I did manage an A level at it (but it was a long while ago).
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New Sat 30 Apr 16 18:40 
 A lighting/physics question..... - Armel Coussine
The worst electric shock I can remember was from a car engine. Very nasty that was.
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New Fri 29 Apr 16 16:06 
 A lighting/physics question..... - sherlock47
Pick an established retailer -some offer a 5yr warranty - 50,000 hr life should take you well beyond 5years!


LED technology is changing/improving all the time and prices have fallen dramatically over the last few years.
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New Fri 29 Apr 16 21:46 
 A lighting/physics question..... - MD
>> ...and a lot of online retailers seem to source abroad and supply things that don't
>> conform to UK safety standards. Be careful.
>>
So do SOME retailers source theirs from say Milton Keynes or perhaps the shores of Lake Windermere?

OR..........By chance do they all originate from the EAST.

Just saying like... :-)
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New Fri 29 Apr 16 22:13 
 A lighting/physics question..... - Ted

I'd go down the LED route if possible. I recently finished fitting the whole house with LEDs. The guy at the electrical wholesaler that I deal with reckoned he was saving 90% on his lighting bill. There's no heat given off at all and the brightness is first class.

Waiting for the first bill now....might be interesting. I'll let you know.
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New Fri 29 Apr 16 22:45 
 A lighting/physics question..... - Dulwich Estate II
"....he was saving 90% on his lighting bill. There's no heat given off at all...."

There's no such thing as a free lunch - your gas central heating bill will go up to compensate for the 500w of energy your lights are no longer pumping out when it's dark.

Yes, yes I know the overall saving is going to be significant - just saying like.
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New Sat 30 Apr 16 07:41 
 A lighting/physics question..... - Dog
Tried a phew of them there LED bulbs, but couldn't get on wivvam, like.

I prefur the ambiance of 'proper' bulbs in my 200 year-old cottidge, with its low beams and general gloominess.

:}
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New Sat 30 Apr 16 08:56 
 A lighting/physics question..... - legacylad
You are Bilbo Baggins and I claim my £5
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New Sat 30 Apr 16 09:06 
 A lighting/physics question..... - sooty123
He does have a point, they are a little too 'clinical'.
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New Sat 30 Apr 16 09:42 
 A lighting/physics question..... - Dog
I bought 4 x GU10 and 1 x 5w R50 spot (£4!!) gorn back to the 'proper' ones now.

I have no doubt they will improve one day, and that mankind will eventually land on Mars.
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New Sat 30 Apr 16 10:16 
 A lighting/physics question..... - legacylad
Matt Damon & his chums already have
Catch up....
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New Sat 30 Apr 16 11:15 
 A lighting/physics question..... - Dog
I wouldn't so no to having a drive around Mars. I suppose I could always (and have done) get in some practice on Fuerteventura :)
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New Sat 30 Apr 16 09:51 
 A lighting/physics question..... - Slidingpillar
He does have a point, they are a little too 'clinical'.

Colour temperature 'wrong' in other words, most are more towards the blue end of the spectrum than would be the case with incandescent bulbs.
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New Sat 30 Apr 16 09:50 
 A lighting/physics question..... - Dog
>>You are Bilbo Baggins and I claim my £5

Gollum more like it, and I'll keep my [soon-to-be] €6
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New Sat 30 Apr 16 18:42 
 A lighting/physics question..... - R.P.
Zero would probably know..
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New Sat 30 Apr 16 20:53 
 A lighting/physics question..... - NortonES2
He's watching and waiting. Possibly.
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New Mon 2 May 16 08:53 
 A lighting/physics question..... - Haywain
Thanks for all the replies on this subject.

We've got LEDs in the kitchen but I think an LED would be unsuitable for the case that I'm talking about because the lamp is fitted with a dimmer-switch and it doesn't look as though the design of the mounting would work either.

I will go down the route of a lower-wattage bulb (probably 120w instead of 300w) ...... and I will watch out for the different sizes that could present themselves (thanks for that tip!).
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