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Topic Title: Dimming a single 50W mains halogen lamp
Topic Summary: suggestions?
Created On: 02 May 2014 04:37 PM
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 02 May 2014 04:37 PM
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BrucieBonus

Posts: 756
Joined: 20 February 2007

Hi folks

Hope you're all looking forward to a bank holiday break!

Have a customer who has a single light (as above) over the bath. He likes it bright to read the paper (!) and his missus likes to keep it on dim at night for those trips to the loo that creep up on us when we get to a certain age.....

It was put in (not by me) on a standard 60W min load dimmer and so not happy.

Any ideas? I was thinking that I could use one of these,
http://www.tlc-direct.co.uk/Products/DNCAPLOAD.html
but the manufacturers deny all knowledge of how their products might work with anything other than their products.

Minimum disturbance required ie no rewiring.

I hope someone can come up with a bright idea!


many thanks

BB
 02 May 2014 04:51 PM
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daveparry1

Posts: 6381
Joined: 04 July 2007

Varilight do 0-120 watt dimmers. Just seen them on Ebay.
Also QVS list a 10-400 watt Varilight dimmer.
 02 May 2014 05:40 PM
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Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

It was put in (not by me) on a standard 60W min load dimmer and so not happy.


Put in where - as in physical location within the bathroom

MK do a 40-250W dimmer (K1531WHI) as a gridswitch module

Regards

OMS

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 02 May 2014 06:14 PM
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aligarjon

Posts: 2923
Joined: 09 September 2005

Toolstation do a dimming pull switch rated between 20w and 150w.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 02 May 2014 06:28 PM
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BrucieBonus

Posts: 756
Joined: 20 February 2007

Ah, thanks for the speedy replies. The switch is there already outside bathroom - 2 gang dimmer, so could change it to a grid OR, if Varilight do a 2 gang one then just straight swap as the other dimmer does 2 x 50W's so would be OK for these

Will check out Varilight

Many thanks all and have a nice evening

BB
 02 May 2014 10:01 PM
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peteTLM

Posts: 3233
Joined: 31 March 2005

Varilight have excellent customer service, tech guy called Tim. I had a similar problem once you see with a minimum load.

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 02 May 2014 10:39 PM
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ady1

Posts: 766
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Won't a dimmable torodial transformer work in this case ?
Regards
Ady

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Resistance is futile.
 02 May 2014 11:24 PM
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westfield6

Posts: 126
Joined: 12 October 2007

Putting a capacitor across a dimmer does not sound like a good idea. Also bear in mind that halogen lamps should not be dimmed. They rely on the recombination of evaporated tungsten which won't happen when they are dimmed so life will be much shorter.
 02 May 2014 11:39 PM
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stateit

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Originally posted by: westfield6

Also bear in mind that halogen lamps should not be dimmed. They rely on the recombination of evaporated tungsten which won't happen when they are dimmed so life will be much shorter.


???

I have dimmers on all halogen downlight lamps in my house.

I've 10 in my kitchen and have only had to replace 3 lamps in 5 years. And they are the lights used (and dimmed up and down) most in the house.

That lifetime seems pretty good to me.

[edit] And as for dimmers : The usual rating I get are 40w-250w. The 60w minimum dimmers are for max load 400w. So ask for a lower rated dimmer. Every wholesaler I use supplies these as standard. I normally have to ask for the 60w-400w purposefully. [/edit]

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http://www.sg-electrical.com
 04 May 2014 07:33 AM
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statter

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If you just want a dim light you could insert a suitable diode e.g. 1N5407 In series with the lamp and arrange a switch to short circuit it for full brightness, quite a bit cheaper than a dimmer and simple switch operation.
The diode will cut out every positive or negative cycle depending on which way round you connect it so power reduces by half. In practice Lamp power will be slightly more than this as filament won't run so hot.

Edited: 08 May 2014 at 10:59 PM by statter
 07 May 2014 12:04 PM
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bowmandj

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If you put two diodes in series then the voltage will drop by another 0.6 volts :-)

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 07 May 2014 12:25 PM
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potential

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Originally posted by: statter

If you just want a dim light you could insert a suitable diode e.g. 1N5407 In series with the lamp and arrange a switch to short circuit it for full brightness, quite a bit cheaper than a dimmer and simple switch operation.

The diode will cut out every positive or negative cycle depending on which way round you connect it so rms volts reduce by half. Lamp power will be slightly more than a quarter rated as filament won't run so hot.

True.
I used diodes/rectifiers for exactly this purpose in 1960.
However the filament lamps will flicker.
 07 May 2014 04:05 PM
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richardcs

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Originally posted by: statter

If you just want a dim light you could insert a suitable diode e.g. 1N5407 In series with the lamp and arrange a switch to short circuit it for full brightness, quite a bit cheaper than a dimmer and simple switch operation.

The diode will cut out every positive or negative cycle depending on which way round you connect it so rms volts reduce by half. Lamp power will be slightly more than a quarter rated as filament won't run so hot.


That's not quite true, a diode (ignoring the forward drop) will reduce rms voltage by sqrt(2) and power by half (for a resistive load), so lamp power will be slightly more than half. On a mains lamp with a thin filament you'd see the flicker, you probably wouldn't see it on a 12V halogen.

Any kind of dimmed lamp is running at very low efficiency, and a night light is on for a lot of hours. A better solution would be a separate 5-10W lamp. You could then switch either or off with a three position switch. I'm not sure where to buy them but they definitely exist - my local hospital uses such an arrangement where the "dim" setting is actually a maintained emergency light in the same fitting and the normal one is the main lamp. Looks like a normal light switch but is centre-off.
 07 May 2014 04:28 PM
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mapj1

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Oh no, actually its more complex than that even.!
When the lamp is under-run the filament is cold, and as a meter tells us the room temperature resistance of a filament lamp is about 1/10 of the resistance at full temperature. With the diode, for sure the current is off half the time, but the equilibrium resistace is lower, so the dissipated power is quite a bit more than half.
However light output falls much faster than linearly with reducing power, as an unfair fraction of the energy ends up in the infra-red . In fact diode dimmed lamps look OK for yellow tinged romantic dinners and make your fairy lights last for years but are not that practical.

As regards blackening halogen laps with dimming, I'd not worry about it, so long as there is comparable duration each day at or near full power to repair the filament. The life will probably be vastly improved by not being direct on line as its all about that cold inrush.

regards M.

(The cold filament problem is also why theatrical dimmers need much bigger thyristors than the lamp wattage might suggest - if the lamp is kept on very dim for an extended period then the very short pulses of many times the hot lamp current conspire to toast the electronics, and can also create an EMC nightmare.)

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regards Mike
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