Dimmer friendly Low V lighting

[Rob Flinn]

I was visiting a friend last night who was showing me a low voltage light he had installed in his kitchen, I think the bulbs were 12v.  the unit had a transformer to drop the voltage.  He pointed out how much the dimmer switch was buzzing, which is something that I have seen/heard before.  I wasn't surewhat causes this & was wondering whether the low V light was a problem ?

Are all low V lights OK to use with dimmer switches ?  I'm sure when I've been looking at them in hardware stores that some units say non dimable & other are ok with dimmer switches.    I couldn't find any info either way for this particular unit.

 

[Ethan]

Cheap triac/thyristor dimmers are often noise generators.
These dimmers chop what should be a gentle mains sine wave at high speed to achieve the dimming.
You could use a big honkin rheostat, or perhaps try to add some smoothing circuitry to the triac dimmer?

 

[JohnRoberts]

Back about a hundred years ago, I did some work with proportional controllers (for heating elements), where the triacs were both switched on and off at zero crossings, so on/off ratio involved full cycles (chopping half cycles introduce DC which is undesirable).  I don't know how well this would work for shorter time constant lighting as there may be some flicker, big dog heating elements have long time constants.

JR

 

[Rob Flinn]

I understand how these dimmers work, as I covered triacs & thyristors when I was at college.  I'm really wondering if they don't like the transformer inductive load ?  Why is it that some of these low voltage lights are not supposed to be used with dimmers ? Is it just the led versions of the bulbs or ... ?

I hear you about using a variac, but it's only some kitchen lighting so not a critical audio mains scenario.

 

[MagnetoSound]

Why is it that some of these low voltage lights are not supposed to be used with dimmers ? Is it just the led versions of the bulbs or ... ?

It's just the LEDs. Halogen doesn't care. You can get dimmable LED bulbs now but they are still quite expensive.

Most of the cheaper SELV 'transformers' for halogen bulbs are actually electronic switchers - as opposed to what we would call a transformer - but are still dimmable. Then there are these constant current LED drivers which are not dimmable.

Confused? .... 

 

[thermionic]

Not to hijack Rob's thread, but I'm in a position where I'd dearly like to fit dimmers to the bathroom. It has low-V lighting in there, but it doesn't have a dimmer and is painfully bright. I've spoken to the landlord and he doesn't seem at all keen to fit dimmers, citing the transformer as an issue. I was planning to speak with a friend who lectures in electrical installation, but maybe I can get advice here? It's definitely not using LEDs.

The transformer will create an inductive load, so I can't just put a dimmer plate in place of the switch. Where to start? Find the transformer and get its spec?

Definitely confused!

Justin

 

[MagnetoSound]

... I'd dearly like to fit dimmers to the bathroom.

I have plenty of 12v halogen downlights on dimmers, fed via transformers, never had a problem.

As you are probably aware though, UK wiring regs will not allow a hand-operated switch plate in a bathroom - you have to use a pull-cord switch fixed to the ceiling for safety, so a standard dimmer plate is not an option.

You can use a dimmer plate outside the bathroom - or you might get away with a wireless remote controlled dimmer plate positioned on the ceiling alongside the pull-switch.  (TBH I've not thought of this before, although I can't really see a safety issue provided there is no rotary control on the front of the plate.)

 

[Spiritworks]

I can't speak for wiring in the UK, but here in the US dimmers are available for low voltage lighting. It is necessary to match the dimmer to the specific type of transformer being used for the lighting, not the bulb itself. Some low voltage lighting can be used with a standard incandescent-type dimmer but it all depends on the transformer.