12V Lamps

SELV Power Supplies and Equivalents

If we want to run 12 or 24V lights from a mains power supply, we need a transformer that will bring the voltage down to the required level, and convert it to DC. There are a number of options here, but the lighting standard specifies SELV. This is the safest option, as the low voltage circuits are totally isolated from the mains circuits with double insulation, and there is no earth connection to the low voltage circuits either. This is the safest option.

Description of SELV transformers from Sunpower: https://www.sunpower-uk.com/glossary/what-is-separated-extra-low-voltage-selv/

Another similar type is PELV. Here the power is isolated, but thee is an earth that extends to the low voltage side. This means that if there i a fault on the mains side this may be carried through via the earth to the low voltage side. So it is not as safe.

Comparison of SELV and PELV transformers: http://www.electricalaxis.com/2016/10/what-is-selv-and-pelv-circuits.html

Finally, there is FELV. This uses the same low voltage levels as SELV and PELV, but there is no double insulation in the transformer. This is the least safe of the three options.

Whilst the lighting standard EN 60598 specifically states that an SELV transformer is required, this specification of transformer is now little used in domestic, IT and office equipment standards, so there is not a wide choice of stand-alone SELV transformers available. But we can argue for an equivalent standard.

Standard EN 60950-1 is the standard for IT and general domestic and office electronics etc., and it specifies an SELV transformer. However it has since been replaced by EN 60238-1, which redefines a safe transformer, and dispenses with the SELV term. The good thing is that Annex W of 60238-1 states that an SELV power supply is equivalent to an ES1 power supply to 60238-1 in terms of safety (ES1 basically denotes that the item is intended for use by anyone, not a technician or an engineer - all external power supplies to 60238-1 will be to this level of protection). Thus I'd argue that in designing a lamp, it is perfectly acceptable to use an external power supply to EN 60238-1, which means that you have a vast range of power supplies to choose from.

Mains to 12V SELV Power Supplies

This section looks at sources of SELV power supplies.

There are a few makers of stand-alone 12V power supplies that can be use external to the lamp:

There are many makers of LED drivers designed to be mounted in the lamp (which negates the benefits of lower regulatory requirements), or in a cupboard or ceiling space in a house.

For both of the above, they want to be fixed voltage drivers, not fixed current.

This is a useful article on mains to 12V transformers for LED lighting, including comparing constant voltage versus constant current models, and dimmable 12V transformers; http://www.reuk.co.uk/wordpress/lighting/transformers-for-12v-led-lighting/

And a pair of articles looking at constant voltage and constant current drivers:

For house installations, there are mains to 12V SELV transformers primarily designed for use in wet areas like over the shower, where the light or fan has to be low voltage with an SELV transformer outside of the wet zone.

For outdoor use, there are several manufacturers of mains to 12V SELV power supplies for outdoor lighting, often using things like waterproof cable glands.

Bulb Holders

Vaious bulb holders (primarily vehicle) for low voltage bulbs: https://bae-bristol.co.uk/Catalogue/Lighting/Bulb-holders



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