Technology Reports:

LED Bulbs - What do I need to know?

TechReport (UK)

‘Like for like’?

Obviously, the easiest route to replacing older bulbs with LED ones is to follow a ‘like for like’ replacement route – so is this always possible? The short answer is that in many cases, yes it is.

Traditional Style Lightbulbs: LED bulbs are now available in a full range of shapes (traditional, golf-ball, candle etc.) and fittings (bayonet, ES and SES) and are now of a similar size to the conventional bulbs that they are designed to replace. Dimmable versions are available also for just slightly more money, but please read the section on dimming below for compatibility.

Spotlights / Reflectors: Once again, LED lights are available as direct replacements for many types of spotlights and reflectors. Fittings include GU10 (Mains voltage), MR11 and MR16 (both 12V). It should be borne in mind that the MR11 and MR16 equivalents may require different mains transformers (these are typically placed in the ceiling void) from those used for 12V halogen bulbs, whereas the GU10 bulbs, as well as being cheaper and more efficient, just require a direct mains connection.

Capsule Bulbs: LED ‘Capsule’ equivalent bulbs (the really small 12V bulbs used in some halogen fittings) are available, but may not fit in all fittings (they are slightly larger) and are typically not dimmable, so it may be more cost effective to replace the entire light fitting.

Mains voltage or low voltage?

LEDs themselves operate off a low voltage supply (can be as low as 1.5V, but typically no higher than 12V), so mains voltage LED bulbs incorporate electronics to bring the voltage down as well as protecting the LEDs from mains ‘spikes’. This circuitry brings further efficiency gains over bulbs designed for 12V which require external transformers to connect to the mains supply. Mains voltage bulbs are also typically cheaper (less than 50% of the cost) when compared with similar 12V bulbs (e.g. GU10 fitting bulbs compared with MR11 bulbs), so if possible, go with a mains voltage option.


Back in the early days of LEDs they were not compatible with dimmers (as CFL lights have never been), but that has now changed. Many types of LED bulb (e.g. traditional style and halogen downlighter replacements) are now available in dimmable versions for not a lot of extra money.

There is a note of caution here though – many existing dimmers are designed for much higher loads (typically 40W upwards) and prefer what is known as an ‘inductive load’ (of which conventional & halogen lighting would be an example, but not LEDs).

If such a dimmer is used, in order to ensure that the stated ‘minimum load’ is reached, it may only be possible to use with a larger grouping of LEDs (for example, with 4W LEDs, 10 would be required to meet the minimum loading requirements.


                                                                                         Mains Voltage Waveform

Dimmer switches work by ‘cutting into’ the normal mains voltage waveform, thereby reducing the amount of energy available to the load (in this case the LED bulb), and consequently lowering its light output.

There are two main types of dimmer switch:

Leading-edge’ and

Trailing-edge’ dimmers.

Historically, most dimmers on the market have been of the leading-edge type – they are cheaper to make and work well with normal light bulbs. In terms of LED lighting, they will work with LEDs using an external transformer, if that transformer is rated as ‘dimmable’, as a transformer represents an inductive load.

Trailing-edge dimmers are more sophisticated (and therefore a little more expensive), incorporating features such as ‘soft-start’, smoother dimming and are silent (leading-edge dimmers can sometimes ‘buzz’). The more gradual approach used by these dimmers works well with dimmable LEDs (typically mains voltage versions) and will actually extend their life still further.

The vendor (or certainly the manufacturer) will normally state the type of dimming used on the specification, so keep an eye out for this when buying new dimmer switches.

‘Back Boxes’ - Depth

If replacing a conventional light switch or an older dimmer with a more modern one, it should be borne in mind that the more sophisticated electronics in these modern dimmers require greater mounting depth than conventional switches (typically 25mm, but sometimes as much as 35mm), which may not be accommodated by many of the older mounting boxes behind light switches in UK houses (some of which may only be 16mm deep).

Digging out a wall and mounting a new box is messy and is not for those averse to DIY (and would require the local mains supply to be off for some time) – please bear this in mind if planning to fit new / replacement dimmers to go with LED lighting.

‘Smart’ / ‘Touch’ Dimmers - Compatibility

Some Smart or ‘Touch’ dimmers are effectively ‘always on’ – they require a very low-power continuous feed in order to be able to function correctly.

Whilst this was not an issue with conventional tungsten / halogen lighting (as the operating current required by the dimmer was at far too low a level for the lights to operate), such a continuous feed may provide enough power for some modern LEDs (or perhaps just one in a group, for example) to partially illuminate.

In order to avoid this issue, if using such an electronic dimmer with LED lights, please check compatibility via the manufacturer’s website.

For example, practice has shown that the Philips ‘Master’ dimmable LED range of low-power downlighters are not fully compatible with the Lightwave RF ‘Smart’ Dimmer range, but the Megaman range of LED downlighters are fully compatible (which is perhaps not too surprising, as they are made by the same company).

Light Colour Temperature

One of the really great things about LED technology is that LEDs can be designed to provide light of almost any colour that we might wish, but just which colour is most appropriate in a domestic environment?

The colour of light is normally specified as a ‘colour temperature’ – this is the equivalent to the colour of light that would be emitted by any object if heated to the stated temperature (quoted in Kelvin, K, the scientific temperature scale) – here is a scale for the range of light colour temperatures that we are familiar with:

From the above you can see that conventional household lighting (incandescent and halogen bulbs) emit a light that is a lot ‘warmer’ (more red / yellow) than normal daylight.

However, this warmer colour is what we have become used to in our homes in the evenings, so to replicate this, LED bulbs with a specified colour temperature of 2700K – 3000K (sometimes called ‘Warm White’) should be chosen.

However, for public areas of a house, bathrooms and kitchens, there is now a strong trend for a cooler, cleaner look to the lighting and if this is what you are trying to achieve, go for bulbs with a colour temperature of about 4000K (also known as ‘Cool White’).

There should be no difference in the efficiency / maximum light output of the two variants.

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