Dimmer Switch: How It Works, LED Bulbs and Fitting

A dimmer switch replaces an ordinary light switch and lets you set the brightness anywhere from a low glow to full light. It works with LED bulbs as long as three things line up: the bulbs are marked dimmable, the dimmer is rated for LEDs, and the total wattage sits between its minimum and maximum load.

Few changes do as much for a room for so little effort. The same pendant gives clear, bright light for cooking or homework, then drops to a soft pool of light for the evening. It earns its place above a table in particular, which is why so many of our dining room pendant lights end up on one. This guide explains how a dimmer switch works, which type to choose, which bulbs to put on it, how to work out the load, why LEDs sometimes flicker, and what the Irish rules say about fitting one.

What is a dimmer switch and how does it work?

The electricity in an Irish home is alternating current at 230 volts and 50 hertz: the voltage rises and falls in a wave, 50 complete cycles every second, changing direction twice in each cycle. A dimmer does not reduce the voltage the way a tap reduces water. It lets through only part of each half wave and blocks the rest, many times a second, far too fast for your eye to follow. The bigger the slice it cuts out, the less energy reaches the bulb and the dimmer the light.

There are two ways of cutting the wave, and they matter for LEDs.

• Leading edge. The dimmer switches on part way through each half wave and cuts the beginning of it. This is the older design, usually built around a component called a triac. It suits traditional filament and mains halogen bulbs, and the wire-wound transformers used with older low-voltage halogen spotlights.
• Trailing edge. The dimmer switches on at the start of each half wave and cuts the end of it. This design uses transistors, runs more quietly, and suits most dimmable LED bulbs and electronic drivers.

Many dimmers now sold in Ireland are described as universal or LED dimmers: they either work in trailing edge mode or detect the load and pick the right mode themselves. Always check what the packaging or the instructions say, because an older leading edge dimmer is the most common reason an LED refuses to dim properly.

One point surprises people. A modern dimmer switches the current on and off rather than burning off the surplus as heat, so very little power is wasted inside it. When you turn the light down, the bulb really does use less electricity. With LEDs, which already draw only a few watts, the saving is small in euros; the real gain is comfort.

What types of dimmer switch are there?

The first question is where the dimmer goes: on the wall in place of the switch, or on the lamp itself. The table sums up the main options.

Type	Where it goes	How you use it	Best for
Rotary push on/off dimmer	On the wall, in place of the switch	Push the knob to switch on or off, turn it to set the level	Pendants, ceiling lights, wall lights
Push-button or touch dimmer	On the wall	Tap to switch, hold to dim; many remember the last level	Living rooms, bedrooms, modern switch plates
Smart dimmer	On the wall	Buttons plus an app, a remote or a voice assistant	Rooms where you want scenes and timers
Inline or foot dimmer	On the flex of a lamp	A small wheel or a foot pedal on the cable	Table lamps and floor lamps

The rotary push on/off dimmer is still the one you find in most Irish homes: it is simple, cheap and fits a standard single switch plate. Touch and push-button models look cleaner and often let you set a minimum level so the light never drops too low. Smart dimmers let you bring the lights up to 30 % in the evening without leaving the sofa, but check their wiring needs before you buy, as we explain further down.

What do “gang” and “way” mean on a dimmer?

On a switch or dimmer, the gang is the number of knobs or buttons on the plate: a 2 gang dimmer controls two separate lights from one plate. The way tells you whether it can work on a circuit switched from two places, such as a hall and a landing. A light switched from two positions is called two-way switching. A dimmer marked “2 way” can take the place of one of the two switches; the other stays an ordinary two-way switch. The light can then be dimmed only from the position with the dimmer, while both positions can still switch it on and off. For full dimming from both ends you need a system designed for it, usually a smart or master and slave set.

LED dimmer switch: which bulbs are compatible?

This is where most dimming problems start. A dimmer works with any bulb built to be dimmed, and trouble begins when the LED is not. A dimmable LED contains a driver designed to cope with a chopped supply. An ordinary LED does not: on a dimmer it may flicker, buzz, glow faintly when switched off, or simply cut out below a certain point, and its electronics can fail early.

How to tell if a bulb is dimmable

Look for the word “dimmable” or the dimming symbol on the box or the product page. Since 2021, European ecodesign rules have required the packaging of a light source sold on its own to carry a warning if it cannot be dimmed, or if it can be dimmed only with specific dimmers; in that case the manufacturer has to list compatible dimmers on its website. So if the pack says nothing and the bulb is not described as dimmable, treat it as non-dimmable.

The bulb fitting has nothing to do with it. In Ireland the standard household fitting is the B22 bayonet, and you will find dimmable and non-dimmable LEDs in B22, as well as in the E27 and E14 screw fittings and the GU10 used in most downlights. When you replace a bulb, match the fitting and the word “dimmable”, then check the colour temperature: a warm white around 2,700 K keeps the soft, amber feel of an old filament bulb as you turn it down.

For a light with integrated LEDs, there is no bulb to swap. The product description has to state that the fitting is dimmable, and sometimes with which type of dimmer. If it says nothing, do not put it on a dimmer. Our LED lights state this on each product page.

Dim to warm bulbs: the closest thing to a filament

An old filament bulb did something LEDs do not do by default: as you turned it down, its light became warmer and more amber. A standard dimmable LED keeps the same colour at every level, so at 10 % a cool white can look grey and flat. Dim to warm bulbs fix this. The European ecodesign regulation describes them as light sources that shift their white light towards a lower colour temperature when dimmed, simulating the behaviour of incandescent bulbs. On the box you will see two figures, for example 2,700 K at full brightness falling to around 1,800 K or 2,200 K when dimmed. They cost a little more, and they are worth it above a dining table, in a living room or in a bedroom, anywhere you spend the evening with the lights turned down.

Swapping halogens for LEDs on an existing dimmer

This is the classic trap when you modernise a room. A kitchen with twelve 50 W halogen downlights put 600 W on its dimmer; the same kitchen with twelve 5 W LED downlights puts 60 W on it. An old dimmer sized for halogen may have a minimum load of 40 W or more, and it may be a leading edge design. The new LEDs then fall close to, or under, its minimum, and they flicker, buzz or never go fully off. If new dimmable LEDs misbehave on an old dimmer, the fix is almost always a new LED dimmer, not another set of bulbs.

Minimum and maximum load: working out the wattage

Every dimmer is rated for a range of load, and the range is different for each type of bulb. An LED dimmer might say something like “LED 10 to 150 W, incandescent 10 to 400 W”. Add up the wattage of every bulb the dimmer controls and check that the total sits inside the LED range: above the minimum as well as below the maximum. Many people only check the maximum, but with LEDs it is the minimum that causes trouble.

Two worked examples. A three-bulb pendant fitted with 7 W LEDs puts 21 W on the dimmer: comfortably inside a 10 to 150 W range. A single 4 W LED in a small wall light puts only 4 W on the same dimmer, below its minimum, so it may flicker however well matched it is otherwise. The answer there is a dimmer with a lower minimum, or more bulbs on the circuit.

Three habits save most of the trouble. Read the LED figure, never the incandescent one: a dimmer sold as “400 W” is usually rated 400 W for filament bulbs and far less for LEDs. Leave some headroom, with a total comfortably under the LED maximum rather than right on it, so the dimmer runs cooler and lasts longer. And treat a double dimmer as two circuits: on a 2 gang dimmer each knob has its own range, so check the load on each one separately.

Fitting a dimmer switch in Ireland: who can do it?

A wall dimmer fits in the back box of the switch it replaces. Before anything else, it helps to know where the law draws the line. Safe Electric, the body that registers electricians in Ireland, explains that Restricted Electrical Works cover most electrical work in a home and must be carried out and certified by a Registered Electrical Contractor. It lists like-for-like replacements, such as switches, among the minor works that fall outside that definition. Any work in a special location such as a bathroom, a new circuit, or a change to the distribution board is restricted work, full stop.

Our advice is simple: have a dimmer fitted by a Registered Electrical Contractor. A dimmer is not quite a like-for-like swap, it has its own load limits and wiring needs, and a registered electrician can test the circuit and confirm that the dimmer suits your bulbs. If your ceiling light is in a bathroom, there is no choice: that is restricted work under the Irish rules, and the switch position itself is governed by the bathroom zones of the National Rules for Electrical Installations, I.S. 10101.

What to check before you buy

You can prepare the job so the electrician fits the right product the first time.

1. Count the bulbs and add up their wattage, and confirm that each bulb is marked dimmable.
2. Check whether the light is switched from one place or two, and choose a dimmer marked “2 way” if it is switched from a hall and a landing.
3. Check the depth of the back box. A dimmer is bulkier than a plain switch, and the instructions state the depth it needs. A shallow box may need replacing with a deeper one.
4. Check whether the dimmer needs a neutral. Ordinary rotary dimmers use only the two switch wires, like the switch they replace. Many smart dimmers also need a neutral wire, and in many Irish homes the switch box holds only the switch wires and the earth, with no neutral. Choose a model that works without one, or ask your electrician before you buy.
5. Look at the switch plate. Match the finish and the size of your existing plates so the new dimmer does not stand out.

Once fitted, many LED dimmers let you set the lowest and highest level. This trimming step, explained in the instructions, is what removes most flicker at the bottom of the range, so ask for it to be done with your actual bulbs in place.

Why do my LEDs flicker or buzz on a dimmer?

Flicker is the most common complaint, and it almost always has one of these causes:

• The bulb is not dimmable. No setting will fix it: change the bulb.
• The dimmer is not designed for LEDs. An older leading edge dimmer made for filament and halogen bulbs often struggles with LED drivers. Replace it with an LED or universal dimmer.
• The load is below the minimum. Too few watts on the dimmer, typically after switching from halogen to LED. Add bulbs, or choose a dimmer with a lower minimum.
• The range has not been trimmed. Each dimmable LED has its own useful range. If the light flickers or drops out at the bottom of the dial, set the minimum level a little higher.
• Mixed bulbs on one dimmer. Different makes or models of LED react differently to the same dimmer. Use identical bulbs on one circuit.

A faint hum from the dimmer or the bulbs has similar causes: the chopped wave makes components vibrate slightly. A good LED dimmer and matched dimmable bulbs are usually silent. A dimmer that gets hot to the touch or buzzes loudly is overloaded or unsuited to the bulbs: switch it off and have it checked.

It is worth knowing what the European rules do and do not cover here. LED bulbs placed on the EU market must keep visible flicker at a Pst LM value of 1.0 or less, the level at which an average observer has a 50 % chance of noticing it, and since 1 September 2024 the stroboscopic effect at an SVM value of 0.4 or less. But those limits are measured at full load. They do not guarantee how a bulb behaves once dimmed, which is why matching the bulb and the dimmer still matters.

Which lights work best with a dimmer switch?

A dimmer earns its keep wherever the mood of a room changes through the day. Above a table, it takes you from clear light for cooking or homework to a softer light for dinner. Pendants with a fabric lampshade spread an even, glare-free light even at a low level, and they hide the bulb so you never look straight at it.

Exposed filament bulbs are a special case: at a low level they give an amber glow close to candlelight, as long as you choose dimmable LED filament bulbs. They suit open designs, from wooden frames to a simple bulb pendant light on a cord. In the living room and the bedroom, a dimmer on the main light, together with one or two lamps, gives you a warm, layered evening light without adding more light points. In a bedroom, a bedroom pendant light on a dimmer can drop low enough for a restless child without waking anyone.

For downlights, a single dimmer can handle a whole ceiling of GU10 spots, provided the total stays inside its LED range; our spotlight ceiling lights take GU10 bulbs, which come in dimmable versions. Whatever the fitting, the same three checks apply: dimmable bulbs, an LED dimmer, and a load inside its range.

Dimmer switch: frequently asked questions

Sources

• Restricted and Controlled Electrical Works, definitions under S.I. No. 264 of 2013, including minor works and the special locations in Part 7 of the National Rules for Electrical Installations. Safe Electric
• Commission Regulation (EU) 2019/2020, ecodesign requirements for light sources, Annex II: flicker and stroboscopic effect limits at full load, and the packaging warning for light sources that cannot be dimmed or need specific dimmers. EUR-Lex
• National Rules for Electrical Installations, I.S. 10101, published by NSAI. NSAI
• Dimmer: phase control, leading edge and trailing edge dimmers, noise and efficiency. Wikipedia