Your complete guide to dimming, dimmers and dimmable light bulbs

I’m not a betting man, but if I was asked to wager why most people choose dimmable light bulbs over their standard counterparts, I’d probably go with their aesthetic potential.

By manipulating a tiny knob on one’s living room wall, it’s possible to completely alter the mood and ambience of a room at the drop of a hat. However, this actually carries with it a host of less obvious but equally favourable benefits such as reduced energy consumption and improved longevity – the longer your bulbs are dimmed, the less energy they use and the longer they last. Got it? Great.

The proliferation of LED light bulbs in recent years has magnified these benefits yet further, with massively reduced wattages and hugely inflated lifetimes meaning that our bulbs are now lasting longer than ever before.

But (there’s always a but), the advent of LED has also completely changed the dimmer game. Dimming your traditional incandescent bulbs is still fairly straightforward, as I’ll explain below, but dimming LED bulbs is somewhat trickier.

This ‘dimming for dummies’ guide aims to address each of the whys and wherefores from the wonderful world of dimming, and it’s my hope that you’ll emerge on the other side a bona fide dimming dynamo.

Before we dive head first into our dimming odyssey though, I’ve included a summary of its main points below. These take the form of our customers’ most common queries and simply clicking on a heading will take you directly to its relevant section.

  1. Can I dim any LED bulb?
  2. Can I use my existing dimmer to dim my LED bulbs?
  3. Leading Edge vs. Trailing Edge Dimmers: What's the Difference?
  4. Dimming traditional bulbs with a leading edge dimmer
  5. Dimming LED bulbs with a trailing edge dimmer
  6. Why are my LED bulbs flickering/not dimming correctly?
  7. What can I do to make sure my bulbs dim properly?

Can I dim any LED bulb?

In a word, no. Dimmable LED bulbs and their non-dimmable counterparts use entirely different componentry, so putting a non-dimmable bulb on a dimmable circuit and expecting it to work is a bit like chucking a toaster into the sea and expecting it to swim – it’s only going to end in crushing disappointment and some seriously damaged electrics.

This isn’t true of the reverse though; a dimmable bulb will work just fine on a non-dimmable circuit, it just won’t dim. But hey, at least your electrics won’t have been blown to pieces.

Can I use my existing dimmer to dim my LED bulbs?

This would entirely depend on the type of dimmer you have. If you’ve been using your dimmer with incandescent or halogen light bulbs in the past, I’d wager (not a betting man, remember) that it’s a leading edge dimmer, though this may not always be the case. This assumption is based on the idea that leading edge dimmers have been around much longer and are therefore more ubiquitous than the newer trailing edge variant. In this instance it's best to check what kind of dimmer switch you have.

Wait, what's the difference between trailing edge and leading edge dimmers?

Okay, so there are numerous types of dimmer switch available, but by far the most popular are the two that we’ll be concentrating on today – leading edge and trailing edge phase-cut dimmers.

As their name implies, both work by trimming the voltage at various phases of an alternating current’s sine wave, thereby reducing the power they send to the lamp. Now, at this juncture I’m aware that there’ll be some amongst you that are itching to know exactly how this works, which is why I’ve kindly included a link to our leading edge and trailing edge dimmer guide here. You’re welcome.

For those of you that are still with us, it’s best to think of things as simply as this…

  • Leading edge dimmers are by the far the more popular of the two. They've been around much longer and are traditionally used to dim traditional incandescent and mains halogen light bulbs. This lends them a much higher wattage range (normally anywhere between 250W and 1000W) making them less likely to work with LED bulbs, though this can be possible if the required tolerances are met.
  • Trailing edge or LED ready dimmers have been designed specifically for use with LED bulbs. As such, they share a number of characteristics that make them much more compatible, such as comparable wattage ranges and an inherently digital make up. The required wattages for these dimmers are much lower, making them more compatible with the lower wattage of LED bulbs. Again though, trailing edge dimmers CAN also be used to dim traditional bulbs if the wattage tolerances are still met, and they do this more effectively than using a leading edge dimmer to dim LED bulbs.

The best way to avoid confusion here is to check which dimmers are compatible with a certain type of bulb. Most manufacturers rigorously test their products for compatibility with a range of dimmer switches and the results are normally readily available on their respective websites.

Dimming traditional bulbs with a leading edge dimmer

Dimming traditional bulbs is relatively straightforward. Any leading edge dimmer carries with it a minimum and maximum load. As long as the bulb or bulbs paired with the dimmer sit within these tolerances, everything will work just fine.

The most common modules available generally limit their loads to 250W, 400W, 600W and 1000W at the top end and 25W, 40W, 100W and 150W at the bottom respectively. So, by way of example, if you use 4 x 100W incandescent bulbs on the 400W module it’ll work just fine, but if you try a single 100W bulb on the 1000W variant it would exhibit a number of problems, from flickering and excess noise to outright failure.

Dimming LED bulbs with a trailing edge dimmer

The mistake that many people make is assuming that trailing edge dimmers work in the same way as their leading edge equivalents. This is a mistake - they are more expensive, infinitely more complex and absolutely brilliant when used properly.

They use a complicated, albeit sophisticated electronic system that yields a whole slew of benefits from silent operation to smoother control. They also generally have a lower minimum load than leading edge dimmers, which is why they’re better suited to the more modest wattages associated with LED bulbs.

How many LED bulbs can I use on my dimmer?

One of the biggest problems people experience when installing trailing edge dimmer systems is working out just how many LED bulbs it will support.

The main root of this problem is the massive disparity between incandescent and LED wattages. When a dimmer’s rated wattage is defined, it doesn’t take account of an LED bulb’s drastically superior efficiency.

So, imagine we have a 400W trailing edge dimmer. It will accept 4 x 100W incandescent bulbs without any issue and you can dim them to your heart’s content.

However, this doesn’t mean that you can then go and stuff 40 x 10W LED bulbs on there and still make up the 400W. Oh no. This isn’t Disneyland. I’m afraid you’ll need to divide the dimmer’s maximum wattage by 10 to get its relevant value for LED bulbs.

In this instance, it would be able to take 4 x 10W LED bulbs or less, providing that the minimum wattage is still met.

The graphic below should illustrate this point further…

 Diimming Guide

However, it’s important to stress that this logic purely serves as a rule of thumb and is by no means exact. This is one of the myriad reasons why full compatibility cannot be guaranteed, which leads me neatly on to my next and final point...

Why are my LED bulbs flickering/not dimming correctly?

1. You’re under-loading your dimmer!

This is a fairly common problem when people remove their old incandescent bulbs and simply replace them with LED equivalents. Imagine you have two 60W filament bulbs paired with a leading edge dimmer that has a minimum load of 25W and maximum load of 250W. In theory, this set up should work perfectly: the combined wattage of the bulbs equals 120W, which is well above the dimmer’s minimum requirement but also well below its maximum. Now imagine that you decide you want to improve the efficiency of your home and invest in some sexy new LED bulbs. These clock in at just 6W apiece and will absolutely eviscerate your lighting bill - result. Unfortunately though their combined wattage of 12W gets nowhere near the 25W minimum and so they’ll most likely flicker, fail or blow your electrics. So what’s the solution? A low wattage trailing edge dimmer of course!

2. You’re over-loading your dimmer!

This is probably the most common mistake that people make when switching to LED, whereby they forget to divide the dimmer’s maximum wattage by 10 when considering its tolerances for LED. Remember to do this before assessing where your bulb sits in terms of its wattage. If you’ve divided the dimmer’s rated wattage by 10 and your LED bulb(s) cumulative wattage sits between its maximum and minimum values, crack on.

 3. Plain bad luck.

Sometimes, you do all you can and the world still just wants to watch you struggle. You see, the virtue of LEDs is that they don’t run on mains voltage alternating current, they instead use low voltage direct current. This bestows a number of their most laudable benefits, but it also means they require an electronic driver to convert the voltage they receive from the mains. These are built into the bulb itself (which is why you can retrofit LED bulbs directly to existing fixtures without having to rewire them) but they need to be compatible with a dimmer’s components to dim properly. If they’re not, they won’t, and unfortunately this isn’t something you have much control over – it all comes down to the manufacturer. This is why most manufacturers will normally provide a list of compatible dimmer switches that have been tried, tested and certified for use with their LED bulbs.

What can I do to make sure my bulbs dim properly?

1. Make sure your LED bulbs are dimmable.

This is by far the easiest bit. A basic ability to read the bulb’s packaging and product description should set you right here.

2. Upgrade your dimming circuit from leading edge to LED-ready trailing edge.

Leading edge circuits CAN work with LED technology, but it’s much trickier to fathom, presents much more of a risk and won’t do wonders for the longevity of your bulbs.

3. Use a compatible dimmer.

For the most part, we would recommend our Varilight range of dimmer switches. They’re compatible with an extensive number of LED bulbs and carry very accommodating tolerances, from low minimum loads to generous maximum capacities.

4. Move to a non-dimmable circuit.

How much do you actually use your bulb’s dimming function? Once a year at Christmas when the family’s round probably isn’t enough to justify the additional headache. The easiest and most efficient way to remove any potential set of issues is to simply remove the environment in which they’re allowed to arise.

5. Ensure that all your bulbs are of the same type and from the same manufacturer.

This ensures that all the drivers in the circuit are made by the same people and therefore remain mutually compatible throughout. If you use a smattering of Philips bulbs that are compatible with a particular dimmer but then intersperse them with bulbs from another brand, you’re going to compromise your circuit as a whole.

6. Replace low voltage bulbs with mains voltage versions.

This removes the need for external transformers and minimises the risk of compatibility issues here also. As above, if ALL of your bulbs are made by Philips but require a third party transformer to convert their voltage, it also needs to be compatible. If it isn’t… you get the picture.


Dimming LED bulbs CAN certainly be tricky, but when done properly the benefits are endless, from the improved efficiency and longevity of the bulbs to smooth, silent dimming that looks infinitely better than its incandescent equivalent.


Just one final note: even though you don’t need to be an electrician to understand why or how certain types of circuit work, we would always advise enlisting their help when actually doing any kind of work on them.