LED Lights Flickering on a Dimmer? Why It Happens and How to Fix It

You’ve swapped the old halogen bulbs for LEDs. Then you turn the dimmer down and the lights start flickering, buzzing or strobing like a nightclub.

This is one of the most common lighting calls I get. Honestly, almost everybody who has put LED bulbs on an old dimmer has run into it, and hardly anybody knows why.

The good news is it’s usually not dangerous and it’s usually easy to sort. The bit worth knowing is when it isn’t the dimmer at all.

Quick answer

LED lights usually flicker on a dimmer because the dimmer was designed for high wattage halogen or incandescent bulbs. Old dimmers often need a minimum load, such as 40W or 60W, and a handful of LED bulbs doesn’t reach it. Fitting an LED compatible (usually trailing edge) dimmer and using good quality dimmable bulbs fixes most cases. If lights flicker without a dimmer, or when big appliances switch on, get an electrician to check the wiring.

Why old dimmers and LED bulbs don’t get on

Most dimmers fitted in houses around Rotherham before LEDs took over were made for old style bulbs. A 60W bulb, a 50W halogen spotlight, that sort of thing.

Those bulbs are simple. They’re basically a hot wire, and a dimmer just gives them less power so they glow less brightly.

LED bulbs are different. Each one has a tiny electronic driver inside it, and that driver has to make sense of what the dimmer is doing. Old dimmers weren’t built with that in mind.

The minimum load problem

This is the big one. Lots of older dimmers have a minimum load printed on them, often 40W or 60W. Below that, the dimmer can’t control the circuit properly.

Think about a typical kitchen with six halogen spots at 50W each. That’s 300W, well above the minimum. Swap them for 5W LEDs and you’ve got 30W. The dimmer is now working below its minimum, so it behaves erratically and the lights flicker, pulse or refuse to dim smoothly.

Leading edge and trailing edge dimmers in plain English

A dimmer works by chopping bits out of the electricity supply many times a second. Less of the wave gets through, so the light is dimmer.

Leading edge dimmers

These chop the start of each wave. They’re the older, cheaper type and suit old style bulbs and some transformers.

With LEDs they often cause flicker and buzzing, because that sudden switch on at the start of each wave is harsh on the little driver inside the bulb.

Trailing edge dimmers

These chop the end of each wave instead. It’s a smoother, gentler way of dimming, and it generally suits LED bulbs much better.

Trailing edge dimmers also tend to have a much lower minimum load, which is exactly what you want when your whole room is running on 30W of LEDs.

A quick way to tell which you’ve got

Older rotary dimmers with no mention of LEDs on the front or back are very often leading edge. If you’re not sure, don’t take the plate off to look. A local electrician can check it in a couple of minutes.

Dimmable and non dimmable bulbs

Not every LED bulb can be dimmed. The box will say “dimmable” if it can. If it doesn’t say so, assume it can’t.

Putting non dimmable LEDs on a dimmer can cause:

• Flickering at anything below full brightness
• Buzzing from the bulb or the switch
• Bulbs failing much sooner than they should

Quality varies a lot too. Cheap multipacks are often the worst offenders.

Common symptoms of a dimmer and LED mismatch

Here’s what people usually describe when they ring us:

• Flickering or strobing, often worse at the lower end of the dial
• Buzzing or humming from the dimmer, the bulbs, or both
• Ghosting, where the bulbs glow faintly even when switched off
• Bulbs failing early, sometimes within months
• Limited dimming range, where the lights go from bright to off with hardly anything in between

Why LEDs glow when they’re switched off

Ghosting catches a lot of people out. Some dimmers let a tiny bit of current leak through even when off. Old bulbs wouldn’t show it, but an LED needs so little power that it glows faintly. A compatible dimmer and decent bulbs usually cure it.

The fix: LED compatible dimmers

In most homes the answer is swapping the old dimmer for a proper LED compatible one, usually trailing edge.

When we choose one, we look at:

• The minimum load, so your bulbs comfortably exceed it
• The maximum LED load, which is often far lower than the old halogen rating. A dimmer rated at 400W for halogen might handle much less in LEDs, so check the LED figure on the box
• Adjustable minimum setting, which many modern dimmers have, so the bottom end of the dial can be set to suit your bulbs
• Size, as some dimmers are deeper and may need a deeper back box

Swapping a dimmer like for like isn’t notifiable building work in England under Part P, but it still involves live wiring. It’s a job for an electrician, not a YouTube tutorial.

Don’t mix bulb brands on one dimmer

This one surprises people. Put three bulbs from one brand and two from another on the same dimmer and you can get flicker, uneven brightness or one set dimming differently to the other.

Each brand’s driver responds to the dimmer slightly differently. My advice is to use the same make and model of dimmable bulb across a whole circuit, and keep a spare or two in the drawer.

Smart bulbs and dimmers don’t mix

If you’ve got smart bulbs that you control with an app or a voice assistant, they shouldn’t be on a dimmer switch.

Smart bulbs do their own dimming and need a steady full supply. A dimmer chopping the power can make them flicker, drop off your WiFi or fail early. Either replace the dimmer with a standard switch or use smart bulbs on a different circuit. There are smart dimmer switches too, which work with ordinary dimmable LEDs instead.

When the flicker is NOT the dimmer

This is the part I really want people to take on board. Sometimes the dimmer is a red herring and the flicker is pointing to a wiring problem.

Signs it could be a wiring fault

• Lights flicker on a normal switch with no dimmer at all
• Flickering happens across several rooms, not just one
• Lights dim or flicker when the electric shower, kettle, oven or another big appliance switches on
• Lights get noticeably brighter or dimmer on their own
• Sockets or switches feel warm, or you can smell burning

Loose connections

A loose connection at a switch, ceiling rose or junction box can cause flicker. It can also create heat, which is why it shouldn’t be ignored. In older properties across Wickersley, Bramley and Maltby we do find tired connections behind switches and light fittings, particularly where things have been changed over the years.

Neutral problems

A poor or shared neutral connection can cause odd flickering and lights changing brightness. If lights around the house start brightening and dimming wildly, switch off what you can and get help straight away. If the fault is in the supply cable rather than your wiring, your electricity network operator deals with that.

Voltage dips from big loads

A slight dip in brightness when a powerful shower kicks in can happen, and LEDs can show it more than old bulbs did. A big or repeated flicker isn’t something to live with though. Our guide to electric showers explains why showers put such demand on a supply.

These faults need an electrician in Rotherham or wherever you are to test the circuits properly. An EICR (electrical installation condition report) can be a sensible step if you’ve got several warning signs, and upgrading an old consumer unit gives far better protection on older installations.

Dimmer heat: what’s normal and what isn’t

Dimmers do get slightly warm when they’re on. Some energy is turned into heat inside the switch, so a gentle warmth on the faceplate is normal.

What isn’t normal:

• Hot to the touch, so you don’t want to keep your hand on it
• Loud buzzing or crackling
• Scorch marks or brown discolouration on the plate or wall
• A burning or fishy smell
• A melted or soft feeling faceplate

If you notice any of these, switch the lights off, stop using that circuit and get it looked at. Electrical Safety First also has a handy tool for finding a registered electrician.

Frequently asked questions

Why do my LED lights flicker when dimmed but not at full brightness?

Usually the dimmer isn’t LED compatible, the bulbs fall below its minimum load, or the bulbs aren’t dimmable. At full brightness the dimmer isn’t chopping much of the supply, so the problem hides.

Can I use LED bulbs with my old dimmer switch?

Sometimes, if the bulbs are dimmable and the dimmer copes with a low load. Many older dimmers struggle, and an LED compatible trailing edge dimmer is the reliable fix.

Are flickering LED lights dangerous?

A flicker from a mismatched dimmer is usually more annoying than dangerous. Flicker with heat, burning smells, buzzing from switches, or flicker across the house can point to a wiring fault and wants checking.

Can smart bulbs go on a dimmer switch?

No. Smart bulbs need a full, steady supply. Use a standard switch, or a smart dimmer with ordinary dimmable LEDs.

Getting it sorted locally

I’ve been an electrician for over 25 years. We cover Rotherham and the rest of South Yorkshire, including Sheffield, Barnsley and Doncaster. You can see more on our recent work page or read a bit about us. Every job is quoted properly after we’ve had a look, and it’s free to ask.

Related services and guides

• Lighting design and installation
• LED lighting upgrades
• Electrical fault finding
• Electric showers: what a proper install involves
• RCD, RCBO, AFDD and surge protection explained

Contact MP Electrical

If you need help with flickering LED lights, dimmer switches or any electrical work in Rotherham or South Yorkshire, MP Electrical are happy to help.

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