Why will my LED lights not dim all the way down?

The dead zone at the bottom of an LED dimming range, the stepping and dropout that go with it, and the ordered checks that fix most cases without replacing anything.

Difficulty: beginner
Time: 20 min
Read: 4 min

Short answer

The dimmer's lower limit and the LED driver's lower limit do not line up, so the lamps cut out or refuse to go below a certain point. Set the dimmer's trim adjustment first, check you are above the dimmer's minimum load, make sure every lamp on the circuit is the same dimmable model, and if it still fails, the dimmer is the wrong type for the driver.

An LED does not dim by getting less voltage; its driver reduces the current or pulses it. Below a certain input the driver cannot regulate, so instead of fading gracefully the lamp cuts out, stutters or jumps back to a higher level. Most complaints about LED dimming are this boundary problem, and most are adjustable rather than fatal.

Step by step

Read the dimmer's instructions for a trim or calibration mode.Most modern LED dimmers have either a small screw potentiometer behind the knob or a press-and-hold setup routine that sets the minimum brightness. This alone resolves the majority of "cuts out at the bottom" complaints and it takes two minutes.
Add up the actual load and compare it with the dimmer's minimum.Five 4W lamps is 20W. If the dimmer needs 25W minimum, no adjustment will make it stable. Either fit an LED-rated dimmer with a low minimum or add another lamp to the circuit.
Check the dimmer's LED maximum too.Overloading works the other way: too many drivers on a dimmer rated for a small LED load causes stepping, buzzing and premature dimmer failure. The LED rating is much lower than the headline wattage.
Make every lamp on the circuit identical.Same brand, same model, same batch if you can. Mixed lamps have mixed driver thresholds, so they cut out at different points and the set appears to step.
Confirm every lamp is marked dimmable.One non-dimmable lamp in a set of six will flicker and can destabilise the whole circuit. It is worth taking them out and reading each one.
Test with a reduced set.Take out all but one lamp and try the range, then add them back one at a time. If the range is good with one and bad with six, it is a load or matching problem. If it is bad with one, it is dimmer type or lamp compatibility.
Establish the dimmer type.If the dimmer predates LED lighting or is marked only with a high wattage range and no LED rating, it is almost certainly leading edge and is the cause. Replacing it with an LED-rated trailing edge or universal dimmer is a like-for-like accessory swap in an ordinary room, with the circuit isolated and proved dead.
Accept a floor if the driver has one.Some LED lamps genuinely will not go below 10 or 20 per cent, because their drivers are not designed to. If you want deep dimming, that is a lamp specification to buy for — look for lamps that state a dimming range down to 1 per cent.

Tips

Buzzing that comes from the lamps rather than the switch is usually the driver reacting to a leading edge waveform. Buzzing from the switch is usually an overloaded or wrong-type dimmer.
Dimming shortens no LED's life when it is done with matched equipment. Dimming badly, with a mismatched dimmer, absolutely does.
Keep one spare lamp from the same batch. Replacing a failed lamp two years later with a different model is how a matched set becomes an unmatched one.

Common mistakes

Assuming the lamps are faulty — In a mismatched system the lamps behave exactly as designed. Replacing them with the same type on the same dimmer changes nothing.
Turning the dimmer down hard to force it lower — Driving the driver below its regulation point is what causes the stutter and, over time, stresses the driver components.
Fitting a second dimmer at the other end of a two-way circuit — Two phase-control dimmers in series interfere with each other. One dimmer, one ordinary two-way switch.

If it doesn't work

Lamps cut out abruptly near the bottom of the range

Cause: Dimmer minimum set below what the drivers can regulate — Fix: Use the dimmer's trim adjustment to raise the minimum until they hold steady.

Lamps step rather than fade

Cause: Mixed lamp models with different driver thresholds — Fix: Replace with a single matched set.

Only half the dial changes anything

Cause: Dimmer range and driver range poorly matched — Fix: Trim adjustment first; if that does not fix it, change the dimmer for an LED-rated trailing edge type.

Audible buzz from the lamps at mid-brightness

Cause: Leading edge dimmer on electronic drivers — Fix: Change to trailing edge. This is not fixable with the existing dimmer.

One lamp behaves differently from the rest

Cause: That lamp is a different model, non-dimmable, or failing — Fix: Swap it for one matching the rest.

Questions people ask

Is my dimmer broken?

Rarely. Almost all of these symptoms are compatibility rather than failure. The exceptions are a dimmer that has been overloaded, which often fails on or fails off, and one that is physically hot.

Why does it work fine in one room and badly in another?

Different lamp models, different total loads, or a different dimmer. Compare the three and the answer usually falls out immediately.

Can I dim LED strip lighting from a normal wall dimmer?

Only if the strip's driver is specified as mains-dimmable with that dimmer type. Most LED strip is dimmed on the low-voltage side by its own controller, and putting a mains dimmer in front of a standard driver will flicker at best and damage it at worst.