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Dimming LED Lamps

September 24, 2010   

Dimming LED lamps is nowhere near as simple as halogen lamps where a simple change in the voltage will provide smooth dimming. LED’s require a constant current and work at much lower wattages than halogen lamps which is of course, one of their primary attractions.

For the purposes of this article we will restrict the discussion to GU10 mains voltage LED lamps.

The problems are mainly to do with ensuring that the dimmer switch matches the lamps – LED’s are not like halogen lamps which will operate with almost any dimmer. Most existing dimmer switches are designed to operate with relatively high loads (typically 40-600w) which is fine for halogen lamps which might consume 50w each but less appropriate for LED’s which could be just a few watts. Additionally there are different types of dimmer – leading edge and trailing edge and 0-10v – so where do you begin!

If you choose a high-quality dimmable LED GU10 such as our Toshiba 9w LED, then all the power and dimming options are dealt with within the lamps and as long as you ensure that the circuit load (the number of lamps * the wattage of each lamp) is greater than the lower rating of the dimmer switch, then it should work without problems – NOT TRUE UNFORTUNATELY – you will find that there are minimum and maximum lamp numbers which need to be on a circuit for everything to operate correctly (if you want to know why then read the footnote at the bottom of this article).

Beginning with the dimmer switch, rather than simply buying a dimmer and then some lamps, the lamp manufacturers should be able to offer a list of tested and compatible dimmer switches which will operate their lamps correctly – if you get the combinations of lamps  and dimmer switches wrong then it’s not dangerous but it just won’t work properly – you will get different effects from no dimming through to flashing.

There are many low cost lamps claiming to be dimmable with any dimmer switch – it’s just not true – if you want them to dim properly then you MUST check the compatibility.

If you have already purchased and are having problems whereby lamps are flickering, you can fix this by adding a dummy load to the circuit – something like the ResLoad from Danlers but be aware that when you add these products you are increasing the load on the circuit and will draw more power!

Why is there a problem with dimming LED lamps?

The problem is caused by it being a legal requirement for dimmers switches to always light if switched on (ie it cannot be on and have zero output). This involves having a resistor detect load. With LED lamps when there are only a few lamps on the circuit the capacitance generated by the lamps is below the minimum trigger level on the dimmer switch which means that the circuitry on the dimmer will always reset to the minimum level which is set to suit the higher wattages of halogen lamps  – 5 watts on a halogen is perhaps 10% of power whilst 5w on an LED could be 60-70% power – hence the dimming function is significantly reduced.

The solution is to choose a dimmer switch which is software controller or intelligent where this problems can be properly addressed. Examples include the J Series from Varilight.

We stock a range of fully dimmable GU10 and low voltage MR16 LED bulbs quality focused brands Toshiba and Maxilux. View our GU10 LED Bulbs.


September 26, 2012 at 5:16 am

i am installing 16 dimmable gu10 led lamps probably between 4 and9 watt each 8 is dimmed of 1 dimmer and other 8 of another dimmer. what dimer switch will i need.i am led to beleive that i need trailing edge dimmers, is this true.many thanks steve.

David Wells
October 28, 2012 at 10:40 am

I’ve just bought a set of dimmable LEDs and an inteligent LED dimmer switch. the total wattage from the 7 LEDs is 42w. On testing I find that the light output (520 lumens each) is barely acceptable and i would never dim these. Can I use these with a normal (non dimmer) switch?

Jason Churchill
June 16, 2013 at 2:50 am

What would be the outcome of putting a leading edge 5w bulb through a trailing edge system and vis versa

June 26, 2013 at 10:47 am

Hi Jason,

As you probably know phase cutting technology works by cutting off part of the mains voltage to vary the RMS voltage fed to the lamp. The brightness of the lamps is then controlled by increasing or decresaing the RMS and so the power supplied to it.

There are two ways of doing this , at the beginnng of the sine wave or at the end, If it is done at the beginning it is called leading edge, and after is called trailing edge. I have never tried your application but I would guess the may be a flicker effect in the lamp through its dimming range, how is the system you have trailing edge?


July 20, 2013 at 1:31 am

I have just enstalled some led dimmable lights and they are too bright even at the lowest setting. What can I do?

July 23, 2013 at 1:15 pm

Hi Marge,

A good dimmer should dim the LED’s to a very low light, perhaps there is a problem with the dimmer not compatible with the lamps?
Let me know what Lamps and dimmer you have a we will try to sort the problem out for you.

Darren Sprawson
November 7, 2014 at 2:12 pm

I have a “1 way 230v AC Remote/Touch Control Intelligent Dimmer switch, max 400w- min 40w”, to operate a 4 lamp ceiling fitting. Unfortunately, it only turns on and off, I’m using 4 Philips 4W Master Dimmable GU10 LED – Warm White (25°) I’ve had some advice from an electrician, he thought I might need some sort of transformer, but admitted he had little knowledge of dimmable LED switches. I know nothing of electrical matters.
Please could you advise?

Yours Hopefully

November 7, 2014 at 2:22 pm

Hi Darren,
The dimmer low operating level is too high for your 4 x 4w lamps, the dimmer operates from 40w, you only have 16w in LED Lamps so the current requirement won’t make the dimming circuit switch and operate. In a way the circuit cannot detect the load. You can either increase the load to over 40w or get a new dimmer that operates at a much lower level. There are many designed to start from a 5w load. You may be able to purchase a Resload (dummy load), but it may have to be matched to the dimmer, check with the manufacturer. I hope this helps?

john barber
November 10, 2014 at 12:19 pm

I have Varilight dimmers in all the rooms in my house. I have started to replace the bulbs with LEDs. I have recently replaced the first circuit using 3 GU10 COB LED bulbs, but I find the dimming range is not great, resulting in not being able to dim to the level I would like. Is this because the dimmer is leading edge type, and if so is my only course of action to replace all my (expensive) dimmers? Thank you

November 13, 2014 at 2:41 pm

Hi John,

Trailing edge are preferred, what is the stated range on the dimming specification for the dimmers you have, for example, 5w-250w etc…

January 16, 2015 at 7:03 pm

I wish to buy a good dimmer to give me a wide range of dimming when controlling 5 x 5w GU10 LED bulbs (Screwfix 3797g), please advise.

January 17, 2015 at 11:25 am

Hi Mike, We can recemmend the Varilight J series as a reliable dimmer which can be purchased on line from reputable retailers.


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