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A Guide to Dimmer Switch Selection

What Dimmer Switch do I need?

A very common problem that faces the enthusiastic home decorator and even seasoned electrician is which dimmer switch is correct for my lights? There are so many out there with different wattages and fancy names that for even technically minded people, this area can become a bit of a mine field.

There are some fairly simple rules to follow that will quickly get you to your answer and before you know it, your lights will be at the perfect dimmed level to show off your hard work!


The key is to be aware of what kind of bulb you are trying to dim. “In the olden days” we had standard incandescent light bulbs and all your dimmer had to do was reduce the amount of power to the light bulb and it would dim nice and smoothly.


But of course we are now in a very different world where due to environmental issues, the majority of the bulbs we use are energy saving and therefore packed with electronics and technologies that require some clever technology in the dimmer switch to perform the same task.



Types of Dimmer Switch


There are three main groups of dimmer switch available:


Resistive DimmersFor dimming lamps with filaments inside them.


There are 2 types of resistive dimmer; Leading Edge & Trailing Edge.


Leading Edge: The most common type on the market and able to dim:

Standard incandescent lamps
Mains voltage halogen lamps & capsules
Low voltage transformers powering low voltage lamps (12V)
Constant current dimmable LED drivers

Leading edge dimmers are rated by the maximum recommended wattage it can dim. Overloading the dimmer or using it to control Inductive loads can cause damage to the dimmer. This dimmer will also dim constant current 230V dimmable LED drivers.

Trailing Edge: Less common, more expensive and incorporating more technology and features. Suitable for dimming:

Older Toroidal & Wire wound transformers
Standard incandescent lamps
Mains voltage halogen lamps & capsules
Dimmable LED lamps

Additional features that can be included in a trailing edge dimmer are soft start, smooth control, silent running and multi-way dimming. These features can all assist in extending the life of the lamps.

Inductive Dimmers – Designed to dim from the low voltage side of a transformer or on LED lighting. They are rated by voltage rather than wattage. Inductive dimmers are already de-rated and are generally not found in the retail market.


Fluorescent Dimmers – Dimming fluorescent lamps is complex. To successfully dim a fluorescent lamp a high frequency analogue 1-10 volt regulated ballast is required. There are some compact fluorescent lamps available on the market that will work with this type of dimmer.


This type of dimmer can also be used to dim LED lamps that are power by a constant current 230V LED driver.



Minimum & Maximum Load Ratings


Dimmer switches have minimum and maximum load ratings which should be observed when selecting the dimmer for a particular application. An LED for example only draws a small amount of power such as 5W. Let’s say you have 5 LED lamps being dimmed. That would be 25W minimum draw. To select the correct dimmer for the LED lamp you must select one that has a suitable minimum loading such as a QADTE12 that has a minimum loading of 10W.


Equally for other types of lamp you must observe the maximum load rating of the dimmer. If you wanted to dim 6 x 50W halogen lamps for example, you would need a dimmer with a maximum loading of at least 300W. In fact for halogen lamps, the dimmer must also allow for an extra 25% load. This is due to a surge in power that occurs as the lamp comes to the end of its life and starts arcing. This arcing can be heard in the form of a buzzing sound emanating from the dimmer. Therefore the dimmer required for 6 x 50W lamps needs a maximum rating of at least 375W. Maximum load ratings tend to come in set sizes such as 250W, 400W, 600W & 1000W.


Non De-Rated Dimmer Switches


Another type of resistive dimmer that does not need this extra 25% maximum load head room is a “Non De-rated” dimmer switch. This type was developed when halogen lamps became particularly popular during the 90’s to allow for the surge in current previously described.


Dimming Electronic Transformers


It is worth checking the manual for electronic transformers used for powering low voltage lamps as they may have special requirements for dimming such as an inductive dimmer. Most electronic transformers can be dimmed by a standard Leading Edge Resistive dimmer.


Dimming LED Lamps


Check whether the LED lamp is dimmable before you try and dim it. If the LED lamp is powered by an external driver check that it is a constant current driver and it is dimmable. If you try and dim a non-dimmable LED or driver you will damage them and the dimmer.


If the LED or the transformer state they are dimmable, a standard resistive leading edge dimmer can be used to dim them.


Remember to observe the minimum load rating of the dimmer when dimming LED lamps.

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