Are LED Light Bulbs Dimmable?

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Are LED Light Bulbs Dimmable - Dimmer Switch


Hi there and welcome to this article that looks at whether LED light bulbs are dimmable.

Recently I wrote a review of LED light bulbs that are 100 percent dimmable (see our Articles page). Which kind of gives away the answer. But I also wanted to explain how the dimmer process works in general. And how it works for LEDs.

So lets start with the answer. Which is yes. LEDs are completely dimmable but in a different way from other light bulbs.

How Do Dimmer Switches Work

In simple incandescent light bulbs, light is produced when the filament heats up. Thus, the hot filament glows, and this glow is the light the bulb creates.

If you reduce the current going through the filament then you get less light. And there are two ways to do this.

Think of water flowing through a pipe, with a little water wheel in the middle. The more water flowing through the pipe the faster the wheel turns. So to slow the wheel down you can either reduce the amount of water or add a blockage into the pipe. Dimmer switches work in the same way. Only the water is electricity and the water wheel is the light bulb.

Older dimmer switches effectively added a blockage to the pipe. In this case by adding a resistor in the circuit. Resisters do exactly what their name suggests. They resist the flow of current. However this is a wasteful method, as it still uses all the electricity. With the resister part simply creating heat.

Modern dimmer switches effectively turn the bulb on and off. Though to understand how they do it we must first look at alternating current.


Alternating Current and Dimmers

The electricity in our homes comes in the form of alternating current. So it is a wave that rises and falls. In the United States the current cycles at 60 Hertz. That is 60 times per second.

Alternating Current Sine Curve
An Alternating Current Sine Wave

Modern dimmers chop out some of this curve. Depending on the dimmer setting. The remaining part of the curve part drives the light bulb. Thus overall less current is supplied, and the light bulb is dimmer. This method is much more energy efficient and also creates much less heat.

As can be seen in the graphic below the dimmer removes a portion of the curve. In this case the leading part of the wave. For this reason this type of dimming is called leading edge dimming. Additionally you can buy dimmers that remove the trailing edge of the curve. In fact trailing edge dimmers are the more popular, as they give smother control and a softer start than leading edge. This is because the bulbs work better with a sudden power-off rather than a sudden power-on.

Are LED Light Bulbs Dimmable - Leading edge dimming
Are LED Light Bulbs Dimmable - Trailing Edge Dimming

How Do Dimmers Work With LEDs

So dimmers work with incandescent, halogen and even fluorescent light bulbs by simply removing some of the current wave. Less current gives less light. However LEDs don’t quite work this way.

Control Chips and Direct Current

LEDs have a control chip that supplies power to the LEDs themselves. This control chip also changes the alternating current (the wave) into a fixed, constant current, called direct current.

So, the control chip has converted our undulating alternative current into a nice constant direct current. Now how does it dim down the light output. Well, again there are two options…

Analog Dimming

Firstly the control chip could use analog dimming. This is pretty simple. The chip simply supplies less current to the LED. And less current equals less light. However, the problem with this method is that some LEDs can change color at lower currents. So you might find that the dimmed light looks different from the non-dimmed light. Often the LEDs will look more yellow at lower current.

Which leads up to the next dimming option…

Pulse Width Modulation (PWM) Dimming

Sounds complicated but in fact it is closely related to the alternative current dimming we looked at earlier. In pulse width modulation dimming the control chip turns the LEDs on and off. As shown below, instead of a constant current we get this step pattern. On-Off-On-Off. As long as this switching is sufficiently fast then our eyes don’t notice. But the LED light bulb is actually only producing half the normal amount of light.

50 percent off switching
The light bulb switches off 50% of the time supplying 50% of the normal light.
20 percent off switching
In this case the light bulb switches off 20% of the time supplying 80% of the normal light.

Pulse width modulation has the advantage that the LED is always running at full current. So no problems with color change as often occurs in analogue dimming.

Are All LED Light Bulbs Dimmable?

In a word, no.

Firstly, the dimmer switch is sending either a lower current or a chopped-up alternating current wave. Subsequently the control chip has to respond and dim the LED accordingly.

To do this the LED light bulb manufacturers have to add additional components into the bulb. And these additional components cost money. So some manufacturers choose not to put them in. Thus these bulbs are not dimmable.

Final Words and Tips on are LED Light Bulbs Dimmable

So we have seen that there are two main types of dimmer switches. And that LEDs can dim in two different ways. So how does it all work together?

Well that depends on what your LED light bulb manufacturer has put in their bulbs. Firstly the LED control chip needs to know that the changing power supply from the dimmer switch means it needs to do something. Secondly it needs to either reduce the current to the LEDs or switch the current on and off. This will depend on how the manufacturer has built their bulbs.

Tips on Dimmable LED Light Bulbs

  • If you are going to use more than one light bulb on a dimmer switch. For example if you have a chandelier. Try to use all the same make and model of bulbs. That way they should dim in the same manner.
  • Buy a good quality bulb. The manufacturer is more likely to have spent money on good components that are required for proper dimming.
  • Check the bulb is compatible with the dimmer switch. If you use a popular dimmer switch and a major light bulb supplier you should be able to find compatibility details on the manufacturers website.

Best Fully Dimmable LED Light Bulbs

If you are looking for the best 100% dimmable LED light bulbs then see our review here.

Additional Tips For Dimming LEDs


Architect: LEDs: A Deep Dive in Dimming