Let`s take a closer look at how a dimmer switch works
By altering the amount of power delivered to a light source, a dimmer directly affects the level of light output from that source.
The more power delivered, the higher the light output. The less power delivered, the lower the light output.
On Off Switch
The two extremes of light output range are of
course achievable by a simple switch.
With a simple switch, we either turn the light on, meaning we close the circuit, delivering full power to achieve the full light output capabilities of the lighting mode or we turn the light off.
Meaning we open the circuit, cutting off power from the lighting load and getting no light output in return.
While this explanation is obvious and simple, the concept is actually foundational to the understanding of how typical dimmers work.
In order to achieve light level from are given lighting load, that is somewhere between fully bright and fully off, what the dimmer actually is doing a switching power on and off at a high rate of switching. Thereby reducing the power delivered to the lighting load and resulting in a lower light output.
High Frequency Switch
The frequency of the switching, is high enough that you cannot see it.
The very nature of the power delivered by the electric company makes this feasible.
This graphic represents a single cycle that delivered sinusoidal voltage signal. This cycle repeats 60 times every second.
Using a simple light switch and turn that switch on, we will be delivering this voltage to the lighting load throughout the entire cycle.
As a result, current would flow through the lighting load.
The product of the voltage applied and the resulting current through the lighting load to determine the power delivered to the lighting load.
To reduce the light level output again, we need to reduce the amount of power that’s delivered. We will do so by switching twice,
off and on during a cycle. Using power electronics, such as a triac, we can turn the voltage off, where the sine wave crosses the zero line. Then we leave it off for certain delay, then back on in order to deliver some power, reduced power to the load.
And we do that again on the negative side. The longer the delay before turning on, the less power we will be delivering and the lower the light level of the light source.
Forward Phase Dimmer
So this then would be the modified waveform coming out of the forward phase dimmer. So this is how a standard dimmer works.
A dimmer with the output characteristics explained here, is known by many names, including standard dimmer, household
dimmer, incandescent dimmer, triac dimmer, magnetic dimmer, forward phase dimmer, leading edge dimmer. I will refer to this throughout the remainder of the article
as a forward phase dimmer.
Keep in mind that these dimmers were developed for dimming conventional light sources. Particularly incandescent lamps.
Resistive light sources
These light sources are resistive in nature and they don’t care about the shape of the wave applyed. They will react to how much power is delivered to them, regardless of the waveform.
When the lamp is low voltage and a transformer is used to convert the line voltage to 12 volts, for example to apply to that lamp, then the addition to the lamp itself, some complexity is introduced by the transformer.
Now conventional wire rounded magnetic transformers are compatible with this type of dimming or forward phase dimming,
so long as certain factors are implemented in the dimmer.
For example:
The power delivered during the positive half of the cycle, must equal the power delivered to the negative cycle. So that any DC component the power delivered is within tolerance of transformers, and out of tolerance, DC component would cause the transformer saturate, leading potentially to its failure.
Incandescent dimmer vs magnetic dimmer
So the difference between an incandescent rated dimmer and a
magnetic rated dimmer, is the ladders ability to perform without detriment to the transformer -they are both forward phase dimmers.
Electronic dimmer switch
At one point the history of light source development, the electronic
transformer was introduced, with a very different set of characteristics from those of a magnetic transformer.
One key difference is that, it is not all compatible with a sudden voltage rise, that occurs twice during each cycle.
So there’s a compatibility issue between the electronic transformer and the standard dimmer – you cannot use the two together.
The solution to dimming electronic transformers is, to swap the sequence of power delivery to the lighting load each half-cycle, instead of opening the switch at the beginning of the cycle,
the switch is closed and at that point the power is delivered for a time.
Then after a predetermined delay, the voltage is turned off and power is no longer delivered for the remainder of the half cycle.
In this way, no sudden voltage rises occur, so it works with the electronic transformer.
Elv or electronic voltage dimmer
Dimmers that produced this backward wave form, are more known by the following terms: elv or electronic low voltage dimmer, simply electronic dimmer, reverse-phase dimmer
or trailing edge dimmer.
I will refer to these from here on out as reverse phase dimmers.
Again the incandecent light source, without a transformer doesn’t care about the waveform. So we can then dim line voltage
incandescents with either forward Phase or reverse-phase dimmers.
However the following differences exist.
The triac that is used to perform forward Phase dimming, cannot be operated in reverse phase matter and so different electronics are used for reversed-phase dimming – typically a FET or an IGBT.
These electronics are comparable more expensive than a triac and their current ratings are typically lower. So if you use reverse phase dimming you will pay a premium and your load capacity will be
somewhat lower, compared to the forward phase dimmer.
Compatible dimmer switches
I should mention here, that some electronic low voltage transformers have been modified, specifically to be compatible with forward phase dimmers.
The manufacturer of the elv transformer must state, that it is compatible with forward phase dimmers or a electronic transformer should always be paired with a reverse-phase dimmer.
As these two introduced dimming methods share a common
concept, of switching power on and off during each half-cycle and because they’re not the only means of dimming, we’re going to classify them together and refer to them collectively as phase counter phase control dimming.
While phase control is still the primary dimming method, it is not the only method. Historically there came a point, when it became desirable to dim fluorescent light sources, but fluorescent lights don’t typically respond well to phase control dimmers.
So other methods of dimming became commercially available.
All of which have carried forward in one form or another to LED dimming.
These other methods of dimming differ from phase control dimmers, in that the dimmer is not actually directly modifying the power delivered to the lighting’s load.
Rather it’s sending a signal, a control signal to an intermediary electronic circuit, known as a dimmable ballast in the fluorescent fixture or is a dimmable driver in an LED fixture.
Indirect dimmer switches
The signal is used to communicate to what level the light output should be set and the ballaster driver uses some mechanism beyond the scope of this article, to achieve that lighting level. For this reason I refer to dimmers that use these methods as indirect dimmers.
Indirect dimmers are of two major types: analog and digital.
I will discus the analog indirect dimmers first.
A light source which includes indirect dimming capability, will be looking for a variable control signal, separate from the power delivered, that power typically being switched on and off by the controlling dimmer, in addition to the variable control signal.
The first type of control signal I will discuss, is the variable low voltage output – sometimes referred to as zero to ten dimming.
Such a device will react to a control signal, that varies between 0 and 10 volts DC, to regulate the light output.
If the control signal received is 10 volts, the device causes
the light output, to go to its full on value.
If the control signal received is zero volts, the device causes the light output to go to its minimum or off value.
A control signal value of 5 volts, being at the midpoint of the
control signal range, would cause the light output, theoretically to go to the fifty percent output level.
The second method of analog indirect dimming is pulse width modulation or PWM.
In pwm a constant voltage square wave, a repeating square wave as a control signal.
Now the percentage of the time that the voltage is on, is actually the variable.
We don’t change the frequency, we don’t change the voltage, we just change the duration, then it is on. And that goes between 0 and 100 percent.
Now there is a standard that applies to dimming pulse width modulated control light sources.
That standard is actually opposite of what you might expect. Meaning that if I were to actually apply zero percent of the time
voltage, then the light output is actually going to be a hundred percent and reduce down linearly to 0 percent when we apply a hundred percent of the time the voltage – opposite of what you expect.
Now because it is opposite of what we expect, you do find from time to time manufacturers of PWM control light sources that don’t comply and go more with what you would expect.
With those light sources, if I apply a zero percent of the time the voltage, then I will get 0 light output.
And I will increase linearly up to a hundred percent or full output when I get hundred percent of the time that I’m applying voltage.
A third and final method of analog indirect dimming, uses a
line voltage phase control waveform as a control signal.
Such a device, is characterized by a neutral conductor, a conductor for power switching and a conductor for dimming control.
So it’s known as a three-wire balusters – a three-wire driver.
While the power delivered to the light sources switch
between the switch leg and neutral, in addition a face control signal is applied between the third conductor neutral, to signal the desired light output level.
Finally there is indirect digital dimming, where in we will send a series of bits, according to a preset protocol to communicate commands and status updates over a two serial communication port.
Two examples of this are DMX and Dali
while these technologies are mentioned to round out the discussion of dimming control, they fall outside of the scope of this article.
I hope you understand a little bit more about dimming now.
Led lights can be dimmed using any methods discussed in this article.
That`s pretty much how a dimmer switch works.
