When you're furnishing a home, light is
everything. The light level in a room dictates what you can
and can't do, and it has a huge effect on how you feel. You
can't read very easily by a single candle, for example, and a
romantic dinner for two isn't so romantic under a 1,500-watt
halogen
lamp.
The
problem is that people need to use some rooms for multiple
purposes, and these different functions call for varying
amounts of light. Enter the dimmer switch, a handy
electrical component that lets you adjust light levels from
nearly dark to fully lit by simply turning a knob or sliding a
lever.
In this edition of HowStuffWorks,
we'll look inside one of these everyday devices to find out
how it controls lamp fixtures. As it turns out, the inner
workings are pretty cool -- and surprising. Modern dimmer
switches work in a totally unexpected way.
The Old Way Early dimmer switches had a
pretty straightforward solution to adjusting light levels -- a
variable resistor. An ordinary resistor is a
piece of material that doesn't conduct electrical current well
-- it offers a lot of resistance to moving electrical charge.
A variable resistor consists of a piece of resistive material,
a stationary contact arm and a moving contact arm.
In this design, you vary the total resistance of the
resistor by adjusting the distance that the charge has to
travel through resistive material. If the contact arm is to
the left, charge flowing through the circuit only has to
travel through a little bit of resistive material. If the
contact arm is all the way to the right, charge has to move
through more resistive material.
As charge works to move through the resistor, energy is
lost in the form of heat. When you put a resistor in a series
circuit, the resistor's energy
consumption causes a voltage drop in the circuit,
decreasing the energy available to other loads (the light
bulb, for example). Decreased voltage across the light
bulb reduces its light output.
The problem with this solution is that you end up using a
lot of energy to heat the resistor, which doesn't help you
light up the room but still costs you. In addition to be being
inefficient, these switches tend to be cumbersome and
potentially dangerous, since the variable resistor releases a
substantial amount of heat.
Modern dimmer switches take a more efficient approach, as
we'll see in the next section.
The New and Improved Way Instead of
diverting energy from the light bulb into a resistor, modern
resistors rapidly shut the light circuit off and on to reduce
the total amount of energy flowing through the circuit. The
light bulb circuit is switched off many times every second.
The switching cycle is built around the fluctuation of
household alternating current (AC). AC current has
varying voltage polarity -- in an undulating sine
wave, it fluctuates from a positive voltage to a negative
voltage. To put it another way, the moving charge that makes
up AC current is constantly changing direction. In the
United States, it goes through one cycle (moving one way, then
the other) 60 times a second. The diagram below shows this
sixtieth-of-a-second cycle.
A modern dimmer switch "chops up" the sine wave. It
automatically shuts the light bulb circuit off every time the
current reverses direction -- that is, whenever there is zero
voltage running through the circuit. This happens twice per
cycle, or 120 times a second. It turns the light circuit back
on when the voltage climbs back up to a certain level.
This "turn-on value" is based on the position of the
dimmer switch's knob or slider. If the dimmer is turned to a
brighter setting, it will switch on very quickly after cutting
off. The circuit is turned on for most of the cycle, so it
supplies more energy per second to the light bulb. If the
dimmer is set for lower light, it will wait until later in the
cycle to turn back on.
That's the basic concept, but how does the dimmer actually
do all of this? In the next couple of sections, we'll look at
the simple circuitry that makes it work.
The Triac In the last section, we saw that a
dimmer switch rapidly turns a light circuit on and off to
reduce the energy flowing to a light switch. The central
element in this switching circuit is a triode alternating
current switch, or triac.
A triac is a small semiconductor
device, similar to a diode or transistor. Like a transistor, a
triac is made up of different layers of semiconductor
material. This includes N-type material, which has
many free electrons, and P-type material, which has
many "holes" where free electrons can go. For an explanation
of these materials, check out How
Semiconductors Work. For a demonstration of how these
materials work in a simple transistor, see How
Amplifiers Work.
Here's how the N-type and P-type material is arranged in a
triac.
You can see that the triac has two terminals, which are
wired into two ends of the circuit. There is always a voltage
difference between the two terminals, but it changes with the
fluctuation of the alternating current. That is, when current
moves one way, the top terminal is positively charged while
the bottom terminal is negatively charged, and when the
current moves the other way the top terminal is negatively
charged while the bottom terminal is positively charged.
The gate is also wired into the circuit, by way of a
variable resistor. This variable resistor works the
same basic way as the variable resistor in the old dimmer
switch design, but it doesn't waste nearly as much energy
generating heat. You can see how the variable resistor fits
into the circuit in the diagram below.
So what's going here? In a nutshell:
The triac acts as a voltage-driven switch.
The voltage on the gate controls the switching action.
The variable resistor controls the voltage on the gate.
In the next section, we'll look at this process in greater
detail.
The Circuit When there is "normal" voltage
across the terminals and little voltage on the gate, the triac
will act as an open switch -- it won't conduct electricity.
This is because the electrons from the N-type material fill in
holes along the border with the P-type material, creating
depletion zones, insulated areas where there are few
free electrons or holes (see this page for
a full explanation of depletion zones).
If you apply a strong enough voltage to the gate, it
will disrupt the depletion zones so electrons can move across
the triac. The exact sequence varies depending on the
direction of the current -- that is, which part of the AC
cycle you're in. Let's say the current is flowing so the top
terminal is negatively charged and bottom terminal is
positively charged. The circuit is arranged so that the
voltage boost on the gate will have the same charge as the top
terminal. So we get something that looks like this:
When the gate is "charged," the voltage difference between
the gate and the lower terminal is strong enough to get
electrons moving between them. Moving electrons out of the
N-type material -- area e -- disrupts the depletion
zone between areas e and d. Introducing more free
electrons into area d disrupts the depletion zone
between d and c. Electrons from area c can move
toward the bottom terminal, jumping from hole to hole in area
d. This introduces more holes into area c, which
gets electrons moving out of the depletion zone between c
and b. The voltage is strong enough to drive electrons
from area a into the holes in area b, disrupting
the last depletion zone. With the depletion zones
dispersed, electrons can move freely from the top terminal to
the bottom terminal -- the triac is now conductive! (Note:
Some dimmer switches also contain a similar semiconductor
device called a diac, in addition to a triac.
These circuits work in the same basic way.)
In order for the triac to start conducting electricity
between its two terminals, it needs a voltage boost on
its gate. The required voltage level doesn't change, but you
can adjust how long it takes the gate to "charge up" to this
voltage. This is where the variable resistor and the firing
capacitor come in.
Current passes through the variable resistor and
charges the firing capacitor (current builds up
electrical charge on the capacitor's plates -- see How
Capacitors Work for more information). When the capacitor
builds up a certain amount of charge, it has the necessary
voltage to conduct current from the gate to the bottom
terminal. It discharges, making the triac conductive.
Turning the dimmer switch knob pivots the contact
arm (or contact plate) on the variable resistor,
increasing or decreasing its total resistance. When the knob
is set to "dim," the variable resistor offers greater
resistance so it "holds up" the current. As a result, the
necessary boost voltage doesn't build up as quickly on the
firing capacitor. By the time the capacitor is charged enough
to make the triac conductive, the AC current cycle is well
underway. If you turn the knob the other way, the variable
resistor offers less resistance and the capacitor gets up to
the necessary boost voltage earlier in the fluctuating cycle.
The variable resistor from a basic dimmer
switch
As soon as the current fluctuates back to the zero voltage
point, there is nothing driving current through the triac, so
the electrons stop moving. The depletion zones form again, and
the triac loses its conductivity until the boost voltage
builds up on the gate.
This system works very well, but it does create an odd
problem: It tends to produce a distinctive buzzing in
the light bulb. In the next section, we'll find out why this
is.
Dimmer Buzzing If you hook up a really cheap
dimmer switch, you may notice a strange buzzing noise. This
comes from vibrations in the bulb filament caused by the
chopped-up current coming from the triac.
If you've read How
Electromagnets Work, you know that electricity flowing
through a coiled length of wire generates a substantial
magnetic field, and fluctuating current generates a
fluctuating magnetic field. If you've read How Light
Bulbs Work, you know that the filament at the heart of a
light bulb is just a coiled length of wire. It makes sense,
then, that this coiled filament becomes magnetic whenever you
pass current through it, and the magnetic field fluctuates
with the AC current.
Normal undulating AC current fluctuates gradually, so the
magnetic field does, too. The chopped-up current from a dimmer
switch, on the other hand, jumps in voltage suddenly whenever
the triac becomes conductive. This sudden shift in voltage
changes the magnetic field abruptly, which can cause the
filament to vibrate -- it's rapidly drawn to and repelled by
the metal arms holding it in place. In addition to producing a
soft buzzing sound, the abruptly shifting magnetic field will
generate weak radio
signals that can cause interference on nearby TVs or radios!
Better dimmer switches have extra components to squelch the
buzzing effect. Typically, the dimmer circuit includes an
inductor choke, a length of wire wrapped around an iron core,
and an additional interference capacitor. Both devices
can temporarily store electrical charge and release it later.
This "extra current" works to smooth out the sharp voltage
jumps caused by the triac-switching to reduce buzzing and
radio interference. (See How Inductors
Work and How
Capacitors Work for more information.)
The guts of a basic dimmer
switch
Some high-end dimmer switches, such as the ones commonly
used in stage lighting, are built around an
autotransformer instead of a triac. The autotransformer
dims the lights by stepping down the voltage flowing to
the light circuit. A movable tap on the autotransformer
adjusts the step-down action to dim the lights to different
levels. Since it doesn't chop up the AC current, this method
doesn't cause the same buzzing as a triac switch.
There are a lot of other dimmer switch varieties out there,
including touchpad dimmers and photoelectric
dimmers, which monitor the total light level in a room and
adjust the dimmer accordingly. Most of these are built around
the same simple idea -- chopping up AC current to reduce the
total energy powering a light bulb. At the most basic level,
that's all there is to it.
For more information on dimmer switches, including an
installation guide, check out the links on the next page.
