By Kimberly Quang

Light dimming is based on adjusting the voltage that gets to the lamp to emit only small amount of light. Light dimming has been possible for many decades by using adjustable power resistors and adjustable transformers. Electronics controlling also made possible to make them easily controllable from remote location.

Most homes have lamps or fixtures that can be made brighter or dimmer by rotating or sliding a control on their on-off switch. Years ago, this was done using a device called a rheostat–a large variable resistor. This method wasted electricity and generated a lot of heat. To control the amount of energy going to the light the rheostat had to throw a lot away, turning it into heat. For example, at half brightness a 100-watt bulb would waste about 20 watts to heat in the rheostat.

Early light 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.

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). 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 early dimmer switches tend to be cumbersome and potentially dangerous, since the variable resistor releases a substantial amount of heat.

Modern dimmers take a more efficient approach. 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.

Modern light dimmer switches use a transistor like device called a TRIAC to switch the electricity on and off very rapidly–120 times each second. Because they sort of ‘chop up’ the electrical power this way they are sometimes called ‘chopper switches.’ The current doesn’t change suddenly. It rises and falls or, undulates.

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. A modern light dimmer switch “cuts off” 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.

The knob or slider on the dimmer switch is also a variable resistor but, in this case it’s just used as a signal to move the turn-on point –it’s not redirecting the flow of current as the old time rheostat was. With the dimmer circuitry very little energy is wasted. A typical modern dimmer control is more than 99 percent efficient–less than 1 watt is wasted controlling a 100-Watt bulb.

If we hook up a really cheap dimmer switch, we may notice a strange buzzing noise. This comes from vibrations in the bulb filament caused by the chopped-up current coming from the triac (triode alternating current switch).

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.

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.

Most of light dimmer switches are built around the same simple idea — chopping up AC current to reduce the total energy powering a light bulb.

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