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Just about all inexpensive light dimmers (wall switch type, turn knob type, X-10 lamp modules, line cord or built in a lamp type) all work the same way. Some AC/DC variable-speed motors used in consumer equipment like treadmills use the same basic devices to control the speed of the equipment.

They have an electronic device called a Triac that switches the AC power wave on with various timing throughout the cycle. Triacs act like a switch in that they can only be ON or OFF. So how does that dim a bulb?

Delivering full voltage cycle to a bulb generates full brightness. Reducing brightness requires either reducing the voltage or the duration the bulb sees the voltage. Dimmers use triac circuits because they are inexpensive and deemed "good enough" for the consumer market.

AC power in North America is delivered in a 60 cycle sine wave where a complete cycle lasts 1/60th of a second. (50 or 60 cycle in other countries) Each wave has a Positive half and a Negative half, so each half is at 1/120th of a second. A sine wave is nice, orderly, and fairly quiet in the world of Radio Frequency Interference (RFI). Most RFI-sensitive equipment knows how to filter 60 and 120 cycle hum.

A triac defaults to blocking current. At some time into the AC cycle, depending on how a person sets the dimmer, the triac is switched on and allows current to flow to the bulb for the rest of the half cycle. A sine wave starts at zero volts, climbs to its peak positive voltage, drops back down to zero, then starts its negative side where it drops to its peak negative voltage and goes back up to zero and repeats the cycle.
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For a dim setting, the triac keeps the current off until well-after the wave passes its peak, and switches on to allow only the lower voltage side of the wave through. The bulb sees a pulse of the lower voltage side of the wave, and glows dimly. When the wave reaches its zero point the triac switches off and is ready for the next wave pulse.
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Increasing the dimmer control tells the triac to switch on earlier in the wave, allowing higher portions of the wave through, delivering more voltage, and the bulb burns brighter.
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After all this, how does this mess up DSL?

A regular sine wave is a fairly quiet waveform as things go. However, chopping the sine wave with the triac produces a modified waveform that is very noisy in the electronics world. Worst case is the triac about midpoint, when the wave is at its peak sends a jolt from no voltage to full voltage. While those bursts 120 times each second might make the bulb glow at half brightness, it also produces massive amounts of RFI. Better dimmers have circuitry that suppresses some of this RFI, but you get what you pay for. An $11 dimmer will not have $25 worth of RFI suppression built into it.

RFI is insidious. It can sneak around no matter what you do. As you vary the brightness, thus varying when the triac switches on in the wave, you also vary the RFI. Some brightness settings may not affect anything, while some other settings could totally wipe out your DSL.

Noisy RFI from dimmers can reach your computer or modem through the AC lines, but more likely by radiating from the AC lines to your phone line. Dimmers on full brightness can still radiate RFI if there is even the slightest chop to the wave. Anything that uses a triac (or its cousin the SCR) to control a power line can radiate RFI and cause problems. RFI can interfere with various sections of the spectrum needed for DSL, causing lower sync rates and throughput, and in some cases repeated interruption or even total failure to sync.

Motion detection lights outside, even if they dont dim, likely use triacs to switch on and off. Check wall switch dimmers and line cord dimmers. Check desk lamps and anything else that uses electronic control, including fans. If you suspect a dimmer/RFI problem, either unplug the device, or totally disconnect power from that circuit and see if it stops. Turning the dimmer switch to off may not be sufficient!

Depending on how the RFI is received, you might try re-routing the DSL line, making certain the run is on a twisted pair like Category 5 cable, or you might have to replace or eliminate the noisy dimmer. For more information on how to track down RFI, see this "How can I detect interference with an AM Radio?".

Lastly, if you suddenly start having sync or throughput problems and you suspect RFI might be the cause, check and double check for anything around the home that has changed. Don't just assume that because you have not changed your computer setup that nothing is different. Did you have guests that visited and used the dimmer lights in a little used guest room? Did you just fix the attic fan? Did you install a motion sensor light on the garage? Did you get a new electric grill? It can be using triacs to control the heat. Sometimes RFI is hard to spot, but it can be solved.

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by Riss_Centaur See Profile edited by lev See Profile
last modified: 2006-03-30 15:25:51