Through most of my nearly four decades in the power quality industry, one oft-quoted statistic is that 70 percent of power quality problems originate within a facility, despite the prevalent belief that the utility company or the electricity supplier is usually the culprit.
Fast-forward to last decade, and it seems there is a PQ problem that clearly originates from within the facility, and it is somewhat intentional by the rapid deployment of energy-saving devices that aren’t fully compatible with the electrical world they are to operate in—namely, LED lamps on dimmer circuits. While such a statement will raise the ire of lamp manufacturers, there is enough evidence from a quick internet search to confirm the scale of the problem. Of course, there is the disclaimer that not all manufacturers of LED lamps and dimmers exhibit this problem, but it is real and self-induced.
Until about a decade ago, most dimmer switches used the forward triode for AC (TRIAC) method for dimming the lights, which were predominately incandescent lights consisting of a predominately resistive-type load. The TRIAC is a semiconductor device that has two silicon-controlled rectifiers (SCRs) connected in parallel in reverse directions with a common gate or control. Each SCR conducts in one direction of current flow when told to turn on by the proper gate voltage applied. It turns off when the current flowing through it goes to zero, which will happen twice in each AC voltage cycle.
To get the lights at half-power level, the control signal turns the SCRs on half the time. In this type of dimmer, the turn-on is delayed from the start of a normal cycle (0 degrees) until sometime later, such as 75 degrees for reduced power on the positive half cycle, and 255 degrees on the other half-cycle (see the graph above). Incandescent lights didn’t mind this treatment because their thermal mass (which made them so inefficient) was happy to ride through and smooth the light output proportionally.
Along comes the LED lamp. It is not a “dumb” resistive load; rather, it is a DC-operated light source. LEDs require a constant-current source to maintain a constant light output. What goes in, comes out; there is no thermal smoothing. Directly connecting an LED light source to AC would have it changing its light output with the power frequency, or 60 times per cycle in North America. While this rate is above the human perception of flicker, the LEDs also can only tolerate a small voltage level, not the 170-volt (V) peak on a typical 120V AC circuit. So, in goes some electronics, first to turn the AC into DC, then lower the voltage level to a regulated 3–5V DC, typically. The result is a stable light output at 120V.
But, what happens if you want to dim the lights, as is often the case in restaurants, theaters, residential dwellings and other establishments? The regulated supply keeps the power source to the LEDs constant until the input voltage gets too low, then it decays rapidly and eventually goes too low and things go dark. Many LED lamps operate in this manner.
What about the “dimmable” ones? Inside the lamp is additional circuitry to determine that there is a “phasing” change, meaning the voltage is not turned on for the entire cycle, and to adjust the light output of the LEDs inside the lamp by proportionally changing the current through them. While this seems reasonable, the circuitry that determines this can often have problems determining exactly what light output is requested. The result is a nonlinear relationship, and one that can even modulate at certain points within the dimmer setting range. The result is light flicker from the dimmed LED lamp, even though the voltage provided by the utility company is perfectly sinusoidal.
A newer type of dimmer was introduced to help mitigate this phenomenon: the reverse-phase dimmer. Instead of turning on the voltage part way through the sine wave curve, it starts up the voltage normally on but then turns it off proportionally to the dimmer setting. This seems to correct the flickering of some LED lamps because it is said to be “less confusing” to the control circuitry within the lamp.
In response to these issues, the IEEE formed a working group that resulted in IEEE Standard 1789 2015, Recommending Practices for Modulating Current in High Brightness LEDs for Mitigating Health Risks to Viewers, to inform and provide guidance and recommendations for the lighting industry, ANSI/NEMA, IEC, EnergyStar and other standards groups about the emerging concern of flicker in LED lighting. The standard produced pushback from some LED lamp manufacturers and NEMA, but it did succeed in raising the awareness and acceptance that it is a significant problem.
About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.