The Utility of Dimming

TRIAC dimming isn't all that. It costs us a large amount of power, and the situation only gets worse as more constant-current LEDs are introduced.

The conventional wisdom says that CFLs failed in the marketplace because they weren't dimmable at first (well, that and their color wasn't any good). Despite the energy savings offered in many other CFL bulb applications, where TRIACs and electronic trailing-edge dimmers were not being used, the inability to dim became a rallying cry. This, despite the fact that there are about 2 million dimmers worldwide among tens of billions of A-lamps -- TRIAC dimming is needed in fewer than 0.1% of A-lamp applications.

The LED industry was determined not to follow in the CFL's footsteps. A sort of dimming snobbery has emerged, where a non-dimmable bulb is regarded as inferior. It is far from it.

Upfront cost

Dimmable CFL bulbs will set you back about $10 and use 14 W of power. This pricing is rarely used as a baseline of comparison with the dimmable 800-lumen LED bulbs being decried as too expensive versus CFLs. The LED bulbs now also cost less than $10.

Dimmable LEDs are now at parity with dimmable CFLs in upfront cost, and they come out at a third of the cost over their lifetime in home or office.

Australia's Hazelwood 1.6GW brown coal power plant.

Energy cost

TRIAC dimmability does carry a price, not only in component cost but also in power use. The incandescent was always on and had a negative temperature coefficient of resistance. This meant that a resistor could be used to keep the RC circuit in the TRIAC dimmer discharged by trickling 20 or so milliamps through it (do the math, and that's 2.4 W with the light off), and it had sufficient current flow at 0.5 A RMS to hold the TRIAC on. Beauty.

LEDs and CFLs have to emulate these trickle and hold currents, increasing power and decreasing wall-plug efficacy for 19,998,000 sockets to accommodate 2 million. That's 20 gigaWatts of added power worldwide -- or 10-40 power plants -- for dimming snobbery.

Most LED bulbs' drivers are designed to supply constant current to the LED, which also means constant power draw -- about 8-12 W for an 800-lumen bulb, depending on the design and whether it's dimmable. The substantially lower power draw, whether it's a CFL or LED, means a trickle and a hold current need to be introduced to prevent flickering by keeping the TRIAC happy, usually costing 1-3 watts of additional power. An LED driver typically relies on the phase cut of a TRIAC to perform a duty-cycle-based dimming function, where an incandescent functions on both duty cycle and applied voltage.

If we have an LED string voltage of, say, 20 V, that means that, at just above the string voltage (say, 24 V), we are delivering 0.5 amperes (12 W) to the LED lamp. Compare that to the 60W incandescent (calculating the hot resistance as V²/P or 120²/60 or about 240 ohms). At 24 V, it would draw about 100 mA, or five times less grid current than an LED.

This LED behavior at lower voltages is a nightmare for power generation. At lower voltages, five times the current needs to be sourced by the grid, and, worse, 25 times the energy needs to be delivered. LED bulbs draw constant power, so lowering the grid voltage by 30% increases the current draw by 40%. Our 100mA LED bulb now draws 140 mA at 85 V. Add in the trickle and hold currents for a dimmable bulb, and we are closer to 175 mA. Compare this to the inherently dimmable incandescent, where lowering the voltage by 30% reduced the current by 30% to about 350 mA, halving the power consumption.

Line-voltage-based load shedding with a constant-power LED design cannot be done, and the LED really saves only about half the power of an incandescent (versus the 85% claimed) at maximum load shed, though at voltages below about 60 V, the LED actually draws more current and instantaneous power from the grid than an incandescent.

To mitigate the shortcomings of LEDs in terms of load shedding performance and dimming curve emulation of an incandescent, the folks at NliteN Inc. (disclosure: I have a financial interest in this company), creators of the first flat LED A19 lamp, use a patent-pending technique that differs significantly from the paradigm used in conventional driver design. Conventional LED drivers hold LED current constant, resulting in constant power draw -- even in the presence of power-factor improving boost stages. In the NliteN IP, the ratio of line voltage to LED current is held constant, which presents itself as a fixed resistor. The LED intensity is modulated by a TRIAC's phase cut and voltage waveform, and utility load shedding is made possible, where dropping the applied voltage by 30% reduces the power drawn by these LED lighting loads by half.

With 20 GW of additional power being consumed to accommodate TRIAC dimming versus the inability of LED lamps to shed 100 GW of peak grid lighting demand, why are we so crazed about wall dimming?

Related post:

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