The word is officially out—inefficient light sources are not cool. Pardon the pun, but we’re rapidly becoming aware of the impact our energy consumption has on the environment. Lighting applications consume a large portion of our overall energy consumption. According to the U.S. Department of Energy, lighting consumers 12% of residential energy and 25% of commercial energy. So, there’s certainly a significant amount of energy savings to be made by using lighting technology that’s more efficient.
The incandescent bulb has hardly changed since it was initially designed, and it has an efficacy of approximately 8 lumens per watt. Efficacy is a measurement of efficiency used by lighting gurus that specifies how many lumens of light output are produced for each watt of input power. Approximately 95% of the power put into an incandescent bulb creates heat, not light. Alternatives to the incandescent bulb, on the other hand, can easily provide efficacy values that are two to 10 times higher.
I’m not trying to pick on the incandescent bulb. If efficacy were the only important quality of a light source, incandescent bulbs would have long since vanished. Other parameters such as lifetime, durability, and quality of light are important, depending on the type of lighting application.
For optimal efficiency, all lighting technologies can benefit from switch-mode power-supply (SMPS) systems. Furthermore, intelligent control can be applied to any lighting technology to minimize energy loss through active conservation. Therefore, intelligent embedded-control systems that include SMPS-control features are a necessary component for developing energy-efficient lighting applications.
“Dim the Lights, Please”
Incandescent bulbs have remained desirable for their high color-rendering index (CRI). The CRI of a light source is a measure of its ability to faithfully reproduce the colors of an object that’s illuminated by the source. A monochromatic light source would have a CRI of 0, since only one color can be reproduced. Incandescent bulbs have a CRI very near 100, the maximum possible value.
It’s obvious that we have grown accustomed to the warm, pleasing light that incandescent bulbs provide in our homes. Incandescent technology is also popular in retail lighting applications, where it increases the appeal of products on display.
Adding an embedded processor to a lighting application doesn’t have to be complex (). Simply dimming an incandescent bulb can save lots of energy. And your mother was right—you could even turn it off when you don’t need it! The circuit in uses a six-pin PIC10F200 MCU to control a triac circuit. The triac controls the light intensity by controlling the amount of conduction time in each half-cycle of the ac-input voltage. In effect, the triac performs a pulse-width-modulation (PWM) function on the incoming ac voltage. Waiting longer before turning on the triac at the start of the ac cycle reduces light intensity ().
Two I/O pins are required to control the triac. The MCU monitors a sample of the ac-line voltage on an input pin to obtain zero-crossing information. It can then use the zero-crossing information to implement a variable triac firing delay.
Some engineers might say they don’t need an MCU to control a triac, since they can do that with a simple RC-delay circuit. However, the MCU offers some advantages. A triac requires a certain amount of gate-bias current to enable current to flow. Also, a triac has a minimum holding-current specification. When the amount of current flowing through the triac exceeds the holding current, the gate bias can be removed and the triac will continue to conduct.
What this means is that the triac can be energized with just a short pulse on the gate when using a MCU. Therefore, the bias circuit will have a very low current when averaged over each ac cycle. This makes it possible to use smaller and less expensive bias-circuit components. The MCU will need a 5-V power supply as well, but it turns out that a very inexpensive resistor and Zener-diode circuit can be employed, since the MCU draws less than 500-µA average current. Two 11k, 1/8-W resistors are used in this example application to generate the MCU bias supply.
Now that you have an MCU in the circuit, you can add additional functions, including remote control, motion sensing, and timing-related functions. In addition, the dimming control can be made linear. Because the ac voltage has a sinusoidal profile, you won’t get a linear relationship between the triac firing delay and light intensity. This can be easily fixed using a lookup table to translate the requested lamp intensity into an appropriate firing angle.
If you want to control the circuit with a photocell or IR sensor, it’s best to power these devices using an I/O pin on the MCU. This way, the sensing devices can be enabled only when required to conserve power drawn from the 5-V bias circuit.