Microcontrollers provide power control for lighting applications

Tag: Pulse Capacitor

Technical Staff Engineer, Microchip Technology Inc.
The word is officially out – inefficient light sources are not cool! Pardon my poor pun, but we are rapidly becoming aware of the impact that our energy consumption has on the environment. Lighting applications consume a large portion of our overall energy consumption. According to the US Department of Energy, 12 percent of residential energy and 25 percent of commercial energy is consumed by lighting. So, there is a significant amount of energy savings to be made by using more efficient lighting technology.

The incandescent bulb has hardly changed since it was designed and 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 percent of the power put into an incandescent bulb creates heat, not light. There are alternatives to the incandescent bulb that easily provide two to10 times higher efficacy values.

Now, I am 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 upon the type of lighting application. For best efficiency, all lighting technologies can benefit from switch-mode power supply (SMPS) systems. 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.

I am going to show you some examples of how embedded control can be applied to different lighting technologies – even the incandescent light bulb, providing control of SMPS circuits and adding intelligence.

DIM THE LIGHTS, PLEASE

Incandescent bulbs have remained desirable for the high color-rendering index (CRI) that can be obtained. The CRI of a light source is a measure of its ability to faithfully reproduce the colors of an object that is 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, which is the maximum possible value. 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.

The addition of an embedded processor to a lighting application does not have to be complex, as shown in the schematic of Figure 1. A lot of energy can be saved by simply dimming an incandescent bulb. The circuit in Figure 1 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 PWM function on the incoming AC voltage. The light intensity is reduced by waiting for a longer time from the start of the AC cycle to turn on the Triac, as shown in Figure 2.

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.

Now some of you engineers might say, “I don’t need a MCU to control a Triac. I can do that with a simple RC-delay circuit.” However, the MCU offers some advantages here. A Triac requires a certain amount of gate-bias current to get 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 a MCU is used, so the bias circuit will have a very low current when averaged over each AC cycle. Therefore, smaller and less expensive bias-circuit components can be used. The MCU will need a 5V power supply as well, but it turns out that an inexpensive resistor and Zener-diode circuit can be used because the MCU draws less than 500ìA average current. Two 11K, 1/8W resistors are used in this example application to generate the MCU bias supply.

Now that you have a MCU in the circuit, you can add additional functions, including remote control, motion sensing, and timing-related functions. Additionally, the dimming control can be made linear. Since the AC voltage has a sinusoidal profile, you will not 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 using a photocell or IR sensor, it is best to power these devices using an I/O pin on the MCU. In this way, the sensing devices can be enabled only when required, to conserve power drawn from the 5V bias circuit.