Demonstrating Green Technology with Apples, Oranges, Lemons, and Limes

Jim Henson's creation, Kermit the Frog says, "It's not easy being green." We concur that saving energy is difficult, but very necessary. Smart circuit designers and progressive companies are meeting consumer's expectations in this aspect. And the smallest details are critical: power efficiency is one measured in microamperes (µAs), one-millionth of an ampere. For comparison, a 60W incandescent light bulb draws 0.5A. That is 500,000µA.

Why is it necessary to measure so precisely? Because it is the sum of all currents that count and like any budget, one must reduce every cost, no matter how small. Obviously in a battery-powered device, customers are sensitive about battery life. Not so obviously, plug-in appliances also have a cost associated with just being plugged in. When a device is "off", but displays a power indicator while waiting for a remote command, button, or timer, it is consuming standby power.

What is the cost of standby power? That 60W bulb can cost $14.65* a month if it runs 24/7. An appliance that draws 1W in standby power can cost $0.25 without doing any practical work. Walk around a typical home and count the appliances, TVs, radios, stereos, computers, garage-door openers, microwave ovens, washing machines, dryers, forced air heaters, and lawn sprinklers. Then add the battery chargers. You can easily have 20 devices on standby power, and most draw more than just 1W.

That is the good news. The bad news is that a cable TV box can draw 80W while in operation and 79W while in standby (costing $19.53 and $19.30, respectively). A satellite DVR box can consume 120W while in operation or standby, so just add $29.30 to your monthly subscription rate.

This is relevant not only as a monthly cost; as a citizen of Earth, we must be conscious of our environmental impact. As circuit designers, we can choose carefully, and thankfully low-energy parts are not necessarily more expensive. In fact, because newer parts are made with smaller geometry IC processes, they tend to be more efficient. Furthermore, Analog Devices circuit designers have long taken the lead in designing with energy efficiency in mind.

Table 1 illustrates ICs that can be used to reduce power costs in household appliances, computers, and set-top boxes. Today both designers and consumers have choices; we all can influence our Earth's future by the little things we do each day.

What does this have to do with apples, oranges, lemon, and limes? And how do these fruit demonstrate green technology? For a story to attract media attention, it needs a catchy headline. To demonstrate how little current is used by Analog Devices ICs, our parts can be powered by batteries made from apples, oranges, lemon, and limes. Figure 1 shows such a circuit.

This 3.6V battery is made of 4 lemon cells. Each cell produces 0.9V at approximately 100µA. The MAX6029 voltage reference produces a precise 2.5V while drawing a maximum of only 5.5µA. Figure 1 graphically demonstrates the tiny current draw of an IC.

The battery voltage is primarily determined by the chemistry between the copper and zinc. Many fruits and vegetables will operate as batteries. Apples, oranges, lemons, limes, grapefruit, and potatoes all produce voltages between 0.88V and 0.95V per cell. The color of the battery really makes no difference, but saying we are getting greener by using limes does make cute story.

It is estimated that 10% of household power consumption is lost to standby power.² The Energy Star^® program, first created by the U.S. Environmental Protection Agency and the U.S. Department of Energy, estimates that there are 129 million households spending an average of $2200 each per year on electric power.³ Putting the numbers together, households spend approximately $28.3 billion per year on standby power in the United States. That is a staggering number, considering it just allows us to become coach potatoes! If we just would get up and turn off a switch, we could save that money.

All of a sudden, green sounds pretty good when we realize our household can save $100 per year. Now that power efficiency is personal, we can redouble our environmental saving behavior. The Analog Devices website has live parametric tables that allow us to interactively select parts. We can pay attention to power consumption as we select the appropriate part for our circuit. Though a microampere is a tiny amount, when multiplied by the huge number of people in the world, it does make a difference.

*The residential retail price of electricity varies by location and over time. These prices were accurate for residential customers in Northern California at the time of this application note's posting.

参考电路

¹"What are energy vampires and what can I do about them?" ENERGY STAR (accessed September 27, 2011).

²"Standby Power" in Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/wiki/Standby_power (accessed September 27, 2011).

³"Where Does My Money Go?" ENERGY STAR, https://www.energystar.gov/products/where_does_my_money_go (accessed September 27, 2011).