Posted by Cary Chin on April 22, 2009
Cary Chin (CC): Welcome everyone to the inaugural edition of our low-energy blog, Absolute Power! I’m Cary Chin, a Synopsys low power engineer and long-time computer nerd (Apple II through MacBook Pro, and a few PCs along the way) along with fellow Synopsoid engineer, Darin Hauer, a techno-geek in good standing (Apple IIc owner now sporting a home-built multimedia PC ), and we’re here to take an interesting and entertaining look at Low Power design, tools, methodology, and requirements.
Darin Hauer (DH): Hmmm. That’s entertaining? Wouldn’t getting a root canal be more entertaining than babbling on about low power?
CC: But power has become such a big issue in electronics design in recent years that the discussion itself is interesting. And most people, even many engineers, don’t have a good basic understanding of power.
DH: you mean, you want to talk about how power is measured in units of watts, and how one watt is by definition one Newton meter per second (or one Joule per second), which makes power a measure of energy per unit time. Electrically, power is the product of voltage (volts) and current (amperes).
CC: Exactly right.
DH: So I guess the next logical question to ask is: How many volts, applied across my temples, would produce enough amperes to have the same impact as reading your next line?
CC: OK, OK. Let’s look at this from a more “entertaining” perspective. I’ve got an AM radio that can run for days or even weeks on a single AA battery. Why can’t I power my iPhone off of the same AA battery?
DH: Well, a typical alkaline AA battery stores approximately 3 watt hours of energy, so ignoring losses in conversion, and assuming an average power consumption of 1 watt for an iPhone 3G in relatively high power operation (watching YouTube videos or surfing the web), that’s around 3 hours. Of course, your mileage will vary greatly depending on your application..
CC: Now that’s downright interesting! And how about jogging? If I converted the energy I use to jog for 30 minutes on a treadmill – how much talk time would I get?
DH: That depends on how much you weigh…but on average a jogger expends a little over 100 calories (technically kilocalories – used from now on as simply “calories”) per mile. So jogging at a pretty reasonable 6 mph for 30 minutes would burn about 300 calories of energy. For me, since I am not a reasonable jogger let’s say 5 mph for 30 minutes which amounts to 250 calories of energy. One calorie (really kcal) is about the same as 1.16 watt hours in energy terms (the conversion exercise is left to the reader), so that 30 minutes of jogging for most joggers could theoretically power your iPhone for almost 350 hours!! For me, on the other hand you can reduce that to around 290 hours.
CC: Wow. That’s either a testament to the inefficiency of mechanical work, or the incredible amount of work done by today’s electronic devices with very little energy! No wonder the U.S. Postal Service is having problems.
DH: Now here’s what really sucks. That same 300 calories that you just spent ½ an hour to work off on your treadmill can easily be undone by eating just one average-sized jelly donut!! Don’t ya’ hate that!
CC: Yikes! Keep that jelly donut away from me! On second thought, if I could hook it up to my iPhone…
DH: Actually what you want to hook to the iPhone is a twinkie – it holds a charge for at least 1000 years!
DH & CC: Well folks, more educational and fun power-related tidbits coming in our next posting. Until then, “Corrupt absolutely, read Absolute Power!”