Tag: Efficiency

Categories
Analog Electronics

What Is A Power Budget?

Boy is it hot in here or is it just me? Why’s that gizmo over there giving off so much heat?

Power budgets are a necessity these days. Due to increasing regulation, we’re seeing devices that must comply with efficiency limits in their power conversion (using a switching power supply or otherwise).

So what is a power budget? Much like a budget you might have for your personal finances, a power budget shows where all the possible power will be used by a device to by breaking it down into components and categories. In some situations, you might be told up front that you will have 3W available to run your design. However, sometimes as designers we start by calculating the total power a system needs and then taking actions such as replacing parts or redesigning circuits to cut back power to an acceptable level. So why might someone want to do a power budget from day one?

  1. Power availability — While you might have more power today, it doesn’t mean you’ll have it tomorrow. Designing a system for 3 W power consumption may be acceptable now, but designing a lower power system may meet future regulations. And the trends in the industry point in that direction.
  2. Battery Life — If your device is running off a battery, you likely do not have a choice whether you are doing a power budget. You want to maximize the life of your device on a single charge (assuming it is using rechargeable batteries) and your customers want the same. Just a few weeks ago I was complaining publicly on The Amp Hour about my new device with poor battery performance. Doing a power budget will point to the components consuming the most power so you can later optimize for longer battery life (hopefully this was a design constraint from the beginning).
  3. Heat generation — Heat is an unfortunate side effect of working with electronics. However,  it also has a 3 direct effects on your product and how it is used.
    1. User discomfort — No one likes having a hot laptop sitting in their lap. Nor a cell phone that is uncomfortable to hold.
    2. Circuit robustness — An often quoted specification of an op amp is the voltage offset drift. This sensitivity to temperature can have dire effects in systems that rely on analog accuracy. However temperature changes can create conditions that are unfavorable and could even cause device failure (such as thermal runaway). The heat of the whole system can end up affecting individual components as the nominal temperature inside your device rises.
    3. Product lifetime — The lifetime of a product can be drastically reduced by higher than normal temperatures inside the device. Extreme temperatures can begin to dry out capacitors and cause others to fail catastrophically. While it is possible for systems to fail in a drastic manner, the more likely outcome is a product that does not last for its specified lifetime. An example might be a TV that has a less vibrant LCD after 5 years due to excessive heat and component drift and fatigue. If the product was designed to have lower heat, the product would have lasted longer. For more on how to design and prevent early failures, check out Dave’s video blog about heatsink design.
  4. Cost (sizing) — More power means you need larger components. Other than the obvious requirement of needing more space (duh), it often correlates to higher cost components. Not only will you need larger packages for your components such as op amps and comparators in order to better dissipate heat. You’ll also need a larger power supply with more reliability. If a 5W and 20W power supply with 12V output are compared, the 5W supply has smaller magnetics and less wiring because there is less total current that needs to pass through.

So let’s look at an example power budget (click for a larger version):

As you can see, not much more is required than your datasheets and a spreadsheet type program. Even simpler is a piece of paper but I prefer the built in math functions of the spreadsheet program. The first two columns (A&B) are simply identifiers to allow you to recognize which components correspond to which set of data. The next two columns (C&D) determine the multiplicative factor. If you have 5 components that contain 4 op amps per, then that will consume 20x the power of a device that has the same supply current needs but only one op amp per and there is only one on the board.  The next two columns (E&F) show how much current each individual component contributes and then the sum of all the components of that type contribute. Note that this parameter on a data sheet would be listed as “supply current” or “active current”. The “quiescient” number is when the device is in a resting state and will likely be much less than the active number (and not relevant for this example). Finally, the supply voltage is listed (in column G) to calculate power (using the formula P=I*V) which is listed in column I per device. All of these contributors are summed, an efficiency is estimated (I assumed a poor efficiency linear type supply) and the total power required input to the device is given. Further calculations could result from much of this initial data.

I would be remiss without mentioning something about power budgets: you’re still going to guess about certain things. In fact it will be many different things. You might not have perfect data about your components. You might not completely trust the “typical spec” of one of your components. This is the point where you design in a margin of error. However, just like many other aspects of engineering, this is where tradeoffs come into play. You might want to design in 4 times more power capability than you calculate (to feel safe), but there are cost and spec requirements to consider. You will have to determine how confident you are in your design and how many resources you have available to your design. In the above example where the 5V parts require 408mA from the supply (~2W), I might over spec the part by designing in a part that is capable of supplying 600mA. The (50%) margin of error allows for future expansion (might need to solder in an extra part or two) and also gives a cushion if anything was miscalculated. In some situations this 50% might be too much (think a very low-cost, high volume design) or might be too little (think a military, high reliability design). It all depends on the situation and requirements.

Power budgets can be very powerful depending on the amount of time and effort you put into them. Otherwise they are educated guesses which may or may not be helpful to your project; how helpful they are might also depend on where you are in the design cycle. As stated before, these budgets are more and more of a necessity in a world more power conscious and with devices that continue to shrink. Your customers will expect longer battery life and your products to have yet more features. Teach yourself how to do power budgets now and it will pay dividends for you in the future.

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Categories
Engineering House Life

Energy Consumption Improvements in the House

Now that we’ve made it through a bit of the summer, I think we really need to focus on something important.

Winter is on the way!

If I recall correctly, my home is still not quite up to snuff in terms of how much money I’d like it to cost me to heat and energize my house. I realize my home will never be 100% efficient. I also realize I’m not going to plop down money to get to 100% because that’s silly and sort of impossible. Instead, I have to pick my battles with my own castle and decide what will produce the biggest returns.

  1. Insulate
    • This is the number one project for the late summer/fall for me. I have very poor insulation in my upper floor. In fact, when we bought our house we could actually see the snow melting on the roof where the heat was escaping. Talk about watching your money fly out of your pocket! Check with a local contractor to see how much insulation you’ll need to really see some energy savings. Also, don’t forget the federal credit when you’re finally cutting that check…you could get up to a $1500 tax rebate.
    • But wait…there’s more! Don’t think that whole house insulation is the only thing to focus on. Oftentimes, the biggest culprits of letting expensive, hot air out (or in really) are the small cracks around windows and doors. Spending 40 bucks on some expandable foam, a tube of caulk, a water heater blanket and some new winterizing doorstops can go a long way.
  2. Turn it off
    • There’s no denying that the most effective way to cut energy consumption is by turning devices and lighting off when not using it. This idea, coupled with using energy when it is cheapest and most abundant, is the crux of the “smart grid” idea. For devices that aren’t managed by a central management unit such as the one in the article, most devices now have a “sleep” mode that has reduced processing instructions; the device periodically “wakes up” to check to see if anyone has requested its services and if not it’s back to sleep. Devices with low quiescent current (or the current while not doing much of anything) can show large energy consumption savings.
  3. Buy/Replace
    • Even though people probably don’t relish the idea of throwing away (or hopefully recycling) their old appliances, this is sometimes the best option. Your old freezer in the basement might be saving you trips to the grocery store (good) but might be doing it at the environment’s and your expense by increasing your electricity bill(bad). Pick up a Kill-a-Watt meter to see how much power your old junker is really pulling out of the grid; if it’s considerable, think about pulling the plug.
  4. Inspect your ductwork
    • Oy, with the not-electronics already! I know, it’s not glamorous, but it’s often the simple things in houses that can really cost you. This is a big weakness in my house and something I will have to address before this winter. Back in the 50s and 60s they must have thought it fashionable (or at least cheap) to attach boards to the underside of the crossbeams of my floors. As such, the air actually being pulled down through the cold air returns is minimal, most of the air is actually pulled down through the floorboards and back into the cold air intake of my furnace. It’s a good time in the summer to check out where your ducts are leaking air so that you can save big dollars in the winter months.
  5. Junk Water Dump
    • I saw an article a while back about the waste water from your tub also wasting energy. Think about how much natural gas/electricity it takes to get your water heater to temperature. Now think about how warm the water still is when it’s washed away all the nasty off your body. Finally, think about how cold the tap water can e in the winter. If you have a reservoir underneath your tub collecting warm wastewater and then coil the incoming cold water through it on the way to the water heater, you could possibly retain some of that usually wasted energy. Check out the link and check to see if you ever have that kind of option the way your house is set up. This could be the same for the dishwasher and the washing machine while the water is on “warm”.

I know you’ll see a lot of this information elsewhere but I’d feel silly not to encourage readers here to try it out for this coming winter. As I said above, there are many different monetary incentives to do so, both in rebates and power savings. I plan on getting the jump on these updates now so I can take advantage of the energy savings for cooling my house as well as heating it later on. If I find out about or come up with any other ways to save money and energy in the future, I’ll be sure to post them here.

What about you? Have you decided to do any updates to your home (energy-wise) while the weather is still nice? Do you have any tips you’d like to share? Just leave them in the comments!

Categories
Analog Electronics Renewable Energy

Can DC power an entire home?

AC power vs. DC power: Both are necessary in our everyday lives and switching between the two causes a great deal of strife in electronics. Why do we need both?

As some of you may or may not know, there was a long standing battle between the two types of power raging back in the 1880s between two giants. The proponents of this war knew that whoever won would determine the future of the power distribution in the United States and possibly the world. In the first corner was Thomas Edison and his company that would eventually become General Electric; Edison wanted the world to run on DC. In the other corner was Westinghouse Corporation, funded by George Westinghouse and led (intellectually) by Nikola Tesla. Westinghouse represented AC power and would be the eventual winner. You can read more about the battle HERE, but I thought it would be interesting to point out that this battle eventually became a political one. Edison even started fighting dirty, secretly funding the invention and use of the first electric chair powered by AC, in order to give some bad press.

AC of course won out over DC as the power distribution of choice, mainly because of the ability to have large generators in a central location and then transmit the power efficiently over power lines to homes and businesses. DC would have required local generators on every street or even every home, which was not possible nor economically viable at the time.

Hang on a second though…a DC generator on every home…sounds familiar…where have I heard about something like this before? Oh right, solar power. However, even more interesting than the fact that solar power produces DC power output is that any kind of storage will have to be in DC. So THAT means if you have any kind of renewable energy resource on your premises (wind, geothermal, any kind of generator which will have an AC output) and it’s not continually supplying power to your home, you will likely need to store it somewhere (assuming you are not selling power back to the power company, which is the case in some areas still and a must in the remote areas). Further, barring any possibility of storing AC power (a huge inductor?), you will need to store that power in DC. So let’s look at a theoretical wind turbine on a theoretical property:

The wind blows –> wind turbine spins –> motor in turbine creates AC power –> AC converted to DC –> DC stored in a battery –> DC converted back to AC when needed –> AC powers devices in a home –> (possibly) AC converted back to DC for use in consumer devices

That’s a lot of steps! Not only are there a multitude of steps to convert wind into air conditioning (heh, the electrical way…the natural way is opening the window), there are lots of places that you will be losing energy to inefficiencies. These occur in the power generation (motors have friction), the storage in the batteries (heat and losses due to chemical impurities in the wet cells), the AC to DC conversion and the DC to AC conversion (both processes lose energy to heat in the electronics). All told, it’s not hard to see why this is not the preferred method of powering ones’ home.

So now the real question: Can we take out some of these steps?

Other articles on this site will deal with improving efficiencies of each of these steps, but the simplest method for improving overall efficiency would be to remove one or more of those steps. The way I see it, one of these ways would be to convert a power scheme in a house. Let’s look at all the ways a DC power system in a house could be beneficial or detrimental to ones’ living situation:

Concerns about DC wall power

  1. Many devices have different voltages
    • This would be a definite issue. Have you ever had to power a guitar pedal board? Random question perhaps, but if you saw what the power strip looks like, you’d catch my drift. Every one of those little electronic devices is too small for a transformer, so they all have AC-DC converters which can power the device with a different required voltage. Now take this idea and expand it to all the doo-dads in your house. I would be willing to guess that there are at LEAST 5 different required DC voltages for all of the normal devices in a home.
  2. Converting devices
    • Conversions would be required from DC->DC instead of AC->DC. A possible solution would be to set up the wall sockets to have selectable DC output (perhaps the home runs on 100V DC and each socket can convert this down to 24V, 12V, 5V, 3V).
  3. Selling power back to the power supply company
    • One of the most popular notions in renewable energy today is the idea of selling your excess power back to the power company, hopefully at a decent rate. Then when your device is not outputting power, you simply switch to grid power and start buying it from the power company. This is great because it does not require battery systems. And while this exercise excludes that option (for people living in the middle of nowhere or with unaccommodating power companies), it would be nice to sell any excess power back to make a small profit.
  4. Economies of Scale
    • This is possibly one of the biggest problems that an all DC power system would face: No one does it yet! All parts would have to be custom made and you couldn’t just call an electrician to come out and fix your stuff.
    • This also means that you would have a tough time buying consumer goods. Nearly every device has an AC plug, because that’s what everybody has! Not to mention all of the internal components for AC conversion and occasional power filtering (some devices need very clean DC power). Let’s just say you couldn’t go buy a TV and plug it in…
    • Government regulation would also limit any kind of large scale implementation of DC power sockets. It is almost guaranteed that it would require government certifications on many levels to allow manufacturing large enough quantities to bring the cost down for Mr. John Q Everyman.
  5. Conversion to AC for certain devices
    • Motors are the first kind that come to mind. This is basically how Nikola Tesla got started onto AC, proving that it is much more efficient when using AC than DC AND that these motors do not rely on voltage level (DC motors’ speed can be controlled by the voltage applied). This would mean you would either have to convert your DC back to AC to run the vacuum cleaner or you would have to make sure that your DC could supply constant DC and the whopping currents that those kinds of devices use.
  6. Step up/down transforming
    • You know those big garbage can looking things that are attached to power line poles? Those are changing the ridiculously high voltages in the power lines (done for transmission efficiency) down to something that we can use in our houses. Further, these are VERY high efficiency devices. For power in general, you really can’t beat AC-AC conversion; the system proposed here would have to use transistors (note: not transformers) which will have some amount of heat loss associated with them. So even though we wouldn’t be using the AC power from the power company, we would be losing a critical tool in the electrician/electrical engineers’ arsenal, the transformer.
  7. Leakage currents and phantom power consumption
    • No transistor is perfect, they all let just a little bit of current through. The more components in a system or the higher voltage you run at, the more leakage you will tend to have (Ever wonder why electronic devices run out of batteries eventually, even if you don’t use them for a long time?). This would apply to any DC system too and when you don’t have the lights on or anything running, there’s still a chance that the power devices are leaking. This will cut into overall efficiency.

Benefits of using DC instead of AC:

  1. Higher efficiencies off of battery power
    • This point was discussed above, but is THE main point of the article and for going to all this trouble. The less you need to convert between AC and DC, the less energy will go to waste. And if you do need an AC power source, the inverter could be much smaller, in order to handle smaller loads or in order to sell power back to the power company (once the battery is fully charged)
  2. LED Lighting
    • Currently any LED fixture installed in homes requires an AC-DC converter. Using a DC wiring system throughout a home would allow easy installation of LED fixtures and elements (the LEDs themselves)
  3. No 60 Hz hum
    • I’m sure most of you know what this sounds like from a faulty light switch, an older device with poor power supplies or even by sticking a fork in the wall. The native frequency of power coming out of the wall is 60Hz in the US, but varies by region. Either way, this is something that I’ve had to deal with at my job and that all electronics designs have to deal with. With an all DC system there would be other issues such as power filtering and voltage stability… no hum though!
  4. Shrinking power supplies
    • As devices continue to get smaller, the power supplies are reaching a lower limit. 1.8V is currently the lower end of DC supplies for microchips. This allows for less power consumption, as is governed by the formula P = V² * f * C (where P = power, V = voltage, F = frequency and C = capacitance). Have you ever noticed how they stopped increasing the frequency of microchips past a certain point (~3.5 GHz)? Yeah, it was because they started getting so hot you could fry eggs on the processors. Plus mobile processors became much more prevalent. As more and more devices go towards these lower voltages, there will be less need for conversion (or alternately, more need for AC-DC converters if wall power remains as AC).

So the final question comes back to that posed by the giants of the 19th century: AC or DC power? Well, really the answer will be both, as history has shown. Perhaps over time we’ll see a shift back towards DC power as devices continue to shrink and manufacturers don’t want to include bulky transformers or as people hopefully begin producing their own power at home; but one thing that is for certain is this battle will continue raging for a long time and hopefully we’ll help renewable energy find it’s place.

I welcome any and all comments on this idea and if you know of something being developed similarly, please let me know!

“If I have been able to see further than others, it is because I have stood on the shoulders of giants.” ~Sir Isaac Newton