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AC Rectification

It’s back to usual this week for my article, because I thought of a good topic. I’ve talked many times about AC power and how and why we use it, but many things don’t use AC power. In fact, they can’t. These are things we use every single day. Virtually all digital electronics cannot use AC power directly. It needs to be converted into DC power. Remember that AC means the current is alternating and DC means the current is flowing in one constant direction.

Any time you convert electricity from one type to another, or one voltage to another, there will be losses. Nothing is free. Every time you change something about electricity, you have to pay a tax. The goal in electronics design is to minimize those losses. AC to DC conversion is one of the simplest and most efficient conversions we do on a regular basis. DC to AC conversion is also possible (accomplished by inverters). Most of these processes are on the order of 95% efficient or greater. So how do we convert AC to DC?

First off, the process of converting AC to DC is called rectifying. Devices that accomplish this task are often called rectifiers. A very common rectifier design is called a bridge rectifier. To understand how they work, we have to recall that AC is a wave. Sometimes it’s voltage value will be positive, other times it will be negative. This doesn’t work for DC since the voltage has to always be positive or always be negative. Rectifiers take the negative voltage swing of the wave and flip it up top so that it’s positive. Now instead of a sine waveform, the output of the circuit will look like mountains. The voltage will always be positive, but it will still vary considerably, from the peak (approximately 170 volts on grid-level 120VAC) to zero and then back up again.

How is the voltage rectified? In a bridge rectifier a ring of diodes is used. Recall that a diode only allows current to flow through it in one direction. By arranging the diodes in a ring we can create a “draw off” point for the current where the voltage will always be positive. Instead of being allowed to “pull” the current backwards during the negative voltage swing, the diodes redirect it back to the output point. Of course, this only means that the negative voltages get flipped up, creating the mountain-like waveform I mentioned earlier. This is not good for DC power, so how do we fix it?

There are many ways to minimize this issue, but it can’t be fixed perfectly. One way is to use capacitors to buffer the output voltage. Instead of sinking when the mountains go back down to zero, the capacitors prop the voltage up during that period, until the next mountain peak arrives. Depending on the load, this could mean large capacitors to hold the load long enough. Remember also though that AC has a frequency on the order of 50-60Hz. This means there will be around 120 of those mountain peaks every single second, so the capacitors don’t need to hold for too long.

The other issue we have to contend with is how to reduce the voltage from anywhere between 120-240VAC to something DC electronics might expect, like 5VDC. For AC to anything, this is actually very easy; we use transformers. A transformer has two sets of wire windings, a primary winding and a secondary winding. The primary winding contains the input current, usually fluctuating AC. This fluctuating current induces a magnetic field that also fluctuates in a metal core that extends from the primary winding to the secondary winding. The changing magnetic field in the core induces a new current in the secondary winding. The induced current will necessarily be greater than the input current if the windings are configured to reduce voltage. The overall power has to remain the same. So if the input were 120VAC at 1 amp (120 watts), and the transformer was reducing the voltage by a factor of two, the output would be 60VAC at 2 amps (still 120 watts). The output of a transformer will still be AC, but this is where we would apply rectification.

This method of power conversion is extremely efficient, but as transformers are highly inductive, power companies would hate it if every single device used one. The reason for this is because inductive loads shift power around a lot without actually consuming that much. So the power company has to pay to send the power all the way from their generating station, to you, only to have your device reject almost all of the actual energy in that power. Most homes don’t have solely inductive loads though, so most of the time it’s not an issue. However, certain industrial processes rely heavily on inductive load machinery. In these cases, the power company may request that they burn off a certain amount of power in dump loads (basically huge resistors) in order to keep the grid functioning normally.

So there you have it. How we take that high voltage raw AC power from the wall and tame it to safely power all of your devices.

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Heading Into The Library Stacks

I remember way back when, that Grace and I would go to the Chicago Public library to “do our homework”. When we actually did some homework the reference section was my favorite. That and looking at the huge microfiche library of old copies of the Chicago Tribune.

Did you know that Apple hid a huge reference library in your Mac?

You’re probably used to Mac apps using red underlines to mark misspelled words, but did you know that macOS has long included a fully-featured Dictionary app as well? It provides quick access to definitions and synonyms in the New Oxford American Dictionary and the Oxford American Writer’s Thesaurus, along with definitions of Apple-specific words like AppleCare and MacTCP. But that’s far from all it can do.

First, let’s make sure we’re all on the same page. Launch the Dictionary app from your Applications folder and then type a word or phrase into the Search field. As you type, Dictionary starts looking up words that match what you’ve typed so far—you don’t even have to press Return. If more than one word matches what you’ve typed, click the desired word in the sidebar.

Notice the lozenges below the toolbar, representing the references that Dictionary can consult, and no, your eyes aren’t deceiving you—Dictionary can look things up in Wikipedia if your Mac has an Internet connection. In short, Dictionary gives you instant access to a dictionary, a thesaurus, and an encyclopedia containing over 5.4 million articles in English alone! You can click a reference’s lozenge to limit your search, or select All to scan all of them.

If you want to look up words in another language, or even just British English, Dictionary has you covered, with a long list of other reference works. Choose Dictionary > Preferences and select those you’d like to use. You can drag the selected entries into the order you want their lozenges to appear below the toolbar.

Once you’re in a definition, note that you can copy text for use in other apps—always helpful when wading into grammar and usage arguments on the Internet. More generally, you can click any word in Dictionary’s main pane to look it up instantly. If dictionaries had been this much fun in school, we’d have larger vocabularies! Use the Back and Forward arrow buttons to navigate among your recently looked-up words.

As helpful as the Dictionary app is, you probably don’t want to leave it running all the time. Happily, Apple has provided quite a few shortcut methods for looking up words:

  • Command-Space to invoke Spotlight, and enter your search term.

  • Select a word, and then choose AppName > Services > Look Up in Dictionary to launch Dictionary and search for that word. This trick should work in most apps, but won’t work in all. If the Look Up in Dictionary command doesn’t appear, make sure it’s enabled in System Preferences > Keyboard > Shortcuts > Services, in the Searching category.

  • Last but best, hover over a word or phrase with the cursor and either press Command-Control-D or Control-click the word and choose Look Up “word.” If the app supports it, macOS displays a popover with the definition or Wikipedia article. If you have a trackpad, you can also do a force-click or three-finger tap on the selected word—make sure the “Look up & data detectors” checkbox is selected in System Preferences > Trackpad > Point & Click.

Now that you know how to take full advantage of the reference library that Apple has built into macOS, it’s time to get in touch with your inner logophile (feel free to look that one up).

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Stay Cool

Summer officially started this week. Has anyone checked with Mother Nature? Are we sure she got the memo? Seriously, sometimes I can’t tell where we are with the weather. I don’t really mind the hot weather at all, as long as I can do things like jump in a river, or ride my bike. The warmer days had me thinking about an interesting topic that spans a bit more than just electricity and magnetism, but I thought it might be interesting anyway. How do we cool things down?

I’ve always been fascinated by this actually. Even as a child, I could understand, heating things up was easy. Fire is easy. Warming myself up in winter with a heavier jacket was easy. Cooling things down is always trickier, and there are several ways to do it. To get a better understanding of how we cool things down, we have to start at the very basics. What is temperature? Actually, what we call temperature is a measure of motion, specifically of atoms and particles. They’re not moving from A to B, but rather, vibrating in place, or in the case of gases, moving around haphazardly. The faster they move, the higher the temperature of the substance. When all motion stops, the substance if brutally cold. So cold, in fact, that it has a special name: absolute zero. Absolute zero is approximately -273.15 Celsius or -459.67 Fahrenheit. We even have a temperature scale that starts at absolute zero, called the Kelvin scale. 0 Kelvin is absolute zero. The motion of the atoms or particles also gives off something called black body radiation. This is simply thermal-spectrum electromagnetic radiation. As such, it’s part of the electromagnetic spectrum I wrote about some time ago. This is how infrared cameras work as well as any thermal imaging camera.

With that background in place, it’ll make more sense when I explain certain cooling methods we commonly use. One of the most efficient and effective cooling methods is gas decompression. This is how nearly all refrigerators, air conditioners and heat pumps work. In any gas, the molecules comprising the gas are spaced pretty far apart. When you compress them into a tinier volume of space, the gas will increase in temperature. Is this because the particles are now hitting each other more often and creating more frictional heating? It would be nice if that’s how it worked, but it’s actually a bit more complicated than that. The simplest way to understand heating of compressed gas is to understand conservation of energy. Energy cannot be created or destroyed. Compressing a gas requires some amount of energy, and that energy has to go somewhere. It ends up going into the gas causing the particles to move faster, which we observe as heat. From a physical standpoint, the gas particles are interacting with the boundaries of their space more frequently as the space containing them shrinks. The most scientifically accurate explanation of the temperature increase is that by reducing the available volume of space, you’re increasing your theoretical knowledge of where each particle is. Instead of being somewhere in a huge volume of space, each particle is now in some much smaller volume. You’re decreasing the entropy of the gas. That knowledge doesn’t come free though. The particles essentially say, “ok, we’ll let you know more about where we are, but in turn we’re going to let you know less about our speeds, because we’re going to move faster.”

Whew…all of that. Are you still with me? Hadley, you haven’t mentioned a single thing about cooling yet! Just heating! Yes, but now all the pieces are available. When we compress a gas, it heats up for the reasons stated above, but we can do something with that heat. We don’t have to let the gas just stay hot. This is what refrigerators do. Once they compress a gas, they pass it through some type of heat sink. This is a device that allows the heat of the gas/fluid inside to dissipate as quickly as possible to the ambient environment. Once this happens, we have a roughly room temperature compressed gas. If we allow the gas to decompress, its temperature will fall…to below room temperature. The particles are now saying, “ok, you’re increasing our volume, and this means you’ll know less about where we are. So to compensate, we’ll slow down a bit so you can at least know how fast we’re going.” Obviously the trick in any of these refrigeration systems is being able to compress the gas a lot as well as being able to efficiently remove the heat from the compressed gas.

This process will work for any gas, including air. In fact, I encounter this phenomenon every time I air down the tires on my bike. The tires are filled with ordinary air, compressed to between 90 and 110 PSI. When initially pressurized, they do heat up, but over time, they cool to ambient temperature. When I rapidly release the pressure, the valve becomes noticeably cold to the touch. In most refrigeration, we don’t use air, we use some kind of refrigerant, like Freon. Freon is the trademark name for any number of different gasses used as refrigerants known as halocarbons. You’ve probably heard of at least some of these by their scientific name, chlorofluorocarbons (abbreviated CFCs). These are the same CFCs that scientists discovered were causing ozone depletion in the 1980s and 1990s, so they aren’t in widespread use anymore. All refrigerants are just special types of gas that have properties that are beneficial to what we’ll be doing with them.

The important thing to remember is that if you’re unable to remove the heat from the compressed gas, and you let it decompress, all it will do is decrease back to roughly room temperature where it started. So it won’t be cool the way we want it. If you want the gas to be very cold, you have to make its starting temperature less. There is actually no limit to this (beyond absolute zero) and this is how we create extremely cold substances like liquid nitrogen. In fact, you can sometimes create bits of dry ice (frozen carbon dioxide) by releasing a fire extinguisher into a burlap sack. The CO2 in the fire extinguisher is compressed, and if it’s been sitting long enough, it’s also at room temperature. Releasing the pressure causes a large decrease in temperature. So large that the gas actually solidifies into its solid state.

Hopefully this was an interesting slight deviation from my usual topics of electromagentism.

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Remind Me Again

Apple designed the built-in Reminders app as a list-keeping assistant for both macOS and iOS. You can add reminders of any sort to the default Reminders list, or you can create custom lists, like Groceries or Movies to Watch. Plus, if you’ve set up Family Sharing, you also have a shared family list that everyone in your family can access. I can see that being handy if you have kids but I am gonna hide that from Grace because she will fill up my list!

Making reminders is easy enough, but they can be easy to lose track of, and you may have to hunt through a number of lists to find any given one. How can you be certain that you won’t forget a particular to-do item? One technique that works well is to add a time trigger to the reminder. Time triggers cause your Apple devices to alert you to the reminder, and as an added benefit, they make it easier to find associated reminders.

Say you want to remind yourself to buy tickets to Halestorm’s next concert. To include a trigger in your reminder, you can get Siri’s assistance by mentioning a time in your request: “Remind me to get tickets at 10 AM tomorrow.” Or, when you add the reminder manually, pick a day and time. After creating the reminder, hover over it or tap it, tap the i button that appears, and the option to be reminded on a day. Then, on a Mac, click the preset day and time to adjust them. In iOS, tap Alarm and set a day and time. Unless the specific time matters, pick a general time that’s early in the day, like 10 AM.

Because your reminder includes a time, it appears not only in the list where you added it but also in the special Scheduled list. That’s important!

Now imagine that it’s first thing tomorrow morning and you’re trying to plan your day. You can either check the Scheduled list in Reminders or ask Siri: “Show me my reminders for today.” Once you see your day’s reminders, you can just do the easy ones, plan them into your day, or reschedule them for another day.

Of course, since you’ve assigned a time-based trigger to these reminders, Apple’s Notifications feature comes into play. At the appropriate time, your Apple devices can display an alert that you must dismiss, show a banner that disappears quickly, or play a sound.

Reminders can make it easy to remember important tasks, but try these tips if you need help:

  • For reminders created on one device to trigger notifications on another, set up your iCloud account on both devices must have Reminders on. Do this on the Mac in System Preferences > iCloud. In iOS, tap Settings > Your Apple ID Name > iCloud (if your copy of iOS isn’t up-to-date, tap Settings > iCloud). Plus, the reminders must be on a list that’s stored in iCloud.
  • If you use Siri to make reminders, specify the list where those reminders will be added if you don’t speak its name. On the Mac, choose Reminders > Default List. In iOS, go to Settings > Reminders > Default List.
    Configure Mac notifications in System Preferences > Notifications. At the left, select Reminders and then make your choices at the right. The Alerts alert style is the easiest to notice. Set up iOS notifications in Settings > Notifications > Reminders. Turn on the Allow Notifications switch. For best results, turn on Show on Lock Screen and select Alerts under “Alert Style When Unlocked.”

  • On your iPhone, to see a different Reminders list, tap the “stack” of lists at the bottom of the screen.

Remind me again why I need those reminders? Well, since I always have a device handy (literally in the case of the Apple Watch!) it is easy to keep track of my “Honey Dos” and the important stuff, too!

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Capture Perfect Photos With Burst Mode

Have you struggled with getting the perfect photo? How many times have we all struggled with a group photo you have to take multiple times because someone blinked, someone yawned or that toddler in front just wouldn’t stay still? Taking photos of animals like dogs and horses can be equally frustrating. One moment it’s the perfect shot, the next they moved or you just keep snapping that photo at the wrong time.

The more an object for your photo is moving the harder it is to get the perfect shot. Images get blurry, you capture just half of the body or any number of other factors that take your photo memory from frame worthy to trash can bound. I’ve long considered myself an average photographer. I’ve purchased a few high end cameras over the years but I never practiced or used the cameras long enough to understand what the settings really do and why. Thanks to the iPhone I really do look like a better photographer than I really am!

Professional photographers shooting sports or even wildlife have been using burst mode features on their cameras to take photos in quick succession for decades, but it typically meant this feature also went along with a very expensive camera set up. Film cameras required a motor to advance the film quickly enough, but now everything is digital. A burst mode simply needs enough processing power and storage space to record frame after frame. Your iPhone or iPad has this capability and is incredibly easy to activate burst mode.

The secret hidden trick to using burst mode on your iPhone or iPad is less hidden than you might realize and it’s likely you activated the feature a times without even realizing it. Rather than pressing the shutter button once, just hold it. The longer you hold it down, the more burst photos you will get. You’ll hear a distinctive and continuous sound of the shutter quickly taking photos.

Your iPhone can capture 10 frames per second, so just 2.5 seconds of photos can turn into 25 in burst mode. Sometimes the only challenge to burst mode is sorting through all of the photos and selecting the ones that you want. Do note that Photos appear stacked when your looking at them. When you select the photos you will want to then look for the word “select” so that Photos will then display all of the images caught during your burst.

Once you have hit “select” and start to look through all the photos you will see the typical circle image in the bottom right hand corner. As your going through the images simply tap on the circle for any photo you want to keep, it will then display in blue. Once you hit the Done button you will then be asked if you want to keep just the favorites you selected or if you want to keep everything. I normally keep just the ones that I have selected as my favorites.

One warning. If you use iCloud Photo Library or My Photo Stream to sync photos between devices it might take some times for all the photos in the burst shots to move from iPhone to Mac. Whoops, one more! If you are using My Photo Stream to transfer photos to your Mac only those photos that you’ve marked as a favorite will transfer. To set it so entire bursts transfer automatically, open Settings > Photos & Camera and enable upload photo bursts.

Give bursts a try this summer, you never know what unexpected photo moments you might end up discovering.

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