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April 2013

The Beasts In The Wall
Article By Grey Rollins

 

  My mother, despite being a well-educated woman and pretty level-headed in the bargain, had one belief that was suspiciously close to being rank superstition. Although she was perfectly willing to accept the conveniences afforded by modern electrical appliances, those pesky little demons hiding in the electrical outlet filled her with irrational fear. At some point in her life, she must have been shocked badly by electricity, because she was deathly afraid of getting even the slightest tingle, ever again. When I asked her what she was so afraid of, she waved her hands vaguely in the air and tried to describe something with teeth and a vicious temper and...

I frowned, trying to make sense of her description. "Sounds almost like you're describing tiny piranha."

"Exactly," she said, folding her arms and sitting back, lips drawn tight.

 

And I never managed to dissuade her from that view. She went to her grave thinking, somehow, that there was an extra-dimensional portal between a river in the Amazonian rain forest and a power outlet, through which small, toothed creatures about the size of a pea would make their mysterious way to nibble on unwary fingers. Had it been electric eels she was fixated on, I might have been able to see the connection, but it was teeth she saw in her mind's eye and that was that.

My mother was not alone in being afraid of electricity. There are lots of people who might otherwise be eager electronic DIY guys and gals, but for fear of those razor-sharp teeth. So, let me see if I can lay some of your fears to rest.

Folks, here's the plain, unvarnished truth: If you mess with electronics long enough, you will at some point get shocked because you put your finger in the wrong place at the wrong time. I'll be the first to tell you that it can leave a mark. I've got a scar on my right index finger from the time some moron thought it would be hilarious to turn on a piece of equipment while my hand was inside it.

The good news is that it's not the end of the world — elseways, I wouldn't be sitting here writing this.

This is not an excuse to adopt a cavalier attitude towards electricity. It can kill you, and I mean dead, as in forever dead. Kaput.Extincto. Four paws up.

Look at it this way — if you cook, you will eventually get burned. When it happens, you cuss, apply any necessary first aid measures, and go back to cooking. Does the possibility of getting burned stop people from enjoying cooking? Or course not. In like manner, the possibility of getting shocked shouldn't stop people from building electronic circuits.

There's an old saying that fire is a good servant, but a bad master. It can cook your food, it can light your home, it can keep you warm when it's cold... but it can also burn your house down, even kill you. The rational thing to do is to accept that there are risks and use a little common sense. Keep the fire in the hearth, where it belongs. Don't stick your hand into the flames. Don't leave an open flame unattended. When it comes to electricity, keep it in the equipment, where it belongs. Be careful about putting your hand into it. Don't leave a live circuit unattended.

I offer the following guidelines. Note that they are not sanctioned by any official governing body, but then it's damned difficult to get people to talk rationally about live circuits. I've followed these rules of thumb for years and they work.

1) I don't touch any circuit with my bare hands if it's over 75 Volts DC.

2) I don't touch any circuit with my bare hands if it's over 50 Volts AC.

3) I don't touch any circuit over 25 Volts if my hands are sweaty.

4) Be careful sticking tools into live circuits, regardless of voltage.

5) It is a good idea to use one hand, and one hand only, when you're poking around in a live circuit.

 

There, I've said it. I've mentioned the elephant in the room. You will, at some point, need to troubleshoot a circuit and the only way to do that is with the circuit turned on. Sure, you can turn it off. Go ahead, hit the switch. Okay, smarty-pants, how are you going to track down that oscillation with it turned off? The answer is that you can't. So, turn it back on.

Those guidelines weren't taught to me by anyone, because everyone I ever knew (even professors in college) were reluctant to talk about it. It's absolutely necessary to work with live circuits from time to time, but it's not something anyone wants to talk about. I can touch 75 Vdc and not feel a shock. Yes, there's current flowing through my hand, but it's not enough to cause pain or damage. My AC voltage guideline is lower because the RMS (the figure most meters read) value is less than the peak voltage. It is not a straight, mathematical reduction from the DC voltage guideline, but it's a good, practical number that covers the majority of audio circuits, once you get past the incoming line voltage. The sweaty hands rule is because sweat contains a lot of dissolved ions which conduct electricity a lot better than clean, dry skin. The tool rule is because metal conducts electricity and a dead short, even at low voltage, is mathematically an attempt to draw infinite current. You're likely to damage the circuit. The one hand rule is to reduce the chance of electricity going across your chest. Your heart is the organ most likely to go on the fritz if you get shocked, because it is essentially an electrically fired pump. An external electrical signal can confuse your heart, causing it to quit pumping effectively. And if your heart quits...you quit. It's much better to take a shock from, say, the thumb to the index finger of the same hand than it is to take the same shock from one hand, up across your chest, and down to your other hand. Do your best to keep electricity out of your chest... always.

Now, I've seen people do all sorts of things that I regard as crazy and get away with it. An example being the time they changed the light fixtures at work. Did they turn the lights off? No. They pulled the wiring down, undid the wire nuts, pulled the wires apart, took down the old fixture, put the new one up, and wired the new fixture, all the time working with live 120 Vac power. Jeez. Aside from the fact that they were breaking all sorts of rules and regulations, it was foolhardy. Not something I would ever consider doing. But every one of the guys in that work crew left the building alive, in spite of being shocked numerous times by 120 Vac. That said, my advice to you is DON'T DO IT. Just take it as a data point that says that under certain conditions, the human body can take a lot of abuse and keep going. You will touch high voltage enough times by accident — there's no reason to do it on purpose.

I'm going to say it again. Electricity can hurt you. It can kill you. If it wasn't for all the wonderful things it can do for us, it would remain a laboratory curiosity. The trick is to strike a reasonable balance between shivering paranoia and the gung-ho recklessness of that work crew. If you want to adopt a more conservative set of voltages than I'm suggesting, then do so with my blessing. Be really cautious about going above those voltages, though.

So what is electricity, anyway?

It pains me to see the number of people who have no earthly idea what electricity is. In this day and age, given that nearly everything uses electricity in ways both large and small, I feel that it should be mandatory at the high school level to demonstrate at least some rudimentary understanding of electricity. But who am I fooling? Not myself, and certainly not you. Ain't going to happen.

Remember that diagram with the nucleus of the atom in the middle and electrons whizzing around it? That's all you need. No quantum mechanics required. We'll get to tunneling electrons later. For the time being the simple atomic model proposed by Niels Bohr allows us to visualize the electrons orbiting the nucleus, each carrying a negative charge. If the atom in question is a metallic atom — and one of the definitions of a metal is that it can conduct electricity — then you can persuade an electron to jump from one atom to another, and from there to another, and from that atom to yet another. The vast majority of roaming electrons in our power lines were convinced to leave home by magnetic fields inside of generators. There are other ways to make electrons jump about. Batteries do it by chemical means and photovoltaic cells (aka solar cells) use the impact of photons to do the trick, but at our current level of technology, nothing beats a good, old-fashioned generator for moving wholesale quantities of electrons down the line. And that's what electricity is — electrons in motion. It's that simple.

Once electricity is flowing, we need a way to talk about it and measure it. Suppose you were to take an imaginary box and fill it with 6.24 × 1018 electrons. That's a lot of electrons. Fortunately, there's an easy way to refer to them; better than pointing at the box and saying something vague like, "That box of electrons." It's called a Coulomb, named after Charles-Augustin de Coulomb. If you flog those electrons into motion and get them moving past a given observation point at the rate of one Coulomb per second, then that current is called an Ampere (after Andre-Marie Ampere).

The classic way to describe electricity is to compare it to water. A Coulomb is analogous to a volumetric measure of water, say, a quart or a liter. Once the water is flowing, you can quickly get an intuitive feel for the pressure required to make it flow (in electrical terms, this is a Volt, named after Alessandro Volta) and the degree of difficulty involved in making water flow up a hill. In electrical terms this is called resistance, measured in Ohms. As you might suspect, there's a name behind the Ohm, that being Georg Ohm. In fact, if lasting fame is your game, you could do worse than to discover a new principle in electronics. All the important stuff is named after somebody and the names stick, even if the people behind the unit are only known to those of a geeky turn of mind.

Electricity comes in two flavors: Alternating Current and Direct Current, both abbreviated by their initials, AC and DC. Alternating current is electricity that flows one way for a period of time, then turns around and goes the other way for a while. Sounds kinda aimless and wishy-washy when you put it that way, but without that back and forth-ness we wouldn't have music, so it behooves us to allow for alternate lifestyles. As long as the AC is going about its business in a repeatable, rhythmic way, you can assign a frequency to it so as to be able to indicate whether it's going back and forth quickly or slowly. Frequency is measured in cycles per second, or Hertz. No, it's not named after the car rental company; it's named after Heinrich Hertz.

Direct current is just what it sounds like — electricity that flows in the same direction all the time. If you want, you can view DC as a special case of AC; one where the frequency of the AC is zero Hertz.

If some enterprising miller were to employ the flowing water we've been watching to turn a water wheel and grind grain, then the water would be doing work. In electrical terms, you can refer to work as a Watt (James Watt). To calculate Watts, you multiply Volts by Amps. Note that the math involved so far is elementary. Addition, subtraction, multiplication, and division will take you a long ways. Later we'll need a bit of algebra, but nothing too intimidating. Even those who hate math can usually grind through the necessaries without breaking into a sweat.

In closing out this first installment, I'd like to propose a new unit of measure. It would be useful to have an agreed-upon term to quantify the subjective "ouch" factor of getting shocked. It would come in handy when talking to your buddies to have a more scientific means of expression for the discomfort involved. Instead of shaking your head and saying something vague like, "Man that really hurt!" You could, instead, say, "Wow! That was a five Eleanor shock!" Or perhaps, "Nah, it wasn't so bad... only about two Eleanors."

So who's Eleanor?

My mother, of course, soon to be famous for her discovery of the 'piranha' aspect of electricity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     
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