Sunday, April 5, 2009

Electricity for Synth-DIYers: Volts and Amps

This is the beginning of a series that I'm starting to talk about the basics of electricity, as it applies to synth players and DIYers. I'm not going to get too much into theory in this series, but I'll start it with a basic discussion of what electricity is and how it's measured. Later posts will focus on electronics components, what they do, and how they are specified, as well as provide a few basic circuit examples.


What is Electricity?

All materials are composed of atoms, and molecules composed of atoms. An atom is made up of a nucleus surrounded by electrons. In non-conducting materials, all of the electrons are pretty tightly bound to their respective nuclei. However, in conducting materials (metals mainly), each atom has one or more electrons that are loosely bound and can swap around with electrons from adjacent atoms. Normally, they do this at random, zinging around in arbitrary directions. However, if one applies an electromagnetic force to the conducting material, the electrons can be made to move in a certain direction. Since each electron carries a specific amount of electric charge, this movement constitutes electricity.


No, Really, What is Electricity?

Electricity, for most practical purposes, is the flow of electric current from one place to another. Electronics is the engineering craft of designing electrical circuits to create and process various forms of signals that are converted to or from electrical signals. (One of the advantages of electrical energy is that it is easily converted into other forms of energy, and modern industry has invented all kinds of devices for doing so.) In our context, we are mainly talking about electrical currents that will eventually, when piped through an amp and speaker, be converted into sound. Our first concerns about any flow of electrical current are always: (1) Which direction is it going? (2) How much electricity is flowing? (3) How much "pressure" is there motivating the electricity to flow? The quantity of electricity flowing, and in what direction, is referred to as "current" or "amperage", and is measured in amps. The unit is designated with the letter A in a specification, such as 1.5A for one and one-half amps. The "pressure" is referred to as "voltage" and is measured in volts, designed with the letter V in a specification, e.g., 15 volts is 15V. The usual metric prefixes can be used for millivolts/amps, kilovolts/amps, etc, so 30 mA is thirty milliamps.

Water analogies are often used to explain the basics of electricity. It's something one has to be careful about, because the analogy only goes so far, but it does help explain the basics. Electricity is similar to water in that you cannot "compress" it; if you push a certain amount of electrical current into one end of a wire, an equal amount of curent has to come out somewhere else. You can "use up" voltage, but current in always equals current out. If you think about a water supply pipe, there are two things that determine how much water you can get out of it: how much water is in the line, and how much pressure there is making the water move. Voltage is analagous to the amount of pressure in a water pipeline, while current (amps) is the amount of water moving through the pipe. The amount of pressure has an effect on how much water moves through the pipe, but the size of the pipe has an effect too. A bigger pipe can carry the same amount of current as a smaller pipe, at a lower pressure.

Lighting is an example of an electrical current with high voltage but a relatively modest number of amps flowing. Electricity produced by a van der Graaf generator is an even more extreme example: the voltage is in the millions of volts, but the current is very close to zero. At the opposite extreme, in the 1980s, I worked on some computer systems that had power supplies that output 400 amps at only 2.6 volts. This is sort of the electrical equivalent of the Mississippi River -- there's an enormous amount of water flowing through the Mississippi, but at a very low downstream pressure.

How much is a volt? One volt isn't much. Small batteries of the AAA/AA/C/D types put out 1.5V nominally. You'll note that you can hold these in your hand, with one finger on the positive terminal and the other on the negative terminal, and not feel any electricity flowing. "Square" batteries that use the snap-on connectors are 9V; most automobile batteries are 12V. Modular synths often use plus and minus 15V power supplies. The minimum voltage that is usually considered to pose any threat of electrocution to a human (assuming that one is in reasonably good health, and not doing anything stupid like standing in water while working on a circuit) is about 20V. A moderate-powered modern amplifier might have power supplies running at about 60V. Old vacuum tube (valve) circuits needed higher voltages; 200-300V is common for guitar amps. These voltages are definitely lethal and must be respected. Power distibution in homes in most countries takes place at 220-240 volts; 120V is used in the USA, Canada, and a few other countries.

By comparison, one amp is a fairly generous amount of current. On North American power, a 100W incandescent light bulb uses about 1A of current. Power distribution branch circuits in houses are usually in the 15-30A range. Electronics circuits (other than power amplifiers) usually deal with currents in milliamps or smaller.


You Can't Compress Electricity

I already said it above: as in the case of water, you can't compress electricity. If you push an electric current into one end of a conductor, an equal amount of current has to come out somewhere else. This is an important concept, and it leads directly to the notion of electrical circuits. Wherever electric current comes from, it eventually has to go back there, in a closed loop. Water can be "conducted" by almost anything; nearly all substances allow water to flow over, around, or through them, so building a water circuit isn't that hard conceptually. However, there are a lot of things that don't conduct electricity, at least not very well, including the air around us. As in the case of stagnant water, electricity that isn't moving and has no path to move cannot do useful work. So it is always necessary for electrical circuits to be constructed in closed loops. Current in always equals current out; wherever the source of current is in a circuit, there has to be a path for the "used up" current to get back there. That's way batteries and generators have two poles: current comes out of one pole, and goes back in the other one.


Resistance and Ohm's Law

Outside of semiconductors, all electrical circuits have some resistance in them; they tend to resist the flow of electrical current to at least some degree. The amount of current that flows through the circuit is inversely proportional to this resistance, and directly proportional to the voltage that is motivating the current. Turning to the water analogy again, if you consider a pipe that is straight and has very smooth walls, it has little resistance to the flow of water through it. So it doesn't take much pressure to make a lot of water move through the pipe. On the other hand, if the pipe has rough interior walls (e.g., old galvanized pipe) and a bunch of twists and turns, it has higher resistance, and requires higher pressure to deliver the same amount of water flow.

The same thing happens in electrical circuits. Increased voltage will make more current flow through the circuit; increased resistance will lessen the current flow. These relationships are described by Ohm's Law, which is the basic law of electricity:

E=IR

where E=voltage (in volts), I=current (in amps), and R=resistance (in ohms).


A Few Basic Circuits

Below is the most simple circuit possible.  It consists of a battery and a resistor.  The symbol with the short and long lines represents the battery; the plus sign shows which end represents the positive pole of the battery.  The squiggly symbol represents the resistor.  This is standard symbology for electrical circuit drawings.  

This circuit doesn't do much; all it does is create heat when the current flows through the resistor.  The next circuit is more interesting.  It adds a light-emitting diode, or LED, to the circuit.  


You're familiar with LEDs; they generate light of a specific color when electricity passes through them.  The round symbol containing the triangle and line, plus the squiggly arrows coming out, represents the LED.  The arrows have to be there to emphasize that this is an LED and not a regular diode, which doesn't create light and is used for a different purpose.  

What is the resistor doing in this circuit?  It limits the flow of current through the LED.  The LED itself (unlike a regular light bulb) has very low resistance; if it were connected directly to the battery, so much current would flow through it that it would quickly burn up.  

The third circuit is a variation of the above.  It allows the brightness of the LED to be varied:


The resistor circuit with the arrow through it represents a variable resistor, one that has some mechanical means (such as a knob) for adjusting its resistance.  The two resistors together, the fixed one and the variable one, make up what is known as a "voltage divider".  Depending on the setting of the variable resistor, some of the current will flow through the variable resistor and bypass the LED; the rest will instead take the "short cut" through the low resistance of the LED.  You've probably heard of a potentiometer, or "pot" for short.  A variable resistor is one way a potentiometer can be wired up.  


AC/DC (And We're Not Talking About Angus Young)

Here's where we start to make things more complicated, and where the water analogy starts to break down. So far all the circuits we've discussed have been direct current (DC) circuits. In a DC circuit, current flows from the positive side of the current source, around the circuit, and back to the negative side of the current source. Pretty simple. However, in an alternating current (AC) circuit, the current doesn't go all the way around the loop; instead, it constantly changes direction. In this case, the current source doesn't have a fixed positive or negative terminal; at any given time, one side is positive and the other side is negative, then they switch directions.

So what good is this? For one thing, as it turns out, AC is a more efficient way of delivering a lot of electrical energy. The reasons why are complex and I won't get into them now, but this is why the power circuits in your residence are AC and not DC. (Thomas Edison, who designed the world's first commercial electric power distribution grid, tried using DC. But competitors using AC were able to put him out of business.) Of more interest to us is the fact that an AC current is a perfect analog for a sound wave travelling through the air. Sound waves are also alternating waves, and it's easy for an electrical circuit to convert sound waves into electrical waves. The device that does this is, of course, the microphone. And the device that does the opposite, turning electrical waves into sound waves, is the loudspeaker.


Stop, That Hertz

If the current follows a repeating pattern (waveform) alternating at a consistent rate, then one "cycle" is considered the time from some arbitrary starting point in the waveform to the point where it begins to repeat the just-completed pattern. The time it takes for one cycle is known as the "period". Measuring that time in seconds, and then taking the reciprocal, gives the frequency, which is the number of times that the alternating current alternates in one second. The unit of measure for frequency is the "hertz" and is abbreviated Hz; by definition, 1 Hz is one cycle per second. The word "hertz" has only been in common use since the 1960s, so in old radio and TV manuals from before that time, you may see frequency units given as "cps", which is the same thing (cycles per second).

AC circuits have more complex behavior than DC circuits. An AC current may consist of a blend of different componets, or partials, alternating at different frequencies. The device known as the oscilliscope will show you what the composite waveform looks like, but to find all of the individual components, you have to perform a mathematical operation known as a Fourier analysis, which we won't get into right now. The one important point to remeber is that the "purest" type of wave is the sine wave; this consists of exactly one partial at one given frequency. All other waveforms are made of combinations of two or more sine waves. Electrical power is distributed in the form of a sine wave, alternating at 50 or 60 Hz depending on what country you live in. The audio frequency range is conventionally given as ranging from 20 to 20,000 Hz. Commercial FM radio transmission in North America takes place in a frequency band centered around 100 MHz (megahertz, or millions of cycles per second); TV channels 1-6 are at frequencies somewhat below the FM band, while TV channels 7 and higher are above the FM band.


Ben Franklin Flips a Coin, Comes Up Tails

Thus far, we've treated current as if it comes from the positive pole of a battery, and flows towards the negative pole.  You probably know that electrical energy is carried by the atomic particle called the electron; electricity moves by means of electrons moving from atom to atom through a conductor.  Now, here's were we deal with an unfortunate consequence of a random guess.  Someone back in the 18th century (one legend has it that it was Ben Franklin) had to take a 50-50 guess as to whether the electron is positively or negatively charged.  They guessed that the electron is positively charged.  Unfortunately, they guessed wrong, which means that when we speak of current flowing through a circuit in a given direction, the electrons are actually moving in the opposite direction.  So our convention, of assuming that current moves from positive to negative, is exactly the wrong way around.  Nonetheless, electrical engineering, by long-standing convention, continues to treat current this way; "conventional current" flows from positive to negative, while electrons actually flow from negative to positive.  Circuit designs continue to this day to be drawn and the math computed in terms of conventional current, and for basic circuits, this works fine.  It usually only becomes important to understand the distinction when studying the physics of how certain semiconductors, or vacuum tubes (valves) work.  

The next installation in this series will deal with resistors: what they are, how they work, and how they are specified.

2 comments:

yosh said...

This post was great!! I knew a lot of it, but not nearly everything. Looking forward to more.

Benjamin Budts said...

You're getting tired of my comments by now :p . But I insist on saying this guide was helpful for me and very well written !

Will read all you're techings :-)