Thursday, April 23, 2009

Adventures in Backlighting, Part 3: The K5m Lives!

So having figured out the cut-to-size EL sheet, it was time to put it to use. Out came the K5m to the workbench. Once I recalled how to open the jigsaw-puzzle case, I saw this:

The half on the left, laying flat, is the front panel. The right half is the rear half. A closer view of the front:

To cram the power supply into the K5m's case, they had to split it into two pieces. The transformers were crammed into a corner of the back part of the case, and the voltage regulators were put on a separate board, which is the tan-colored board on the left in the photo above. This board sits atop the LCD display board on standoffs, and it also contains the backlight inverter, which you can see below as the white box on the right. In this photo, the power cables have been disconnected from th board and it is in the process of being removed. To the right of the inverter, the small connector and (very short) cable that routes the inverter output to the display board is still connected. Note that if you have the keyboard version of the K5, this board is not here, but there is a ribbon cable that runs across the back of the display board. On the keyboard K5, you have to remove one of the panel boards to get the ribbon cable out of the way; see here. On the K5m, once you have the regulator board out of the way, the display board is free and clear.

Below is the back of the display board, exposed. Note the metal clips holding it in around the edge. To remove it, you must get some needle-nose pliers and very carefully bend those clips out of the way. They will only tolerate being bent so many times, and the pressure they provide to hold the board in is essential to the LCD working properly. Don't bend them any farther than necessary to get the board out. The gray ribbon cable at left is soldered in at this end, but can easily be unplugged from the CPU board at the other end. The dark green board below the display board is the panel board.

The top of the liberated display board, showing the LCD screen. The backlight is soldered in at the top end, in this picture. For the moment, we put this aside; we'll get back to it later.

The next step is to remove the dead inverter and retrofit the new one. Whatever brand of new inverter you buy, it's almost certain to not fit in the same holes as the old one. Here's the regulator board with the old inverter removed. The connector on the left is the inverter output going to the display board; the connector in the right is for the raw juice coming in from the power transformers.

Here's the regulator, which I mentioned in part 1. It's a Hantronix HVI-5E ordered from Mouser. It has three pins, in the same pattern as old inverter:
  • pin 1: +5V in
  • pin 2: common (return for both the +5 and the inverter output)
  • pin 3: inverter output, 90VAC @ 400 Hz

To my surprise, pins 2 and 3 of the new inverter actually fit in the same holes on the board. If I could just figure out what to do about pin 1, I'd be golden. I considered bending it over and running a jumper wire over to the hole, on the component side of the board. But then I thought of a cleaner solution: I could tell that the board was a single-layer board (hold up to a strong light if you have any doubts). Could I drill a hole for pin 1? Looking on the back of the board, there were no traces in the area that I had to drill. So I drilled a new hole. In the photo below, the old hole for pin 1 is below and slightly to the left of the drill bit:

Note: do this trick with a circuit board only if there are no traces nearby, and you are absolutely certain that it isn't a multi-layer board! Else you may cut an internal-layer trace, or accidentally short power to an internal ground plane, both equally nasty and non-fixable.

Success! Here's the inverter pins protruding through the back of the board. Note the lack of a pad around one of them; that's pin 1 in the new hole:

It was a trivial matter to install a jumper to connect the pin to the old hole location. Here it is, with the jumper installed and the pins clipped:

And the result. This turned out to be a good solution; the inverter is secure on the board, and the existing connector and cable to the display board are still functional.

Now, on to the actual backlight replacement. This turned out to be more trouble than I was expecting. First, the end of the existing backlight, which is a plastic piece that just slips in the slot between the LCD and the board underneath. It has these two tabs soldered in at the end, which are connected on the board to the cable that goes to the inverter:

Unsoldering the tabs is straightforward. Once that is done, the old backlight is just pulled out of the slot:

Here's what you see with the backlight removed. The grey are is the actual display area of the LCD. Note carefully the rubber strips at the left and right edges. Those rubber strips contain many tiny electrical connectors, which simply touch (they aren't soldered) many equally tiny surface-mount pads on the board. There is some kind of conductive glue holding the rubber strips to the board, and it isn't very strong. Also note that there are some exposed vias underneath the LCD.

Here's our new backlight, cut down to the proper size, and with two strips of conductive-glue copper foil attached to the tabs on the bottom, and crimped at the ends to form contacts. I took it to work and cut it with one of those guillotine-type paper cutters to get a clean edge.

Problem: the old backlight was insulated on the bottom. The new one has exposed conductive surface on the bottom, and we can't let that come into contact with those exposed vias. So I had what I thought was a brilliant idea: I'll just put the old backlight back in, unconnected, and put the new one in on top of it! That way, the old backlight will act as an insulator. It was a tight squeeze, but I crammed it in. Here's the result, with the copper foil tabs newly soldered to the pads on the board. I was so proud of myself...

... until I reassembled the display to the front panel and found that it the LCD no longer worked properly; it displayed black lines across the face, and had rectangular patches that didn't work. I took it back apart, and found that one side of the LCD contact strip had come unglued from the board. When I took it out, the whole backlight arrangement fell out the side! Disaster, I thought. At first, I thought it was unsalvagable, until I realized that since the contacts weren't soldered, maybe if I just put it back together without the stack of the old and new backlights forcing the LCD up, the pressure from the metal tabs that hold the display board in (and the display itself against the front panel glass) would hold it together.

At this point, it occurred to me what I should have done about the conductive backside of the new backlight: simply cover it with electrical tape. This I did, and once that was done, it easily fit back in the slot by itself (it's actually quite thinner than the old backlight). So, starting over from here again:

I was able to slip the new backlight back in, and since the LCD was coming up on one side, I was also able to get my finger in there with a piece of electrical tape to keep the copper foil from touching the board, while getting the tabs on the end of the backlight stuck back on to the ends of the foil tabs. After a couple of false starts, I got it back in there and got the whole thing reassembled, and all of the cables attached again. Here it is, nearly ready to close up; you can see the power transformers on the right:

Every time I open thing thing up, the metal case gets a little more warped and there are a couple of more case screws that won't go back in:

But it works! I can actually see the display again, and the light is a nice white color! Here it is in all of its additive-synthesis glory. (Weird shadows and colors were caused by reflections of ambient light in the room.) I noticed that the very top row of dots on the display is very faint, but I'm not going to mess with it; I really don't want to have to take that display board out again.

Now, while I had this beast open, there were a few other things that I could have done that are recommended for K5/K5m owners. If you go back to where the panel board is mentioned, you can remove that board and clean the button contacts (click on the link and scroll about 2/3 of the way down) to make the buttons work better. You can also upgrade the OS by installing a new ROM available from the Yahoo K5/K5m user group; the latest version for the K5m is 1.2, and for the K5 keyboard is 1.3. (It displays the OS version at boot time.) The ROM is socketed so it's an easy replacement. I had already done the button cleaning on mine in a previous servicing, but I do plan to buy the OS upgrade ROM and open it back up later to install that. And there is a mod to improve the output level (kfuenf again; scroll down to near the bottom); I had also done that previously.

Sunday, April 19, 2009

Electricity for Synth-DIYers: Resistors

As the name suggests, the purpose of a resistor is to restrict the flow of electricity through a circuit. Here's a few things that a synth-DIYer needs to know about resistors.

What are Resistors For?

Resistors are used to control the voltage in a circuit, or to control the flow of current through a circuit. We saw an example of the latter in the first installment of this series, where we used a resistor to limit the amount of current flowing through an LED. Resistors are often used to create a drop in voltage in a circuit. For example, if two resistors of equal value are wired in series between the two poles of a battery, the voltage at the point in between the two resistors will be half the battery voltage; this type of circuit is known as a voltage divider. Unequal values create different in-between voltages.

Resistors can be used to "convert" a current into a voltage. An example often seen in synth circuits is a resistor wired to the output of an OTA, an IC frequently used in analog synts. The OTA produces a specific output current which is proportional to its inputs. By placing a resistor on the output, a voltage is developed in between the OTA output and the resistor which is proportional to the OTA's output current.

A pull-up resistor is used to apply voltages to the inputs of components which act as electronic switches, such as JFET transistors and bilateral switches. The switch has an "open" or "closed" position, but in order for another circuit to sense that, some voltage must be applied to the switch input. A pull-up resistor between a supply voltage and the switch input provides the necessary voltage. With the output of the switch connected to ground, when the switch is closed, the measured voltage at the switch input will be zero. When the switch opens, the voltage at the switch's input will rise to the supply voltage, due to the pull-up resistor. The pull-up also limits the current when the switch is closed; otherwise, the switch would short out the supply voltage.

Similarly, a resistor wired between a voltage source and ground acts as a "pull-down". A high-value resistor is often wired to the external input of a circuit to provide some protection against static charges. Static electricity may have very high voltage, but very low current. A high-value resistor dissipates the static charge without "loading down" the circuit's normal input signal.

A resistor in series with a capacitory has an effect which is similar to reducing the capacitance value of the capacitor. This will be covered further in a later installment.

Ohm's Law

The degree to which a resistor resists is measured in ohms. The definition of an ohm is: if a voltage of one volt is applied to a resistance of one ohm, a current of one amp results. This leads to a basic formula of electricity, known as Ohm's Law:


where E is the voltage, I is the current, and R is the resistance. (Why is E the voltage and I the current? E comes from an old term for voltage, "electromotive force". You do sometimes see the letter V used instead. "I" for current, I have no idea, other than the fact that "C" was already taken for something else.) Of course, you can do simple algebraic manipulations to find any quantity needed, given the other two; for example, if you know the voltage and the resistance and you want to find the resulting current, divide both sides by R, and you get:

I = E/R

Specifying and Choosing Resistors

In specifying resistance values, the ohm is represented by the Greek letter omega, which is: Ω. The usual metric multiplier prefixes can be added to this for larger values, for instance, 6 kilo-ohms would be written as 6kΩ, or sometimes just 6k if the context is understood. One ohm isn't much; typically, in electronics about the smallest resistance one encounters is 100Ω, and resistors with values in the megaohms range are sometimes called for. In practice, the omega symbol is often omitted when it is understood from context. A convention that is sometimes seen is that the resistance value will be written with either the letter "R" or the metric multiplier prefix placed in the number where the decimal point would go. So for example, 100Ω might be written as 100R; 1000Ω would be written as 1k, and 6800Ω would be written as 6k8.

Two other parameters are commonly given in specifying a resistor: the tolerance and the maximum power dissipation. Resistors are mass produced, in very large quantities, and the manufacturing processes are not perfect. As a result, it's actually rare for a resistor to measure as being exactly the specified value; it may be some small percentage more or less. Manufacturers measure and grade their resistors as to how close to the spec value they actually are, and then "bin" them up into tolerance ranges. As one might expect, the tighter tolerance ranges cost more. The most common tolerance range sold today is 5%, meaning that if a resistor is marked as a 10KΩ, 5% resistor, its actual value may be anywhere from 9500Ω to 10,500Ω. Besides 5% resistors, the other commonly available ranges are 1% and 0.1%. Electronic products built prior to about 1980 may contain 10% resistors, but resistor manufacturing processes have improved to the point where the 10% range is no longer marketed much, since there is no cost savings. When designing a circuit, the circuit designer will determine for each resistor in the circuit how much tolerance is allowable in order for the circuit to work, and then specify the tolerance based on that information.

Power Dissipation

Whenever a current passes through a resistor, some of the energy contained in that current is converted into heat. A resistor can only dissipate so much heat before it reaches a high enough temperature to destroy itself or nearby components. This waste heat, along with all other measures of energy and power production in the electrical world, is measured in watts. Note that the watt is not a quantity of energy; that's a common mistake. It is rather a rate at which energy or power is produced, flowed, or consumed. (In order to drive your car down the highway at a given miles or kilometers per hour rate, the engine must produce power at a certain rate. That rate of power production can be measured in watts, although in the case of automobile engines, "horsepower" is usually used instead. Horsepower is simply a different unit for the same thing: 1 horsepower roughly equals 750 watts.)

The power dissipated as heat will of course raise the temperature of the resisitor. To avoid cooking the resistor, the dissipation needs to be kept below the rating. If you know the voltage across the resistor and the current flowing through it, the dissipation is given by:

Watts = EI

Many times, in circuit design, you won't know the exact current, only the voltage applied to the resistor. Using Ohm's Law, you can substitute for I in the above:

I = E/R


Watts = E(E/R) = E^2/R

The most common dissipation rating for the types of resistors used in electronics is 1/4 watt. 1/8 watt and 1/2 watt types are also common. In general, the dissipation rating is directly proportional to the physical size of the resistor. Types rated for more than 1 watt are usually large and difficult to package; they are also often built as "wirewound" types which have other effects on a circuit that must be taken into account in the circuit design.

Substituting Resistors

A few rules of thumb for resistor substitution:

  1. You can always use a resistor of tighter tolerance than specified. If a parts list calls for a 5% resistor, you can use a 1% or better resistor of the same value.

  2. You can use a higher-dissipation resistor than specified, provided that it physically fits in the space where it needs to go, and as long as you avoid wirewound types. If the parts list calls for a 1/4 watt resistor, a 1/2 watt or 1 watt one will work as long as it fits on the board. Don't use wirewound resistors unless the schematic specifically calls for it.

  3. If you are in a pinch for a resistor value you don't have, or if you need an oddball value, you might be able to assemble using some combination of series and/or parallel resistors; see the next section.
Resistors in Series and in Parallel

Sometimes, you need a resistance value that is not a standard value or not easily available. You can build different values by wiring up two or more resistors in series and parallel. The rule for combining resistors in series is simple: their resistance values add. Example: a 10K resistor in series with a 6.8K resistor adds up to 16.8K. Resistors wired in parallel are more complicated: the rule there is often called "reciprocal sums". Suppose you have three resistors, R1, R2, and R3 in parallel. You first take the reciprocal of all of their values and add those:

1/Rsum = 1/R1 + 1/R2 + 1/R3

Then, take the reciprocal of 1/Rsum to get the result. So, for example, if R1 = 1K, R2 = 2200 ohms, and R3 = 5000 ohms, the calculation is:

1/Rsum = 1/1000 + 1/2200 + 1/5000

1/Rsum = 0.001 + 0.00045 + 0.0002 = 0.00165

Rsum = 1/0.00165 = 604 ohms

Note that the resulting value is less than the lowest-valued resistor in the set. This will always be true for resistors in parallel. The technique can be extended to any number of parallel resistors. Simplification: if all of the parallel resistors have the same value, the result will be that value divided by the number of resistors.

To figure the dissipation of series or parallel resistors, you would have to work out the applied voltage for each resistor, which can be some work. Try this shortcut first: figure the dissipation treating the resulting value of the series or parallel combination as if it were a single resistor. If that is within the dissipation of all of the resistors in the combination, then it's okay.

Resistor Markings

Resistors often do not have their value printed on the resistor body; instead, they use what are known as the RETMA codes, which are the colored rings you see painted on the resistor body. In current usage, this will be a sequence of four or five color bands. Four bands, with one of them being a gold or silver band, indicates a 10% or 5% tolerance. To determine the value, orient the resistor so the gold or silver band is on the right. Then, read the other bands according to this code:

0 = black
1 = brown
2 = red
3 = orange
4 = yellow
5 = green
6 = blue
7 = violet
8 = gray
9 = white

For a 10% or 5% type, the first two bands indicate the first two digits of the resistance value. The third band indicates that the specified number of zeroes should be added on (Note: this is not the same thing as specifying a power of ten! Some resistors use more digit bands, and the same color last band indicates a different power of ten on these. To keep it straight, don't worry about powers of ten; just add on the specified number of zeroes.) So, for example, a resistor with the color code orange/green/red indicates:

orange: 3
green: 5
red: 00 (two zeroes)

The value is 3500 ohms. A gold band at the end indicates it's a 5% tolerance; a silver band indicates 10%.

1% and better types will have five bands instead of four. It can be a bit difficult to determine which end is the left and which is the right, but if you know the type, a brown band at one end will indicate a 1% type. A violet band indicates a 0.1% type; other values are possible. Read the other bands the same way: red/red/violet/orange indicates:

red: 2
red: 2
violet: 7
orange: 000 (three zeroes)

This is a 227K ohm resistor. Below are pictured five resistors. See if you can make out their values.

Top: violet (hard to see in the photo), green, brown, gold = 750 ohms, 5%, 1/4 watt. Second: red, red, red, gold = 2200 ohms, 5%, 1/4 watt.
Third: orange, yellow, orange, gold = 34K ohms, 5%, 1/2 watt (note larger size).
Fourth: red, red, yellow, gold = 220K ohms, 5%, 1/4 watt
Bottom: brown, black, green, gold = 1.5M ohms, 5%, 1/4 watt

Below are some 9.09K, 1/4 watt, 1% types. Note that they use a printed designation instead of the RETMA markings. This seems to be the way the high-precision types are going.

Here are some old 10% types -- note the silver stripes. You don't see these much anymore.

Specialized Resistors

We mentioned wirewound resistors earlier. You normally want to avoid these because they have a property called induction, which will be covered in a later installment. The induction can mess up the circuit that the resistor is in. You will seldom need one of these in music gear, but there is one place where you may run into one -- amplifiers. Reason: wirewound resistors can be made to tolerate very high dissipations, 5W and up. These are often called "power resistors", and sometimes they are needed to control the high currents found in a high-power amp.

A specialized type often found in synthesizers is the temperature-compensating resistor, or "tempco" for short. All resistors are somewhat temperature-dependent. But a tempco's resistance change at different temperatures follows a very specific formula. This can be used to compensate for other circuit characteristics that change with temperature. All VCO circuits that have volts/octave response need a circuit called an "exponential converter" to transform the input control voltage to drive the oscillator core, but the implementation that is generally used in analog VCOs is very temperature-sensitive. A tempco is essential to offset this in order for the VCO's tuning to remain stable as the circuit heats up.


Resistors are one of the most frequently used of the electrical components. They are available in a huge array of types and values. Sorting out the markings can be tricky, so look closely. If you have trouble remembering the color codes, there are "RETMA" calculators that have little wheels where you can dial up the colors and it will tell you the value.

Next installment: capacitors.

Thursday, April 16, 2009

Adventures in Backlighting Part 2

The glowing blob you see above is a piece of a cut-to-size EL sheet, glowing on my workbench. I finally got it to work last night. Here's a pic of it unpowered (and with the lights on):

And here is what it looks like in low light -- kind of spooky:

If you read the previous post, you know that I was having trouble with it: every time I connected a piece of the EL sheet to the inverter, the +5V power supply would shut down. I was getting the power from one block of the Discombobulator, which had a Condor triple-voltage linear supply, with the +5 rated at 3A. There was only one module using the +5 in that block, but I figured maybe it was drawing a lot of current, so I disconnected it. With this, there was nothing but the EL inverter drawing from the +5. Still no joy; it shut down as soon as I connected the EL sheet. The second thing I tried was to go get another Discombobulator block. One of the three has a switching +5 power supply rate at 5A. Trying it with this, the power supply didn't shut down -- the inverter itself did.

Well, this left me completely puzzled. Was the sheet I was trying to light too big for the inverter? I tried cutting off a small piece (the piece you see above) and just using that. No difference. What was going on? I checked the piece with an ohmmeter, and was surprised to get readings ranging from about 1300 ohms to 20K. An EL light, being a capacitive device, should read open on a DC ohmmeter.

Tried it again, under low light. When I did this, I noticed that I could see small arcs when I pressed the EL sheet's tabs against the connector, just before the inverter shut down. The arcs evidently weren't in the EL sheet itself, since they weren't leaving burned or melted spots. They had to be in the connector. I looked over the connector and didn't see any obvious shorts. But, just to see, I removed the connector and rigged up a pair of jumper wires with some copper foil tape to connect the EL sheet to the inverter.

Voila! That worked. Something in the connector is creating a low-impedence short circuit path. Perhaps it's the adhesieve that was used to glue the metal contact surfaces to the plastic case. But in any event, that connector doesn't work. That's okay; I will rig up something to install it in the K5m when I get ready to do that.

Here's a larger piece, illuminated. This is the piece I'm going to cut to size to fit in the K5m's display. The crennelations are where the connecting tabs are.

By the way, if you play around with these things long enough, and you aren't very careful about the conductive surfaces, you will get zapped by the inverter eventually. And it will hurt. 90V at 400 Hz really stings. Lesson learned from recent experience.

Sunday, April 12, 2009

Adventures in Backlighting Part 1

My K5m needs a new backlight.  Has for some time, actually... and, unfortunately, most of the places that were carrying them a few years ago seem to have disappeared.  So I figured I'd try a DIY approach.  

I know from having tested it previously, using a battery-powered inverter and a length of EL wire, that both the backlight itself and the inverter are bad.  I ordered a sheet of EL material and connectors from's small-quantity online store, along with some connectors.  It's a cut-to-size sheet, so I can cut it to fit into the space inside the K5m's LCD display.  For the inverter, I ordered a Hantronix HVI-5E from Mouser; it appears to be about the same shape and size as the existing one, and should be easy to retrofit.  The inverter's output is spec'ed as 90V at 400 Hz.  

First, I set it all up on my workbench:

I'm using the power supply from one of the function blocks of the Discombobulator to power this experiment.  The inverter is shown plugged into the experimenter socket in the center of the picture.  It takes +5V DC.  The EL sheet is the foil-colored panel to the right (this picture shows the reverse side).  Here's a close-up of the connection:

Here's the front side.  It's only pink when it is not powered; it is supposed to light up white.

Unfortunately I haven't gotten it to work yet.  Every time I connect the sheet, the power supply shuts down.  It also does that if I try to probe the inverter output with the scope.  If I just hold the probe close to the connector, I can see the AC waveform on the scope and it looks good, but the moment the scope probe touches the conductor, no more power.  Haven't figured that out yet; I don't know if it's overloading the +5, or if the inverter does something nasty that tricks the protection circuit.  I guess I'll need to try a different supply.  

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:


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.