Learn Electronics Fast

It's like a dump of concentrated electronics knowledge without the cruft and the wastes of time. Because Ron is the best at explaining things simply. You'll actually learn stuff with me.

Units

Voltage or volts are the same as electrical pressure. Amperes or amps are the current. Ohms are a measure of resistance. The current that flows can be calculated as pressure divided by resistance, represented by I=E/R, using the traditional I for amps, E for volts, and R for Ohms. Watts are a measure of power, found by multiplying current by pressure, P=IE. Makes me think of pie. Yum.

Battery

A battery or cell provides current at a certain voltage using a chemical reaction. The voltage depends on the chemical reaction. Where more volts are needed, multiple cells are used in series.

Resistor

This component resists the flow of current. It's one of the most common components. Resistance is not futile. I should probably try to think of more to say about it here.

Capacitor

This stores a very small amount of electricity, like a tiny battery. Thus it can help to smooth out the power supply and provide power for temporary surges in demand. As a side effect, it allows AC to pass, especially high frequencies, but blocks DC. Capacitors are also used in RC and LC filters, for audio or radio frequencies.

Unlike a battery, it doesn't wear out after hundreds or even thousands of cycles. Supercapacitors may someday be able to store power to run electronics for a reasonable amount of time.

Two sheets of aluminum foil separated by wax paper can be used to construct your own capacitor. Twisting two insulated wires together also forms a capacitor, called a gimmick capacitor, of low capacitance but often good enough for radio frequencies.

Some capacitors use special substances called electrolytes between the plates for greater capacitance, and these are called electrolytic capacitors. Electrolytes may unfortunately add noise to sensitive signals, and most electrolytic capacitors can only handle voltage in one direction.

I recall being struck in the shoulder by an exploding electrolytic capacitor when I was 15. It made a big cloud of smoke. Be careful with 'em. Coulda put my eye out.

Two metal plates separated by an air gap can make a high voltage capacitor. The dielectric strength of air can be up to 30000 volts per cm. Pointed objects, edges, and corners require lower voltage to arc. Hot air requires less voltage to arc. Fire will conduct electricity and can be used to make a vacuum tube without a vacuum.

A capacitor works because when voltage is applied to the plates, the electrons in the dielectric, which is any material between the plates, are pulled from their normal positions by the electric field. They want to spring back to their normal places, and this stores a small amount of energy.

We use a unit called the farad, represented by F, to measure capacitance. One farad is actually a large amount, so the microfarad, the nanofarad, and even the picofarad are more commonly used.

Sometimes you'll notice two different kind of capacitors used in parallel because capacitors aren't perfect and can respond differently to different frequencies, so we use two different types where we want to make sure all frequencies are seeing the capacitance.

Inductor

Probably the easiest component to make yourself, an inductor is usually just a coil of wire, sometimes with a ferrite core, though even a straight conductor has some inductance. An inductor resists changes in the flow of current, thus it suppresses AC, especially high frequencies, but allows DC to pass. That's the opposite of what a capacitor does. An inductor generates voltage, sometimes high, to resist changes in current flow. You sometimes have to be careful the induced voltages don't damage other components. A flyback diode is sometimes used for this purpose.

An inductor works because voltage is generated when a magnetic line of force crosses a conductor, even a line of force of a self-generated magnetic field. Every turn in a coil generates voltage in response to a changing magnetic field, so the more turns, the more voltage can be generated.

Sending an AC signal through an inductor can result in the signal being seen in another nearby inductor, especially if the angles are right, and this is called inductive coupling. Sometimes it's desired, and sometimes it's unwanted.

This is also how a transformer works. An old-fashioned wall-wart is a step-down transformer, often 10:1 for 12 VAC output from 120 VAC input. A transformer can have a core of air, ferrite, or laminated steel plates, which reduce eddy currents to improve efficiency and were most common in the wall warts.

Inductors can be useful sometimes, but I worry that overuse or careless use of inductors can cause problems with noise and feedback. Consider adding shielding or avoiding inductors where possible, especially if a circuit has issues.

Bridge Rectifier

Diode

A diode is a valve for electricity. It allows current to flow in only one direction. Now consider this: a water valve has a spring, and the water pressure has to overcome the force of the spring before the valve can start to allow water through. A diode is the same way! This is called the voltage drop. A silicon diode has a voltage drop of about 0.7 volts. Germanium diodes have less drop, usually 0.3 volts, so are used as detectors in crystal sets. With a lower voltage drop, less voltage is required for the diode to begin to function.

A detector is the part of a radio that converts the radio signal into an audio signal to be amplified for your speakers, and a diode is the main part of a simple AM detector.

Diodes are also commonly used to convert an AC power supply, as from a wall wart, into a DC power supply. Typically, 4 diodes are used, often in a single component called a bridge rectifier. You can also rectify AC using a two-diode circuit for applications needing higher voltage and less smoothness.

You can allegedly make a diode by touching a conductor to a metal oxide, such as copper oxide or iron oxide. I've heard stories of people making diodes during WWII by touching a conductor to a rusty razor blade. It requires some effort to find the right spot.

Zener Diode

This is a special kind of diode often used to provide a reference voltage. Zener diodes are produced for a variety of voltages, including 3, 5.1, 12, 15, and more.

Resistances in Series

If we need to find the total resistance of a series of resistances, we just add the resistances together.

Resistances in Parallel

It's sadly not quite as simple as a series, but interestingly, the conductivity is the sum of the parallel conductivities, and conductivity is the inverse of resistance, so we add the inverse of the resistances and invert the result.

For two parallel resistances:
Ohms=1/((1/R1)+(1/R2))

And for three parallel resistances:
Ohms=1/((1/R1)+(1/R2)+(1/R3))

And likewise for more parallel resistances.

Voltage Divider

Voltage Divider

If I have two resistors that are exactly the same, and I apply voltage across them in series, I'd measure half the voltage at the point where they're connected. Of course, any load on that is going to lower the voltage further because it's like another resistance added in parallel, allowing more current in the bottom half of the divider. The current used by the load must be significantly less than the current going through the divider to avoid bringing down the voltage. Recommendations are 1/10th or less.

High-pass RC Filter

RC Filter

Imagine a voltage divider, as discussed above, but replace one of the two resistors with a capacitor. As you may recall, a capacitor provides a variable resistance to AC depending on frequency. Thus we can create a voltage divider that outputs more of the higher or the lower frequencies depending on how we design it.

When using filters other than active filters, it may help to isolate stages with a buffer or amplifier.

LC Filter

A capacitor and an inductor both cause a shift in the phase of an AC signal depending on frequency, but in opposite directions. After passing through a particular capacitance, some specific frequencies in a signal are 180 degrees out of phase with the same frequencies in the same signal that passed through an inductor, thus they cancel. This way, we can design a filter that eliminates any particular frequency.

By placing the inductor and capacitor in series instead of parallel, we can make a filter not for blocking but for allowing only a particular frequency to pass through. This is how many radio receivers tune to a particular frequency.

Vacuum Tube

It will be easier to understand transistors if we talk about vacuum tubes first.

Diode

Most modern diodes use semiconductors, but in a diode vacuum tube, there are two electrodes, and one is heated. The heated electrode emits electrons easier, thus the diode acts as a one-way valve for electricity.

Triode

Make a diode, but include a grid of fine wire between the plates, and by applying voltage to this grid, we can switch on or off the current or amplify a signal. This is the triode. Because electrons are charged, the voltage on the grid affects the flow of electrons through the device. Voltage on the grid creates force on the electrons, the same kind of force that causes lightweight items to move due to static electricity.

The tetrode and pentode vacuum tubes added extra grids for shielding.

NPN transistor as a voltage amplifier and as a current amplifier

Transistor

The transistor has replaced the vacuum tube except in a few specialty applications, like powerful radio transmitters and some fancy guitar amps, but it does the same sort of thing using some semiconductor voodoo that some say was made by reverse engineering UFOs.

NPN transistor as a simple audio amplifier

There are many different kinds of transistors, but perhaps the most common is the NPN, which is a bipolar junction transistor, or BJT. This switches on when the voltage at the base is more positive than the negative side. It's actually a tiny amount of current and not the voltage that operates the BJT. For the current to flow, the base voltage must be more than 0.6 or 0.7 volts above the emitter due to the voltage drop at the PN junction.

The mirror-image twin of the NPN is the PNP transistor, which switches on when the base is more negative than the positive side. But the PNP uses weird things called "electron holes" instead of electrons, and these aren't quite as good, so we try to use NPN where possible. The PNP is also a BJT transistor.

Besides BJTs, there are also field-effect transistors, or FETs, which operate by voltage instead of current, pinching off the flow of electricity via an electric field, offering a higher resistance to input and greater sensitivity than BJTs.

Voltage Amplifier

One common type of transistor circuit is the voltage amplifier, often used to provide a simple way to amplify audio and radio frequencies, AF and RF. It works because when the base voltage becomes slightly higher, the resistance of the NPN transistor drops, so the output voltage drops. And of course when the base voltage becomes lower, the resistance of the transistor increases, so the output voltage increases.

The bottom resistor isn't strictly necessary but makes the transistor operate in a more stable and predictable way by providing negative feedback for the base circuit. The amount of amplification is calculated as the ohms of the top resistor divided by the ohms of the bottom resistor. A single transistor can typically provide up to 50-200x amplification. A darlington transistor is basically two in one, so provides more amplification.

NPN transistor as a voltage regulator, using a zener diode for reference voltage

Current Amplifier

Another common type of transistor circuit is the current amplifier. This is how a voltage regulator works. You create a weak but stable reference voltage, and use a transistor as a current amplifer to allow loads to draw more current. This is also how a buffer works. A buffer allows an audio signal to be combined with others without influencing the signal upstream, it allows filtering with less distortion, and it allows a weak signal to power heavier loads.

Square wave oscillator using NPN transistors

Op Amp

The operational amplifier, op amp for short, is a thing that behaves in a way that allows it to be used for amplifiers, oscillators, signal buffers, and other kinds of circuits. There are inexpensive little chips that contain 2 or 4 op amps found in many electronics.

In addition to the common power supply pins, each op amp has a positive input pin, a negative input pin, and one output pin. When the voltage of the positive input pin is greater than the voltage of the negative input pin, the output voltage will be high. When the positive input pin is less than that of the negative input pin, the output will be low.

Op amps are designed to be very sensitive, offering a high resistance to the inputs and drawing very little current through the inputs.

It's important to avoid over-amplifying any high frequency signals, because the output of the op amp has a limited slew rate, which is the fastest the output can change voltage. A particular op amp by itself might be able to amplify 15 kHz AC up to 1 volt, but not to 5 volts.

Some op amps are made to be able to deliver more current, but an op amp can also be used to drive a transistor, allowing it to control things that require more current than it can output, such as motors or speakers.

Unintended Side Effects

You have to be careful about unintended side effects, especially when dealing with high frequencies, high currents, or high voltages.

Besides carrying current from one location to another, even a simple piece of wire also acts as a resistor and an inductor. And it forms a capacitor with any other nearby conductors. Inter-electrode capacitance is often a serious problem in electrical components, harming high-frequency performance.

And signals can bleed between any two nearby conductors via capacitive or inductive coupling. To prevent crosstalk between traces in circuit boards, additional grounded traces are often required.

Think Modular

If you want to build a complicated circuit, it helps to design, construct, and test it in a modular way. For a radio receiver, you need a tank circuit that resonates at the desired frequency, connected to an antenna and possibly to ground. You also need an RF amplifier stage, a detector stage, an audio amplifier stage, and a speaker or earphone.

Coupling

How will signals pass between your stages? With inductive coupling, a transformer or a pair of inductors is used to pass the output of one stage to the input of another, but this adds a low-pass filter effect. With capacitive coupling, a capacitor is used, which is simpler, but this adds a high-pass filter effect. Combining inductors and capacitors can form a bandpass filter, which eliminates frequencies too low or too high. Otherwise, a direct-coupling design can be tricky but allows all frequencies to be coupled equally.

Harmonics

Anything but a pure sine wave includes harmonics, which are multiples of the original or desired frequency. A square wave contains a lot of harmonics.

Modulation

AM radio uses amplitude modulation, which can be more susceptable to interference than frequency modulation as used in the FM broadcast band, which also benefits somewhat from a higher frequency. In AM, you change the amplitude of the carrier to send the signal. In FM, you change the frequency of the carrier to send the signal. There's also phase modulation, and there are other kinds of modulation.

Bandwidth

If a radio carrier is transmitting on 1 MHz and you modulate it with audio up to 10 kHz, there are sidebands 10 kHz wide, so the transmission is actually spread between 990000 kHz and 1100000 kHz, called the bandwidth. Except single-sideband, SSB, has the bandwidth on one side only, upper or lower, USB or LSB.

The old analog NTSC broadcast TV channels each had 6 MHz of bandwidth to carry the video and audio signals. The audio was in stereo and the video was in color, though only 480 lines.

Mixing

When two frequencies are combined in a mixer, which is any nonlinear component like a diode, new frequencies are created that are the sum and difference of the original frequencies. So mixing 2400 Hz and 2500 Hz will produce 100 Hz and 4900 Hz frequencies.

Heterodyning

Many radio receivers use heterodyning, which is where the receiver is designed to receive only one frequency, yet it's able to tune an entire band by mixing the incoming radio signals with the output of an oscillator.

Suppose the receiver is built to receive 10.7 MHz, a common intermediate frequency, or IF. To tune to 100.7 MHz, we change the frequency of the local oscillator, or LO, to 90 MHz when we turn the dial. This local oscillator is in essence a very small transmitter and can often be detected within a short range, potentially revealing the presence of a receiver and the frequency it's tuned to.

Multiplexing

This is a fancy word for any way of sending multiple signals on the same medium. A coaxial cable, for example, can carry an entire radio spectrum, so you can send almost any number of radio signals through a coax, in which case we're multiplexing by frequency.

In the ancient past when I grew up, long distance phone conversations were sent over cables or twisted pairs on different frequencies, and conversations on adjacent frequencies were often faintly audible. It is said that this was more common when the conversations passed through aging equipment that had become detuned.

Another form of multiplexing is by time. If we have two audio signals, we could send a sample of one signal, then send a sample of the other. By alternating back and forth, we need only one wire to send both signals. The trick is to switch back and forth fast enough. The other trick is for the recieving side to know how to separate the two signals. In this case, you might include a short period of low signal between the two samples, so when the low signal is seen, the receiving circuit knows to switch signals. Maybe we want a 32 kHz sample rate for goodish audio quality. Multiplied by two audio signals, that's a 64 kHz data rate. To achieve this rate, we could send one sample, a low signal, the other sample, and another low signal, each for 3.9 microseconds, and repeat. I say 3.9 millionths of a second because that's 1/64000/4 seconds, for 64000 Hz and 4 stages: sample 1, low, sample 2, and another low.

And More

Coming soon.

Illustrations were created using drawCircuit.