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Basic Electronics

Basic Electronics

An Introduction on how electronic components and circuits work in an easy to understand language.

Presented here is information on the most commonly used components and examples of how to use them in circuits.

Electronics Theory has deliberately been minimized to make it more easy to understand by non-engineers, bearing in mind that the information on these pages is geared toward people with very little electronics experience. The content of the pages will therefore be broad and leave a fairly wide margin for safety.


By the time the true direction of electron flow was discovered, the nomenclature of “positive” and “negative” had already been so well established in the scientific community that no effort was made to change it, although calling electrons “positive” would make more sense in referring to “excess” charge. You see, the terms “positive” and “negative” are human inventions, and as such have no absolute meaning beyond our own conventions of language and scientific description. Franklin could have just as easily referred to a surplus of charge as “black” and a deficiency as “white,” in which case scientists would speak of electrons having a “white” charge (assuming the same incorrect conjecture of charge position between wax and wool).

However, because we tend to associate the word “positive” with “surplus” and “negative” with “deficiency,” the standard label for electron charge does seem backward. Because of this, many engineers decided to retain the old concept of electricity with “positive” referring to a surplus of charge, and label charge flow (current) accordingly. This became known as conventional flow notation:

electronics_conventionalflow
Others chose to designate charge flow according to the actual motion of electrons in a circuit.

This form of symbology became known as electron flow notation:

electronics_electronflow

In conventional flow notation, we show the motion of charge according to the (technically incorrect) labels of + and -. This way the labels make sense, but the direction of charge flow is incorrect. In electron flow notation, we follow the actual motion of electrons in the circuit, but the + and – labels seem backward. Does it matter, really, how we designate charge flow in a circuit? Not really, so long as we’re consistent in the use of our symbols. You may follow an imagined direction of current (conventional flow) or the actual (electron flow) with equal success insofar as circuit analysis is concerned. Concepts of voltage, current, resistance, continuity, and even mathematical treatments such as Ohm’s Law (chapter 2) and Kirchhoff’s Laws (chapter 6) remain just as valid with either style of notation.

You will find conventional flow notation followed by most electrical engineers, and illustrated in most engineering textbooks. Electron flow is most often seen in introductory textbooks (this one included) and in the writings of professional scientists, especially solid-state physicists who are concerned with the actual motion of electrons in substances. These preferences are cultural, in the sense that certain groups of people have found it advantageous to envision electric current motion in certain ways. Being that most analyses of electric circuits do not depend on a technically accurate depiction of charge flow, the choice between conventional flow notation and electron flow notation is arbitrary . . . almost.

Many electrical devices tolerate real currents of either direction with no difference in operation. Incandescent lamps (the type utilizing a thin metal filament that glows white-hot with sufficient current), for example, produce light with equal efficiency regardless of current direction. They even function well on alternating current (AC), where the direction changes rapidly over time. Conductors and switches operate irrespective of current direction, as well. The technical term for this irrelevance of charge flow is nonpolarization. We could say then, that incandescent lamps, switches, and wires are nonpolarized components. Conversely, any device that functions differently on currents of different direction would be called a polarized device.

There are many such polarized devices used in electric circuits. Most of them are made of so-called semiconductor substances, and as such aren’t examined in detail until the third volume of this book series. Like switches, lamps, and batteries, each of these devices is represented in a schematic diagram by a unique symbol. As one might guess, polarized device symbols typically contain an arrow within them, somewhere, to designate a preferred or exclusive direction of current. This is where the competing notations of conventional and electron flow really matter. Because engineers from long ago have settled on conventional flow as their “culture’s” standard notation, and because engineers are the same people who invent electrical devices and the symbols representing them, the arrows used in these devices’ symbols all point in the direction of conventional flow, not electron flow. That is to say, all of these devices’ symbols have arrow marks that point against the actual flow of electrons through them.


Really Basic Electricity

This article provides some really basic information on Direct Current electricity. It might be called “Theory” but will not be very in depth. The diagrams will be basic and the explanations mostly brief and to the point.

BasicElectricity1 Every circuit must contain the following elements; A Source of Electrons and a Load. These elements are able to produce useful work from the circuit and can be combined in an infinite number of ways to form any circuit.

  • SOURCE OF ELECTRONS
    The source of electrons in the circuit can be any of a wide range of devices.
    These include batteries, photovoltaic cells and thermocouples. The most usual source of electrons in model railroad circuits is the secondary of a stepdown transformer.
  • LOAD
    The load in the circuit coverts the flow of electrons into usable work.
    The work can include the production of light from light emitting diodes or the creation of a magnetic field in a motor or switch machine.

To provide a way of determining the work done by a circuit the energy in the circuit can be expressed using the following parameters.

  • VOLTS
    Is the force that causes the electrons to flow through a circuit.
    The greater the force, the higher the number of electrons that can be forced through a given circuit.
  • AMPS
    The rate of electron flow in the circuit.
    The greater the rate of electrons flowing the greater the work that will be done.
  • RESISTANCE
    The opposition to the flow of electrons in the circuit.
    The greater the resistance, the lower the rate of electron (Amps) flow in a circuit for a given voltage.

 


 Diodes and Rectifiers

The primary function of diodes in model railroading is to allow the flow of current in one direction only. This is generally referred to as rectification.

Diodes have many uses such as to convert an alternating current to a direct current for power supplies and throttles and to route current in matrix circuits for switch machine controls.

 

A diode is a one-way “valve” for electric current, analogous to a check valve for those familiar with plumbing and hydraulic systems. Ideally, a diode provides unimpeded flow for current in one direction (little or no resistance), but prevents flow in the other direction (infinite resistance). Its schematic symbol looks like this:

electronics_diode

 

 

 

Placed within a battery/lamp circuit, its operation is as such:

electronics_diode_working

 

 

 

 

 

 

When the diode is facing in the proper direction to permit current, the lamp glows. Otherwise, the diode blocks all electron flow just like a break in the circuit, and the lamp will not glow.

If we label the circuit current using conventional flow notation, the arrow symbol of the diode makes perfect sense: the triangular arrowhead points in the direction of charge flow, from positive to negative:

electronics_diode_working_conv

 

 

 

 

 

On the other hand, if we use electron flow notation to show the true direction of electron travel around the circuit, the diode’s arrow symbology seems backward:

electronics_diode_working_electron

 

 

 

 

 

 

For this reason alone, many people choose to make conventional flow their notation of choice when drawing the direction of charge motion in a circuit. If for no other reason, the symbols associated with semiconductor components like diodes make more sense this way. However, others choose to show the true direction of electron travel so as to avoid having to tell themselves, “just remember the electrons are actually moving the other way” whenever the true direction of electron motion becomes an issue.

 BasicDiode1

 

 

 

 


 Diode Voltage And Current Ratings

Two families of rectifier diodes are widely used in the hobby, the 1N40xx and 1N54xx series.

The 1N40xx diodes have a 1 amp current rating with voltage ratings from 50 to 1000 volts.
The 1N54xx diodes have a 3 amp current rating with voltage ratings from 50 to 1000 volts.

When selecting diodes, two device ratings must be taken into consideration; Peak Reverse Voltage and Maximum Average Current.

  •   Peak Reverse Voltage or Peak Inverse Voltage
    The maximum voltage that a diode can withstand in the reverse direction without breaking down and starting to conduct. If this voltage is exceeded the diode may be destroyed. Diodes must have a Peak Inverse Voltage rating that is higher than the maximum voltage that will be applied to them when reverse biased.

In DC only circuits, diodes should have a Peak Inverse Voltage rating greater than the highest voltage to which diode will be exposed.

In AC circuits, such as power supplies, diodes should have a Peak Inverse Voltage rating greater than 1.4 times the maximum RMS voltage of the transformer secondary.

  •   Maximum Average Forward Current
    The average forward current that a diode can conduct without being damaged.

In DC only circuits the Maximum Average Current is, generally, the current that the diode will continuously conduct.

In AC circuits such as power supplies the Maximum Average Current Rating of a diode should be twice the DC current that the supply will deliver at full load. For example; If a power supply can deliver 1 amp the rectifier diodes should have a 2 amp current rating.


 Bridge Rectifiers

A common use of diodes is a rectifier bridge. A bridge is four diodes configured so that the output always has the same polarity regardless of the polarity of the input. Rectifier bridges are most often used to convert alternating current into full-wave direct current for power supplies and throttles.

Bridges can be made from four separate diodes or the diodes can be in one package. Bridge rectifiers are available in a wide variety of voltage and current ratings.

BasicDiode2

 

 

 

 


 Special Diode Types

Three special types of diodes the model railroader is likely to encounter are the
*Small Signal diode
*Schottky diode
*Zener diode

Small Signal Diodes
Such a the 1N914 and 1N4148 are low voltage and current diodes but are very fast which makes then ideal for use with very high frequency circuits. These diodes are used in timing and pulse generation circuits.

Schottky Diodes
Such as the 1N5817 and 1N5820 are low voltage diodes but can handle currents of 1 amp and 3 amps respectively. Schottky diodes are fast and are used in DCC systems where high frequency operation is required.

Schottky diodes also have a lower forward voltage drop than silicon diodes which makes the useful in low voltage circuits.

Zener Diodes
Are normally used for voltage regulation and are available in a large range of breakdown voltage and device power ratings. The use of this type of diode is not as common now but may still be encountered in older circuit designs.

BasicDiode3

 

 

 

 


 Bipolar Transistors

Bipolar transistors used in model railroad electronics come in two basic types, NPN and PNP, and are used in two basic ways, switching and regulating. These transistors are available in a wide variety of voltage and current ratings.

BasicTransistor

 

 

 

 


 Basic Transistor Switches

MOSFET and Bipolar transistors are often used for high speed switching applications. Below are four very simple examples of this. ON-OFF toggle switches are used in place of an electronic control circuit.

 BasicSwitches

 

 

 

 

 

 

 

 


Stepdown Transformers

The step down transformer is the interface between the household Alternating Current (AC) system and the electronics that run your model railroad. The main purpose of the transformer is to reduce the relatively high voltage of the house (220V in South Africa) to a safer and more practical voltage (5V, 12V, 16V).

There is also a secondary function that often goes unnoticed which is to provide electrical isolation between the household supply and the layout. This, in a properly constructed circuit, prevents the layout from being exposed to a dangerously high voltage if there is a fault on the primary side of the circuit.

This isolation also separates the power supplies of each subsystem on the layout from the others. For example; On layouts with multiple DC throttles, each throttle is isolated from the others by its transformer. This allows common rail block wiring without causing a short circuit.

BasicTransformer The capacities of transformers are often given as a Volt/Ampere rating. This is the secondary voltage multiplied by the secondary current at full load and is roughly equivalent to a wattage rating but makes allowances for the peculiarities of alternating current. These allowances are not relevant to this page however.

The secondaries of many transformers designed for low voltage applications are simply rated as secondary volts at full load amps.

Note: The secondary voltage of a transformer is often significantly higher when there is no load than at full load. This is largely dependent on the the design of the transformer itself and is usually not a factor in model railroad circuits. However, in many low power and cheaply constructed transformers that are used in consumer electronics this voltage drop can be significant and must be taken into account when good regulation is needed.

Transformers are generally, very efficient devices and no allowance is usually required for the small amount of heat that they may generate. It is good practice however to provide space around the transformer for air circulation and ventilation for enclosures.

Power supply transformers should always be protected by a fuse on the primary side of the circuit. If a transformer supplies multiple loads, all loads should be be protected by individual fuses.


Article Credit:  Rob Paisley,  updated by Stéfan