02 May, 2021
This page presents basic information about electronics for Model Railroaders. Information on various components and simple circuits that a model railroaders are likely to use in their hobby will appear here.
Electronics theory will be will be kept to a bare minimum.
Bear in mind that the information on these pages is geared toward people with very little electronics experience and is not meant to be an engineering resource. The content of the pages will therefore be broad and leave a fairly wide margin for safety.
For the purposes of this page, it will be assumed that current flow is from positive to negative.
This page 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 will brief and to the point.
For the purposes of the Basic Electronics pages at this site it will be assumed that current flow is from the positive to the negative.
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 perameters.
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 Is the rate of electron flow in the circuit. The greater the rate of electrons flowing the greater the work that will be done.
RESISTANCE Is 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.
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.
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 is 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 is 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.
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.
Three special types of diodes the model railroader is likely to encounter are the Small Signal diode, Schottky diode and Zener diode. A brief description and some uses of each follows.
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.
Bipolar transistors used in model railroad electronics come in two baseic 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.
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.
The step down transformer is the interface between the household Alternating Current 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 to a safer and more practical voltage.
There is also a secondary function that often goes un-noticed 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.
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 dependant 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.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
02 May, 2021