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PART SIX – BASIC LAYOUT WIRING
By John Firth
Electrical Basics
Safety
Before proceeding with a discussion about layout wiring, the importance of safety must be stressed. Electricity, in the wrong dosage, can be a little bit fatal. The principal dangers are electrocution, fire and explosion.
Mains voltage in the country is 240 volts and having sampled it for myself, it is not an experience I would choose to repeat; indeed, I am now extremely careful.
Fire can arise as a result of overheating due to large currents flowing through inadequately sized cables, power packs and transformers or loose terminations.
Explosions can result from the energy dissipated when a short circuit occurs. If the current is not limited, molten metal can fly through the air.
Having said all of this, the safety equipment built into the electrical equipment used in modelling should reduce the risks to a tolerable level.
So what is electricity?
One problem with understanding electricity is that it cannot be seen. Sometimes (as in lightning) you can see where it has been and what it has done, but you can’t see it.
An analogy with water is a useful aid to understanding it provided the analogy is not taken too far.
Head
Flow
So 1 milliamp is 1/1000 amps or 0.001amps or 1mA, 1 kilovolt is 1000 volts or 1kV and 1 megavolt is 1,000,000 volts or 1MV. It is advisable not to get an “m” mixed up with an “M”.
One of the best metallic conductors (lowest resistance) is copper with only silver better. Nickel silver is an alloy of 60% copper, 20% nickel and 20% zinc and NO silver. Pure copper is about 20 times better as a conductor than Nickel Silver.
We use nickel silver rail for a number of reasons. Apart from its colour more closely approximating the steel rail surface than copper, the most important reason is that when it forms an oxide layer, its oxides are conductive. Brass is 6 times better at conducting electricity than nickel silver but it oxidises more readily and the oxides are less conductive (have a higher resistance) and hence it needs more track cleaning effort.
Recommended Tools
Apart from the normal tools (screwdrivers, etc), the following are recommended for wiring a layout:
Soldering Iron – a basic 30 watt soldering iron is recommended for soldering wires to rails. The iron needs to be able to heat up the area around the joint to the melting point of the solder. The larger the mass that has to be soldered, the more power the iron needs.
Meter – A basic digital multi-meter is useful for testing. These can be obtained for £5-£10. Typically the cheap meters measure AC volts up to 750 volts, DC volts from a few millivolts to 1000 volts, DC current up to 10 amps, and resistance from a few ohms to 2 megohms. The cheapest meters frequently do not measure AC current but you will probably not need to measure AC current very often.
WARNING – most digital meters have different terminals for measuring current to voltage. Attempting to measure voltage with the leads plugged into the current measuring terminals will short circuit the voltage that you are testing. This could damage the meter, the equipment being tested, or YOU.
Crocodile clips – lengths of flexible insulated wire fitted with crocodile clips at both ends are a useful way of making temporary connections whilst testing.
Switches
If you are building a control panel, then you are likely to need switches. Switches come in a large number of varieties.
The simplest switch is the simple On-Off switch. This is termed Single Pole (because it only breaks a single circuit) Single Throw (because there is a single contact to make or break). This type of switch is abbreviated SPST. If the switch has two positions in which it makes a contact and can simultaneously switch two circuits, it is termed a Double Pole Double Throw (DPDT) switch. Switches come in a number of flavours including toggles, rotary or slider switches to name but a few.
Rotary switches can have up to 16 positions and up to 10 poles but there is not a switch currently available that has both of these properties. When choosing a switch, it is worth checking the specification. It is likely to specify the maximum a.c. and d.c. voltage at which it can work, together with maximum a.c. and d.c. current. In
In the diagram above, water will flow from the tank through the pipe. The flow (the volume of water passing a point in the pipe in a certain time) is the equivalent of electrical current. In electricity it is the number of electrons passing a point in a certain amount of time. Electrical current is measured in Amps.
The head (the height of the water column) determines the pressure forcing the water to flow. This is the equivalent of electrical potential difference or voltage. Electrical voltage is measured in Volts.
The third part of this analogy relates to the size of the pipe. If the diameter of the pipe is increased, then more water can flow in the same time. If the diameter is reduced, the water flow is reduced. The electrical equivalent of this is electrical resistance. Electrical resistance is measured in Ohms (symbol Ω).
A good electrical conductor such as copper has very low resistance whereas an insulator such as glass has extremely high resistance.
Electrical measurements often use prefixes associated with size. The most frequently encountered are:
Mega – meaning million denoted with a capital M
Kilo – meaning thousand denoted with a lower case k
Milli – meaning a thousandth denoted with a lower case m