Basic Electronics I: Electrons and Circuits
Electrons are negatively charged particles (parts of atoms) that can move or be moved within certain conductive materials like copper. If a negative charge (from a power source) is applied to one end of a copper wire and a positive charge applied to the other end, the electrons will move away from the negative end toward the positive end. (Opposite charges always attract each other; same charges oppose each other.) This movement happens immediately throughout the course of the wire when the charge is applied. The presence of a negative charge being on one end of the path and a positive charge on the other end is called polarity.
The charge that is applied to the wire is the power source and can originate from a battery, or from a wall outlet, in which case, it usually goes through a transformer which automatically reduces the voltage from 110 down to a much lower level like 12 or 16.
Moving electrons can be put to work. For example if electrons are forced to flow through a tiny filament that has a lot of resistance, then the filament can heat up so much that it produces light. Or if the electrons are forced to go round and round in a circular fashion they create a magnetic field that can be used to run motors.
A circuit is a path that electrons follow. An open circuit is one where the path of electrons is interrupted, like with a switch. A closed circuit is one where the electrons are flowing, like when the switch is turned on.
A short circuit is one where the electrons are inadvertently bypassing the device you want to operate and going directly from the negative to the positive terminal of the power supply. In this case the device, to which you are trying to supply power, doesn’t get any, and the power source gets overheated. If this happens in the presence of a circuit breaker, the power will be automatically turned off and your power source will not be damaged. You then need to troubleshoot your circuit to find where the problem is.
Basic Electronics II: Ohm’s Law
Voltage is the force making the electrons move, sort of like the water pressure in pipes. Current is the rate of flow of electrons through a closed circuit, like the flow of water through a faucet.
Resistance is anything that holds up or slows the flow of electrons, like the valve on a faucet.
The higher the resistance, then the lower the current, assuming the voltage is constant. The more you tighten the valve on the faucet, the slower the water flows.
This relationship between voltage (V), current(I) and resistance(R) is shown in the equation, V=IxR,
where V is the voltage in Volts, I is the current in Amps, and R is the resistance in Ohms.
This equation is also known as Ohm’s Law, and is the most important rule in electronics. In fact, that’s a good way to remember it, as the Very Important Rule (V=I/R).
You can say the same equation in different ways:
Basic Electronics III: Resistors
Say you have a transformer that supplies 12 volts to your circuit, and you have a light that is rated as 2 volts and 20 milliamps (That means that the maximum that bulb can take is 2 volts and 20 milliamps). Then how do you figure what kind of resistor to add to the circuit so your bulb won’t burn out in the first 5 seconds?
Since we want to know what R is, we will use the equation R=V/I.
First you need to subtract 12-2 which is 10. This is the amount of voltage reduction required.
The maximum current is 20 milliamps but the equation calls for amps, not milliamps. So you have to convert 20 milliamps to amps by dividing by 1000. You get 0.02 amp.
Substituting the numbers for the letters in the equation, R=V/I, we find that
R= 10 divided by 0.02,
which comes out to 500 ohms.
So all you have to do now is go to your favorite electronics parts store and ask for a 500 ohm resistor. What if they don’t have one in that size? Then ask for the next highest one, which will likely be a 560 ohm resistor.
Basic Electronics IV: Too Much or Too Little Voltage
If you want your light to last longer, then underpower it by using an even higher resistor or a lower voltage transformer. The light may be dimmer but it will last longer and you actually may like it better if it’s not so bright.
If you overpower a device, by applying more voltage than it can handle, the device may be damaged. If you apply 12 volts to a light bulb that has a maximum rating of 6 volts, it will burn brightly for a short while , but it will burn out quickly. Similarly, if you try to run an N scale locomotive with the higher voltage required to run an O scale locomotive, the motor will burn out quickly. (Don’t ask me how I know this!)
Basic Electronics V: Lights
Incandescent lights can take either AC or DC voltage and can be connected in either direction within the circuit. In other words, it doesn’t matter which wire goes to the positive terminal and which one goes to the negative. It still works.
LEDs (light emitting diodes), on the other hand, can only take DC, and it must be connected in the right direction within the circuit. The LED has a short lead and a long lead attached to it. You have to connect the longer lead on the LED to the positive side of the power source and the shorter lead to the negative side. Also you MUST use a resistor in series with each LED that you use. Otherwise it will likely burn out very quickly. If you use the right resistor, it can last as long as 50,000 hours.
Basic Electronics VI: Other Devices
Capacitors are devices that will hold a charge for a while even when the power supply is off. They also have to be wired in the proper direction. They are not required for most general model railroad wiring.
A relay switch is useful for activating a light or a motor when a turnout is thrown one way or another. For example, if you want your ferris wheel to light up and start turning when the turnout is thrown to that branch, you could use a relay switch. (Also see page on Electrical Switches)
A potentiometer can be wired as a rheostat and is used as a dimmer for lights or to slow an animation device. It basically increases the resistance as you turn the knob. When a potentiometer is used for this purpose, you have to wire 2 of the 3 leads to each other.
Basic Electronics VII: Wires and Connectors
The larger diameter the wire is, the less resistance there will be to the flow of electrons. If your wire is too thin for the amount of current flowing through it, it may become overheated. The size of the wire diameter is referred to as the gauge. The larger the number of the gauge, the smaller the diameter of the wire. 12 gauge is pretty thick. 30 gauge is very thin.
A buss wire is a large diameter wire (like 14-16 gauge), that you can run along the bottom of your layout and attach track feeders to it. Usually you will use one color buss wire for the positive rail and another color for the negative rail. These buss wires will then be connected to your main power transformer that powers your track.
18-22 gauge wire would be okay to use for track feeders, turnout switches and lights.
Stranded wire is less likely to break if it’s in a place where it will be flexed a lot, like near the hinge of a control panel that you open and close. A loose strand, however, could potentially cause a short circuit. Solid wire is easier to loop around a connecting screw and you don’t have to worry about those pesky strands, but it breaks easier if it’s moved a lot.
When connecting 2 or more wires together, twist them in a clockwise fashion and then apply the screw cap in a clockwise fashion. Make sure you can’t pull one or more wires out of the cap when you’re finished.
You can use the so-called suitcase connectors to attach feeder wires to buss wires. but make sure you use the right size for the wires you’re connecting. The appropriate gauge wires will be printed on the package of connectors.
Also, see the page on Model Railroad Wiring Tips.
References on Basic Electronics for Model Railroaders:
“Very Basic Electronics”, by Bob Kendall, N-Scale, March/April, 2011, p45
, by Dickon Ross