Understanding basic electricity can be invaluable in both safety, as well as when troubleshooting and repairing an electrical circuit. We will add more posts soon on many topics such as how to build a solar generator, and this post will help build the basics on which those posts will expand.
Basic electrical theory often compares electrical current to water current. There are many similarities, and it is much easier to visualize the physics behind water current, so it is a great place to start.
Just like water flows from high ground to lower ground, electricity flows from higher potential to lower potential. Electrical potential is measured as voltage, and in the water comparison it would be the equivalent to water pressure.
Just as higher water pressure can increase water flow, the same is true for voltage. The relationship between voltage and current is very important to understand, and is one of the first things to confuse people learning about electricity. Voltage and Current are not the same thing, but they are related to each other. Higher voltage will try to push more current through a circuit, but the resistance of that circuit will limit how much current can flow. Imagine a water tank that is under 50psi of pressure. Connected to that tank is a 1/4” pipe. Water will flow through that pipe, but the size of the pipe is limiting the total amount of flow. Increasing the size of the pipe will increase flow, but increasing the tank pressure would also increase the flow. In this example, the water tank could represent a battery, and the pipe could represent a wire.
A 12 volt car battery has the ability to flow a lot of electrical current, but if there is nothing connected to it, then there is no current. If you connect a circuit to the battery with a high electrical resistance, such as a dome light, then the current flow will be very small. If you connected a circuit with a very low resistance, such as enabling the starter motor, then a very large current will flow.
The human body also has an electrical resistance. This is why we can be electrocuted, although our electrical resistance is high enough that very little current will flow unless the voltage is high. It is harmless to touch both sides of a 12 volt car battery with both hands, but if we were to touch 110V AC mains, we would receive a dangerous voltage shock.
Wires themselves have internal resistance, as do every single connection along the electrical path. Having good solid connections is very important, otherwise the internal resistance will be very high, and prevent the proper amount of current from flowing. A larger, thicker wire has a lower internal resistance than a small, thin wire.
Ohms law is a very good concept to understand. It defines the relationship between voltage, current, and resistance. If you know two of the above values, you can calculate the third with a very simple formula:
Voltage = I (current in amps) x R (resistance in ohms)
You can re-arrange the formula to solve many basic problems. For example, if you knew a heater element had a resistance of 6 ohms, and you wanted to connect it to a 12 volt battery. You can calculate what the current draw will be as 12 volts / 6 ohms = 2 amps of current. Now that you know the current your heater element will draw from the battery, you can make sure any wires, relays, switches, etc. would be rated for the task.
Another very simple but related formula is how to calculate Power, which is measured in Watts. The power generated by a circuit is simply the voltage multiplied by the current:
P (power in watts) = Voltage x I (current in amps)
So for our above example, we can calculate that our 6 ohm heater element when connected to a 12v battery will produce 12 volts * 2 amps = 24 Watts of power.
This leads us to explain why a wire needs to be sized correctly. Remember that all wires have an internal resistance, and that the internal resistance goes down as the wire gets thicker? If a wire is flowing a current, that resistance produces heat. If the current is too much for a wire, it can produce so much heat that it melts the plastic insulation, or sometimes the entire wire will burn and can even start a fire.
An electrical connection between two points is often called a short. If a wire connected to a car battery is accidentally shorted to the chassis (usually connected to the ground side of the battery), it will cause a large amount of current flow and melt the wire. To prevent this happening, a fuse is typically wired into a circuit as close to the positive side of the battery supply as possible. The fuse should be sized just larger than the highest current ever expected to flow for that circuit. If a fuse blows and is replaced by a size too large, the potential for a wire to catch fire before the fuse blows becomes possible.
The primary difference between DC (direct current) and AC (alternating current) like what is supplied by the electrical grid is that with AC the voltage oscillates between positive and negative 60 times per second (in the US). The fluctuation is smooth and follows a sine wave pattern. By doing this, it makes converting the energy to different voltages and transmitting it far distances much easier. This concept was pioneered by Nikola Tesla in the late 1800’s.
220V Single Phase AC is what the vast majority of homes in the US use for power. Single Phase means there is only one sine waveform. Industrial applications will often overlap multiple sine wave supplies, such as 3 phase. This allows them to run large motors more efficiently than can be done with single phase.
220V AC enters a home supplied between two primary wires called the mains. The power is then split in half with another wire in the middle called the Neutral. At the main panel the Neutral is also tied to earth ground. From there the 110V AC is supplied to the various circuits in the house with either one of the two mains, a neutral wire for the return path, and an additional ground wire as a backup safety path for the current. A few circuits that require 220V will supply both mains, so that the full 220V can be utilized for higher power applications.
Troubleshooting An Electrical Circuit
Whenever you are troubleshooting an electrical circuit, try to check as many things as possible while power is removed. For automotive applications, unhook the negative terminal of the battery to remove power. Always unhook the negative cable first, and reconnect it last, because this will prevent you from accidentally shorting your wrench to ground when disconnecting / connecting the positive cable. Some vehicles have multiple batteries, so be sure they are all disconnected when trying to remove power.
When working on AC power, always turn off the breaker for that circuit whenever possible. If you are not sure which breaker, you can turn off power to the entire house with the main breaker. Always double-check with a volt meter that power is off when working with dangerous voltages such as AC.
A lot of electrical circuits can be fixed by simply following the electrical path and looking for loose or broken connections, blown fuses, melted connectors etc. Inspect each point closely as you follow the electrical path of a circuit from positive all the way back to where it returns back to the ground and into the negative side of the circuit. If you still cannot find the source of an issue, you can then try to turn the power back on, and then carefully once again follow the electrical path while wiggling various connectors and components to see if you can find any poor connections. You can also use a volt meter to see if you still have power at the expected points along the path. Always be very careful not to put yourself into the circuit. Remember that higher voltages such as AC in your home or high energy ignition systems on a vehicle can both shock you very easily.
I will add future posts with more specifics for understanding wiring diagrams and schematics, and tips for how to use them to troubleshoot a circuit even further.