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Basic Electricity - DC Circuits

Basic electricity for boat builders, boat repairers and owners. What you need to know about the electrical systems on your boat. Intro to electricity.

(I want to thank Terry King http://terryking.us/, a member of BoatDesign.net for help editing these pages). 

I am going to try to keep this a simple as possible, not because I think you can't understand it.  I assume that to master the art of boat building you have to be smart. I want to this to be simple, but complete enough to give you a basic understanding of how this electrical stuff works. This does not qualify you as an electrician.  Electricians must take years of study and on the job training to pass certification exams.  It will give you a basic understanding of what is going on and how simple circuits are wired.   When I was much younger I worked on radars and computers on Coast Guard ships and when people asked us "how does it work?" we would say, "magically, mystically, wonderfully, electronically".  But it's actually a lot simpler than that. However, to some it still seems like magic.

So what is electricity?  As we all learned in school the world is made up of atoms. Atoms are made up of electrons, protons and neutrons. What we are interested in, is the electrons, surrounding the nucleus of the atom. Electrons can be dislodged from their atom and attached to another atom giving it an excess of electrons. Excess electrons give an atom a negative charge.  Atoms with not enough electrons have a positive charge. Through a chemical reaction or other force such as magnetism, electrons can be made to flow though a conductor from the negative to the positive and used as electricity.  So electricity is really just a stream of electrons flowing through a conductor from point A to point B and back to where they came from. (a complete circuit) This flow is called current.

Basic Battery

All of us have experienced this either by getting a static discharge off of a door knob or other metal object, or by seeing nature's ultimate display of electricity, lightning. This is a simple exchange of electrons from one point to another. We use this every day in our homes, our cars, our cell phones, and every other electric or electronic device we use, but we give little thought to what is actually happening.

Most of us had a science teacher show us an experiment where a strip of lead with a wire attached, and a strip of zinc with a wire attached, were put into a glass jar filled with acid. This is what is called a voltaic cell, or battery. The ends of the wires were connected to a light, and lo and behold it lit up!

 Battery History. http://en.wikipedia.org/wiki/History_of_the_battery

 That is about as basic a battery as there is. What is happening here is not magic, just simple chemistry.  The acid is an electrolyte. An electrolyte is a fluid that allows electrons to flow through it from one pole to another. Salt water is a pretty good electrolyte. Fresh water is not but will still conduct current and is actually more dangerous if current gets into the water. Dilute hydrochloric acid, used in most 12V lead-acid batteries is a very good conductor. (Here is a very technical definition of electrolyte https://en.wikipedia.org/wiki/Electrolyte. Especially, see the section on Electrochemistry.)

When you put two dissimilar metals near each other in an electrolyte, one of the metals gives up electrons to the other metal. But this only happens if there is a complete circuit, which is the wire and the light. So the electrons flow from one plate to the other through the wire and the light and back to the plates. This doesn't go on forever though.  One plate  will gradually disappear and soon there will be no more electron flow. Then the light goes out. Where does the material on the plate go? It gets deposited onto the other plate.  Remember this, because this is crucial to how batteries work, and how galvanic corrosion works. In fact this is called the Galvanic Process.  (See Galvanic Corrosion https://en.wikipedia.org/wiki/Galvanic_corrosion)

If we didn't have a light in the circuit, just a wire, electrons would still flow, but it would happen so fast that our primitive battery would be dead in no time at all, or the wire would get too hot and melt. This is called a short circuit, meaning the positive side of the battery is connected directly to the negative side. To prevent this we put in a load, something in the circuit that creates a resistance in the circuit. In this case the light is the load.  It provides some resistance to the flow of electrons and slows down the process. Also the filament in the light gives off some of the energy of the electrons in the form of light and heat. Otherwise no work would get done by the electrons. So for electricity to do work there needs to be a load in the circuit.  This can be lights, appliances, electronic equipment, or motors, all of which put the electrons to work.

The wire getting hot and melting is the basic principle behind a fuse. If too many electrons are flowing through the wire, the fuse is designed to get hot and melt at a predetermined amount of current flow. This breaks the circuit, or what is called an open circuit, and stops the current flow, the same way throwing a switch does. So a fuse is a safety device to prevent the wire from getting too hot and maybe causing a fire. Note; the fuse is there to protect the wire, not the load. 

Circuit breakers do the same thing as a fuse but can be reset and used again.  How they do this is different than a fuse, but the important thing is they perform the same function;  breaking (opening) the circuit and shutting off the power.

So now we know what current is. It is electron flow. The amount of current is measured in Amperes, or milliamperes. The symbol for Amperes is A or milliamperes mA.

What drives this current is called voltage. Voltage is a measure of potential energy contained in the battery and is measured in volts. The symbol for volts is V.

The load is resistance to current flow and is measured in ohms, named after the guy who discovered that different loads create differing amounts of resistance. The symbol for this is either R or the Greek letter Omega (Ώ). I will just use R.

Types of Electricity

Electricity comes in different flavors, AC and DC. No that is not a rock group.  AC stands for alternating current, which is what we all have in our houses, and most appliances run on AC, and is usually 120 volts, or 220 volts for some large appliances such as clothes dryers.  Where does AC electricity come from?  See https://www.saveonenergy.com/how-electricity-works/

DC stands for Direct Current and is usually low voltage such as 6 volts or 12 volts. Many of the small electronic devices we commonly use, such as cell phones, calculators, and IPods all run on DC.  Your car also uses DC and most small boats use only DC.  However, as the boats get bigger they may use both AC and DC, until you get into the ship sized yachts that use only AC. First though, I will deal with DC because it is simpler to explain and what most small boats use.

Direct current comes primarily from batteries. You can get it by converting AC to DC but for now I will stick with batteries.  The simplest batteries are a single cells such a the D cells used in flashlights, or AA and AAA cells used in most small electronic devices.  (Today many devices use lithium batteries but I will get to that latter) A long time ago, someone discovered that any single cell battery, no matter how big, puts out about 1.2 volts.  Over the centuries by improving the chemistry,  that has been upped to about 2 volts.  The larger you make a cell, the longer it will last, and the more current you will get out of it (amperes) but you still won't get any more than about 2 volts.

So how do we get typical lead-acid cells to be 12 V and 6 V batteries? These are lead-acid type batteries and have about two (2.0) volts per cell, compared to the 1.5 volts of most flashlight-type cells. If you tie a lot of these cells together in series (I'll explain series in a minute) they add up. So three cells combines to be six volts, and 6 cells adds up to 12 volts. So actually that 12 V battery in your car or boat is really six cells (count the vent caps) wired together to make twelve volts. These cells are all inside one case, so it looks to be one battery.

Series means battery cells that are connected together thus; The positive pole on the first cell is wired to the negative on the second, and the negative on the first is wired to the positive on the second, and so on until you have the voltage you need. See the diagram below.

Parallel means just the opposite. All the positive poles are connected to each other and all the negative poles are connected to each other. This still only gives you about two (2.0) volts per cell, but it increases the amount of current. This is commonly done in boats and recreational vehicles to make a large battery bank to power all the DC equipment on board.  Two or more twelve volt batteries will be connected in parallel to increase the current, to run more equipment, or to run your equipment for a longer time. 

However, if you have something that runs on 24 volts or 48 volts, you can connect 12 volt batteries in series to get the desired volts. In series you add the voltages. In parallel you add up the amperages.

See the Figure below, that shows three batteries.  The two on the left are deep cycle batteries and are the "house batteries"  that run most of the equipment on board, and are wired in parallel. The battery on the right is the starting battery and not connected to the two deep cycle house batteries. However, in this instance there is a switch which allows the house batteries, in an emergency,  to be used to start the engine or the generator.

circuit4.jpg (28379 bytes)

Battery1.JPG (175320 bytes)

So why DC? DC has certain advantages. It comes in convenient sized packages; for example, your typical automotive batteries. Also, DC does not present the big shock hazard that AC does (until you get over 50 volts).  Plus that, most equipment used on small boats is designed specifically to run on DC.  You don't need a large generator to produce DC, and you can easily recharge the batteries from an alternator on your engine, or with a charger from shore power.

Why is it called direct current? Because DC current flows in only one direction, as opposed to AC which flows both directions.  For low power devices, DC is more than adequate to supply your power needs. See Wikipedia on Direct Current http://en.wikipedia.org/wiki/Direct_current

So what about AC.  It's called Alternating Current because the current flow switches direction every 1/120th second, meaning it takes 1/60 of a second to go through a complete cycle, or 60 cycles per second, now called Hertz after the inventor, and shown as Hz. See Wikipedia on Alternating current. http://en.wikipedia.org/wiki/Alternating_current

Alternating current's biggest advantage is that it can be easily transformed to higher or lower voltages with a transformer. To send lots of power a long distance, a high voltage up to hundreds of thousands of volts is used ("High Tension" lines). The current is much less, so the voltage drop is much less.  Some of the newest long-distance lines now use DC, as new methods of converting DC up and down in voltage have been developed.

Someone discovered that electrical equipment such as AC motors run best at between 50 and 60 cycles. We in America chose to use 60, the rest of the world opted for 50. Also, most of the world outside the US uses 220 volts instead of the 120 we usually use. So if you go anywhere else, to run your US electrical equipment, you need an adapter that changes 220 volts to 120 volts. In most cases the difference between 50 and 60 Hz is not a problem.  That's why when you go to Europe and take your laptop computer or a shaver, hair drier, or curling iron, you need appliances that have a switch for 120V to 220V, or a separate converter. Recently though many laptop chargers and small appliances will run from 120 or 220V and 50 or 60 Hz. Read the small print on the back of your charger! 

AC can be changed easily to DC of various voltages as well, by power supply devices that change the voltage and rectify AC to DC.  See definition of Rectifier https://en.wikipedia.org/wiki/Rectifier

See Wikipedia on Electrical Wiring  http://en.wikipedia.org/wiki/Electrical_wiring
See Wikipedia on Electrical Wiring (USA) http://en.wikipedia.org/wiki/Electrical_wiring_(United_States)
See Wikipedia on Electrical Wiring (UK) http://en.wikipedia.org/wiki/Electrical_wiring_(UK)

Power Pole Transformer

Also AC can be transformed, that is changed easily to another voltage by use of a transformer, so you can have 120 volts AC, 220 or 440 AC, or whatever you need.  It can be changed easily to DC as well, by a converter. Out on the power pole, the power in the lines coming into your neighborhood is around 10,000 volts AC.  But the utility company mounts transformers on the poles to transform it to 120 volts AC, 220 or 440 AC so you can use it in your house, or on your boat.

Why AC on a boat?  As boats get bigger the owners want the convenience of appliances that run off of AC.  Probably number one is air conditioning.  There are 12 V air conditioners but AC ones are more available and more powerful.  Also, house type appliances such as large screen TV's, VCR's, computers, refrigerators, etc., all run on AC.  So AC has come aboard our boats and is here to stay. It is also appearing on smaller and smaller boats every year as the market gets more competitive.

newboatbuilders.com 2007 All rights reserved. revised 01/27/2017

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