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| A good friend of mine named Roy put it very clearly in my mind when I was about fifteen years old. He said "When the electricity is sitting in your wall outlet without anything connected to it, it's voltage just sitting there, waiting to be used. When you plug something in that wall outlet and turn it on you are now pulling current out of it." To this day I remember the analogy. |
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| Get this straight in your mind for now and forever (until we all become any smarter in understanding electronics theory, that is.) Voltage is a source of energy sitting by it's lonesome waiting to be used. The higher the voltage, the more current will flow when a given circuit is connected to the voltage source. Voltage is static, current is dynamic. Current flows. Voltage represents our concept of how much POTENTIAL electrical energy is available for us to use. In fact, other descriptive phrases that can be substituted for "voltage" are "electrical pressure" or "electromotive force". Imagine diving down to the bottom of a dam with a pressure gauge and measuring how much the water pressure is at the bottom. Such a measurement might give you a pretty good idea of how much water is behind the dam. If a hole is drilled through the dam at that point and the water starts to flow out, well then, think of that as the current. The heavier the pressure the more water will flow out of a given size hole. This is like more current flowing through a given circuit when having more applied voltage to begin with. Got it? |
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| DC (Direct Current) and AC (Alternating Current) pretty much say it all in themselves. DC is current that flows in one direction only from one end of a DC voltage source, through a circuit, to the other end of that same voltage source. Anything that has a positive and negative pole is a DC device. Your car battery works on a DC principle. If you could see the current flowing in and out of the battery, you would see electrons coming out of the negative pole and returning to the positive pole. Yes, DC electric current flows from negative to positive. (In Ben Franklin's day early experimenters thought it was from positive to negative.) |
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| AC current actually reverses the direction of its flow many times per second. Any source of AC current does not have two different positive and negative poles. With AC, it doesn't matter since the direction of current flow gets periodically reversed. The blades of an AC wall plug might have two different sized blades for reasons other than polarity. The electricity coming out of your wall outlet is AC. (There are some countries that do have DC current at the wall outlet. RV's may also have DC outlets.) I'm talking about the regular wall outlet we have here in the USA. The voltage at the wall outlet (and therefore the direction of the current we pull out of the outlet) reverses itself sixty times per second. They used to call this 60 CPS (Cycles Per Second) voltage, but now it's called 60 Hertz (which means Cycles Per Second). In a car, they used to have generators and now the same item under your hood is called an alternator. Which one do you think refers to a DC item and which one is an AC item? |
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| So now you're asking "Why do they have DC and AC anyway?" It wasn't anyone's choice. It's like asking why is there hot and cold rather than one ideal temperature to do things at. It is the nature of electricity. In fact, everything that is DC is not totally DC at all times and everything that is AC is not totally AC at all times, but the definition works in the real world. We work with both DC and AC circuits today in the ways that make the best use of both kinds of electricity. |
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| In the case of the car alternator, they switched over to an AC charging system because it's a better system to recharge a car battery (even though the battery itself is DC). Why did they have DC generators to begin with? Mainly because they were easier to make. The modern alternator in a car uses small, solid-state devices called "diodes" that weren't available when cars were first being made. The construction and physics of an alternator are more complicated than a generator, too. Were there any advantages to the DC generator system in your car? To someone plagued with alternator noise in their ham receiver installed in their car, they might say yes! |
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| The main advantage of an AC system over a DC system becomes apparent when a lot of power has to be supplied over a long distance. If you have an RV where the DC voltage coming out of your RV's battery only has, at most twelve feet of wire to go through to hit your lantern, all may work well. The more power you draw in a DC system the worse the losses become. In the case of supplying power to your house, if it were a DC system there wouldn't be enough energy left to hit your house from the power plant unless you were very close to it. In places where they have tried (and still do) have a DC system of power, by necessity, all users have to be close to the source of power and all wires have to be quite heavy and thick as compared to an AC system in order to avoid heavy losses in the system. Still, it's much easier to connect a storage battery to some wires than to have to build an AC generator and keep it running to supply power, at least for an individual. |
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| Look at a hand held walkie-talkie. You know that the small battery in it is a DC source. (Believe me, it is.) Yet, the sound coming out of the speaker of the walkie-talkie and the signal that it sends and receives are AC. Clearly, the circuitry inside the walkie-talkie must be able to take DC and convert it to AC in some ways. That is exactly what happens. |
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| Look at the electronic things around you every day and think about whether they are primarily a DC device or an AC device. How about the picture tube in your computer monitor? The high voltage needed to produce the picture on the screen is a DC voltage on the order of thousands of volts. The 117 volts AC from your wall outlet is converted to DC and, along the way, back to AC, and then DC again to produce that high voltage. This is why the subject of electronics can be confusing to a non-techie type. If I'm talking about the way my oscillator circuit drifts in my radio, chances are I'm talking to somebody who understands that the circuit I'm talking about takes a DC voltage and forms a very stable, repetitive AC voltage of relatively high frequency (Hertz) out of it. If you, as a neophyte, were to hear the conversation you might be at a loss to understand where I'm coming from. Now is your time to start getting it all straightened out in your mind. The signal output from an oscillator is AC, by the way. |
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| Even that last statement may be an opening for dispute. There is a type of oscillator called a "flip flop" that actually can be considered a device that puts out varying levels of DC. Still, there are AC components to be found in the sought-after DC output of a flip-flop. How about this one ? Although a battery puts out a DC voltage, certain batteries may put out too much noise to operate certain very low signal amplifiers properly. Noise is AC, so the battery must be putting out some AC too! This is why you should get the two worlds of DC and AC straight in your mind right now, before you attempt to handle tougher concepts. |
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| The gateway to understanding circuit actions is the concept of Ohm's Law. This is a simple equation that tells us how much current we can expect to flow in a given circuit with a certain amount of applied voltage. If you can grasp the concepts of Ohm's Law, you can work the equation in your head and decide what should be happening in a particular circuit. If you measure something different than what is supposed to be happening from that calculation in real practice you get a valuable clue as to whether or not there is an unknown short, open or component failure in the circuit that you are measuring. Study Ohm's law and get a feel for what should be the predicted behavior of a circuit and you'll be on your way to becoming a good electronics repairman. Ohm's Law is handled in the next "Basics" section. |
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