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| What would you think if somebody asked you where they could buy a gallon of sugar or if they could borrow a pound of milk? Would you think that they didn't understand the language they were speaking so well or perhaps they had made a slip of the tongue? Of course, everybody knows that there are different forms of measurement in the world and that one talks about the length of an object in feet or centimeters and the weight of an object in pounds or kilograms. |
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| What would you tell a young child who asked you why we use so many different forms of measurement in this world. Perhaps you might tell that child that these different forms of measurement help to order your mind and that in the long run it makes things go a lot easier to have different forms of measurement. |
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| Likewise, you can't begin to understand and discuss Ohm's Law, which tells you about the relationship between resistance in a circuit, applied voltage and the amount of current that results from applying that voltage until you get straight in your mind the different forms of measurement that are used to measure resistance, voltage and current. |
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| Voltage is measured in Volts. Current is measured in Amperes (amps). Resistance is measured in Ohms. It's that simple. You will no doubt notice that the unit of resistance, the Ohm, uses the same name as the name of the law your are endeavoring to learn about. Perhaps this gives you a clue that this man, Ohm, was important in formulating some basic electronic theory quite a while back. I shall leave the history of a man clarifying one of the most fundamental electrical laws at a time when accurate electrical measurements were hard to come by up to you to look up. I'll help you a bit, though. The name you want to look up is Georg Simon Ohm. While you're at it, look up Count Alessandro Volta and Henry Ampere. Now, back to the learning. |
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| If you look at your VOM (Volt-Ohmmeter) you will notice three distinct areas of measurement. Your VOM can measure volts, ohms and amps. Starting to come together for you now? |
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| There is a Laboratory in France that has a small quantity of mercury kept at a precise temperature. This is the world's standard Ohm. The amount of resistance that this lump of mercury has to electrical current flow is exactly one ohm. Oh, we haven't talked about resistance yet? There's not too much to talk about. You probably know intuitively what "Resistance" is. It is the amount of control that a circuit places on current flow. Remember the hole-in-the-dam picture where the pressure of the water is the voltage and the water flowing out is the current? It is not the hole itself that is to be considered the resistance, it is the SIZE of the hole. Bigger hole- more water. Lower resistance- more current. |
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| Now that you have an idea of the concept of resistance, let's focus our minds on remembering the units of measure that have to come as second-nature to you if you ever want to learn electronics. Voltage is measured in units called Volts, millivolts, microvolts, kilovolts, etc. Current is measured in units called Amperes (amps), milliamps, microamps, etc. Resistance to current flow in a circuit is measured in Ohms, kilohms, megohms, etc. Did you ever get confused when you hear terms such as so many microvolts or so many milliamps? You don't have to be confused any more. If the phrase "volt" is in the phrase, you know you're talking about some amount of voltage. If the phrase "amp" is in there somewhere, you know you're talking about some amount or current. If the phrase "ohm" is used, you know you're making a measurement of resistance. One milliamp is one thousandth of an ampere. The most often used units of these measurements that you will run across are volts, milliamps and kilohms. Well, maybe a tie with ohms. These are the terms you often hear in discussions of electronics theory. |
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| How can I give you an appreciation for what all this means in relation to other things in the world? How can you develop an appreciation for how many volts being measured is to be considered a relatively high voltage and how much current is a formidable amount to consider? The long answer is that this is a skill that you will come to develop after you have measured a few electronic circuits and such. I mentioned in an earlier article that a man once electrocuted himself with a 42 volt DC source. I can also tell you here that you can stop your heart if you allow fifteen milliamperes of current to flow through your body in a path that includes the heart. These amounts of lethal electricity come as a great surprise to most electronics technicians who have worked with electricity for a while. They seem remarkably low to experienced test technicians who never really considered the subject of lethal shocks. The point is that all electricity must be respected. I once got a shock that was so bad that it burned my finger once from an amplifier that had its plug removed from the wall outlet for five full minutes. The reason this was so was that there were three large filter capacitors inside of the amplifier's high voltage DC power supply without a bleeder resistor across them. (It's OK if you don't understand this yet.) Just know for now that one way of getting an appreciation for how much electricity is a lot that you don't want to experience , or experience as the last thing in your life is by getting shocked by placing your fingers across the wrong points in a circuit. |
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| There is a short answer to this, though. You can figure how much voltage you need to adequately push some electrons along in a circuit by considering available Voltage sources in the world around you and what that source of energy can accomplish. Flashlight batteries put out about one-and-one-half volts of voltage. Your car battery measures about thirteen and a half volts at rest (with no current being used). Your wall outlet measures about 117 or 120 volts AC. Rarely will a small transistorized circuit draw more than a current measured in milliamperes. Most small resistors that you will work with for your electronic experiments will measure in the tens of ohms to one megohm (one million ohms). Getting a "feel" for what you are working with as opposed to a strict measurement of what you're working with and having no appreciation for that value will only come to you with experience. In the meantime, you can talk to me or the guy in the Radio Shack store about buying a 10K (ten thousand) ohm resistor. We'll know what you want. |
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| Now that you have it fixed in your mind that you don't talk about a current flowing in a circuit that measures so many volts, you can go right on and learn Ohm's Law in the next article. |
| -George Beloin |
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