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Electricity for Beginners: Resistors & Diodes
©1992 Chris Greacen
The last Electricity for Beginners article in Home Power #31 used a plumbing analogy to explain how current flows in parallel and series circuits. We looked at Kirchhoff’s Laws which tell us how to predict how much current will flow in different legs of a circuit. Now let’s look at the fun part — the pieces and parts that you can fit together to build circuits.
Playing with these electronic parts is to play with one of the pleasant successes of capitalism. These little pieces are cheap, and you can freely build whatever your mind can dream up. The only rules you need to follow are the rules of electrical physics. So, my electronic revolutionaries, lets get a closer look at two of these proletariat LegoTM blocks: the resistor and diode, and how we might put them together to build some circuits.
Some philosophy: an understanding of electronics is built up from lots of little understandings. Each of these components is explained starting with a plumbing model (compliments of our fictitious inventor, Dr. Klüge). Stop there if you want. This level of understanding will give you enough to often interpret what’s going on in an already designed circuit. After the plumbing analogy I’ll discuss some caveats — usually limitations to the device. An understanding of these restrictions is necessary for designing circuits that work or work reliably. Circuits are holistic systems — it is important to know who affects whom, how, when, and why.
Resistors restrict the flow of electrical current. In the last issue we looked at a plumbing analogy for resistors: a section of narrow pipe or a pipe filled with gravel which restricts water flow, resulting in a loss of pressure or “head”. A wide pipe, or a pipe with a small amount of coarse gravel is like a resistor with lower resistance. A narrow pipe, or a pipe with lots of fine gravel corresponds to a resistor with high resistance.
voltage loss = current x
Figure 1: Think of a resistor as a section of pipe filled with gravel. The greater the voltage, the greater the flow of current through the resistor.
The greater the flow in the pipe, the greater the pressure loss from one side to the other. Mathematically, V = IR, the voltage drop (V, measured in volts) is equal to the current (I measured in amperes) times the resistance (R, measured in ohms). This is Ohm’s law, formulated by the German Georg Simon Ohm in 1827. It’s not exactly a law. Ohm’s law works very well for resistors and wires, but for other things (like diodes) it’s not true at all.
An ampere of current flowing through a one ohm resistor has a voltage drop of one volt. Voltage is named in honor of the Italian physicist Allessandro Giuseppe Antonio Anastasio Volta (1745-1827). The “I” for current comes from the French intensité. The Greek omega (Ω) is the symbol for ohms. Never thought basic electricity was so international, did you?
Joule’s Heating Law
The voltage loss in a resistor causes heat. How much heat? The answer is given by Joule’s law (the last law you’ll have to look at in this article): P = IV. P is for power, measured in watts. For any component or appliance using DC electricity, Joule’s law is always true. For ac electricity, Joule’s law is true if I and V are measured at the same instant. One ampere of current at one volt will make one watt of heat. A car head light draws about 10 Amperes at 12 Volts, so it uses 10 x 12 = 120 Watts.
All electronic components make some heat. This Macintosh computer, from an energy perspective, is a glorified 40 Watt heater. Unfortunately, nearly all
62 Home Power #32 • December 1992 / January 1993
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