Electricity for Beginners Resistors Diodes

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Chris Greacen

©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 Lego™ 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. Kluge). 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

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.

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.

higher voltage voltage loss = current x resistance (V=IR)

lower. lower

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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 (W) 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

Basic Electricity electronic components have degraded performance when they are hot, and if they get too hot, they die. Use Joule's law to figure out the heat a component will produce. Multiply the voltage drop across the component by the current through the component. Design your circuits so that they are comfortably below the maximum wattage ratings. Sometimes this is no problem, but often it determines what is practical and what is not.

Joule's law is often called the "I squared R law" since for resistors (and wire) you can substitute V=IR (Ohm's law) into P = IV to get P = I(IV) = I2R. For more on wires and I2R losses, see Elliot Josephson's article in this issue.

Diodes

On to more interesting components! Diodes are one way valves for electricity.

Figure 2a. Above, a typical diode, the 1N4001. Below: the symbol for a diode. The arrow always points in the direction of forward biased current flow.

Arode +

Cathode

Check out Dr. Kluge's plumbing diode, in Figure 2b below. If the voltage is higher on the anode (the side of the electrical symbol with arrow) than the cathode (the side with the vertical line), the diode lets the current flow through (Figure 2.b). In this case the diode is said to be "forward biased".

Figure 2b: The plumbing analogy of a forward-biased diode. Pressure higher on the left (the anode) opens the ball valve: current flows.

Figure 2c: A reversed-biased diode. Pressure higher on the right (the cathode) closes the valve: no current flows.

Figure 2c: A reversed-biased diode. Pressure higher on the right (the cathode) closes the valve: no current flows.

If the voltage is higher on the cathode than the anode then the "valve shuts" — current cannot flow (Figure 2.b). In this case we say the diode is "reverse biased". If you're curious what substances allow current to flow one way and not the other, see "How Photovoltaic Cells Work", Home Power #23.

Where would you use a diode? The current in a typical American home is alternating current (ac) (figure 3a). It sloshes back and forth sixty times a second. Computers and radios and many other consumer electronics need current that flows in only one direction, called direct current (dc). Diodes are used to "rectify" the ac current to a rough dc current. Capacitors, which I'll cover in a future article, are used to smooth this waveform.

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