# Electronics for dummies - McComb G.

ISBN: 0-7645-7660-7

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Calculating Units of Energy

The watt-hour is one of the most practical units of measure of energy; it’s the ability of a device or circuit to do work. You calculate watt-hours by multiplying the power of the circuit, in watts, by the length of time you have the circuit on. The formula for calculating watt-hours is

Watt-hours = P x T

In this formula, P stands for power, in watts, and T represents time in hours that it takes for power to dissipate. To calculate watt-seconds, also known as the joule, divide watt-hours by 3600.

Calculating RC Time Constants

Electronic circuits often use time constants to provide time delays or stretch the timing of signals. You most often construct them using a resistor and capacitor — hence the use of the term RC.

To complete the circuit, you connect the resistor and capacitor, as you can see in Figure 18-2, to some form of active component, such as an inverter or a transistor. You can select the values of the resistor and capacitor to produce a signal that lasts a specific amount of time.

Figure 18-2:

A resistor and capacitor used to make a timing circuit.

—VW

R1

C1

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Chapter 18: Ten Electronics Formulas You Should Know 381

RC circuits work because it takes a certain amount of time for a capacitor to discharge through a resistor. The larger the value of the resistor and/or capacitor, the longer it takes for the capacitor to discharge. Circuit designers use RC networks to produce simple timers and oscillators or to change the shape of signals.

So how do you calculate the time constant for a resistor-capacitor circuit? These circuits combine a resistor and a capacitor. Note that the capacitance value is in farads. Typical capacitor ranges are in microfarads and even smaller units, so the capacitance value is a fractional number.

T = RC

In this formula, T represents time (in seconds), R stands for resistance (in ohms), and C signifies capacitance (in farads).

For example, with a 2000-ohm resistor and a 0.1-uF capacitor, the time constant is 0.002 of a second, or two milliseconds. Table 18-2 shows some examples so that you can get the zeros right.

Table 18-2 Examples of Capacitance Value

Capacitor Value Capacitance Value for Calculation

10 uF 0.00001

1 uF 0.000001

0.1 uF 0.0000001

0.01 uF (0.00000001)

Calculating Frequency and Wavelength

The frequency of a signal is directly proportional to its wavelength, as the formulas in the following sections show you. You may find these formulas handy if you experiment with radio circuits (for example, when cutting a wire to a specific length to make an antenna). The following formulas express wavelength in meters and frequency in kilohertz.

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382 Part VI: The Part of Tens_

Calculating frequency of a signal

Let’s suppose you’re interested in learning electronics so you can gab to folks all around the world on an amateur radio set. It would be useful for you to know all about radio frequencies. Radio frequencies, and the wavelength of the signals carried by those frequencies, work hand in hand. In amateur radio, you’ll hear people say they’re operating at such-and-such a wavelength. Here’s how to calculate the frequency of that wavelength.

frequency =

300,000

wavelength

Wavelength is stated in millimeters, not feet, inches, or a multiple of bunches of bananas. Frequency is stated in megahertz.

Calculating Wavelength of a signal

You can use the same basic formula to calculate wavelength if you already know the frequency of the radio signal:

300, 000

wavelength = -e---

frequency

The result is stated in millimeters. The frequency value is stated in megahertz. Here’s an example. Suppose you’re communicating with beings from another planet on 50 megahertz (50 million cycles per second). Plugging those numbers into the formula, you get:

6000 (millimeter) =

300,000

50

Most folks talk about wavelength in meters, so there’s one final bit of math to perform. As there are 1000 millimeters to a meter, the result is 6 meters. It seems you’re talking to E.T. on the six-meter amateur radio band. Cool!

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Appendix

Internet Resources

t

Ø n this appendix, we present a gaggle of interesting Internet sources for all things electronic. Businesses operate some of these sites, and individuals are at the helm of the others. We've taken the time to find what we consider the most useful resources to save you the time and bother.

Be aware that Web sites may come and go over time. If you try to visit a site and your Web browser can't find it, the site owner probably has moved on. That's life on the Internet! Try search engines, such as Google and Yahoo!, to find additional resources.

Figuring Things Out with Calculators

You can perform calculations on the sites in this section without having to look up equations or pick up a handheld calculator. Choose a Web site that covers the particular equation that you want to use:

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