Doug Lowe - Electronics All-in-One For Dummies

Power is measured in units called watts (abbreviated W ). The definition of one watt is simple: One watt is the amount of work done by a circuit in which one ampere of current is driven by one volt.

This relationship lends itself to a simple equation. I promised myself when I started this book that I would use as few equations as possible, but I knew I’d have to include at least some of the basic equations. Fortunately, this one is pretty simple:

In other words, power ( P ) equals voltage ( E ) times current ( I ).

To use the equation correctly, you must make sure that you measure power, voltage, and current using their standard units: watts, volts, and amperes. For example, suppose you have a light bulb connected to a 10-volt power supply, and one-tenth of an ampere is flowing through the light bulb. To calculate the wattage of the light bulb, you use the P = E × I formula like this:

Thus, the light bulb is doing 1 watt of work.

Often, you know the voltage and the wattage of the circuit and you want to use those values to determine the amount of current flowing through the circuit. You can do that by turning the equation around, like this:

For example, if you want to determine how much current flows through a lamp with a 100-watt light bulb when it’s plugged into a 117-volt electrical outlet, use the formula like this:

Thus, the current through the circuit is 0.855 amperes.

Here are some final thoughts concerning the concept of power:

The term dissipate is often used in association with power. As the energy carried by an electric current is converted into another form such as heat or light, the circuit is said to dissipate power.

Did you notice that current and voltage are represented by the letters I and E , not the letters C or V as you might expect, but power is represented by the letter P ? Sometimes you wonder if the people who make the rules are just trying to confuse everyone.Maybe the following table will help you keep things sorted out:ConceptAbbreviationUnitCurrentIAmp (A)VoltageE or EMFVolt (V)PowerPWatt (W)

Earlier in this chapter, in the section “ Understanding Voltage,” I say that I can’t define “one volt” until you know what power is. Now that you know, you can see that the definition of a volt is simple: One volt is the amount of electromotive force (EMF) necessary to do one watt of work at one ampere of current.

Calculating the power dissipated by a circuit is often a very important part of circuit design. That’s because electrical components such as resistors, transistors, capacitors, and integrated circuits all have maximum power ratings. For example, the most common type of resistor can dissipate at most ¼ watt. If you use a ¼-watt resistor in a circuit that dissipates more than ¼ watt of power, you run the risk of burning up the resistor.

Chapter 3

IN THIS CHAPTER

Finding a place where you can build your mad-scientist laboratory

Investing in good tools

Picking out a good assortment of components to get you started

I loved to watch Frankenstein movies as a kid. My favorite scenes were always the ones where Dr. Frankenstein went into his laboratory. Those laboratories were filled with the most amazing and exotic electrical gadgets ever seen. The mad doctor’s assistant, Igor, would throw a giant knife switch at just the right moment, and sparks flew, and the music rose to a crescendo, and the creature jerked to life, and the crazy doctor yelled, “It’s ALIVE!”

The best Frankenstein movie ever made is still the original 1931 Frankenstein, directed by James Whale and starring Boris Karloff. The second-best Frankenstein movie ever made is the 1974 Young Frankenstein, directed by Mel Brooks and starring Gene Wilder. Both have great laboratory scenes.

In fact, did you know that the laboratory in Young Frankenstein uses the very same props that were used in the original 1931 classic? The genius who created those props was Kenneth Strickfaden, one of the pioneers of Hollywood special effects. Strickfaden kept the original Frankenstein props in his garage for decades. When Mel Brooks asked if he could borrow the props for Young Frankenstein, Strickfaden was happy to oblige.

You don’t need an elaborate mad-scientist laboratory like the ones in the Frankenstein movies to build basic electronic circuits. However, you will need to build yourself a more modest workplace, and you’ll need to equip it with a basic set of tools as well as some basic electronic components to work with.

However, no matter how modest your work area is, you can still call it your mad-scientist lab. After all, most of your friends will think you’re a bit crazy and a bit of a genius when you start building your own electronic gadgets.

In this chapter, I introduce you to the stuff you need to acquire before you can start building electronic circuits. You don’t have to buy everything all at once, of course. You can get started with just a simple collection of tools and a small space to work in. As you get more advanced in your electronic skills, you can acquire additional tools and equipment as your needs change.

First, you must create a good place to work. You can build a fancy workbench in your garage or in a spare room, but if you don’t have that much space, you can set up an ad-hoc mad-scientist lab just about anywhere. All you need is a place to set up a small workbench and a chair.

I do most of my electronics work in a spare room in my home, which also doubles as a display area for some of the Halloween props I’ve built over the years for my haunted house. Thus, as Figure 3-1 shows, my Mad-Scientist Lab really is a mad-scientist lab!

Here are the essential ingredients of any good work area for electronic tinkering:

Adequate space: You’ll need to have adequate space for your work. When you’re just getting started, your work area can be small — maybe just 2 or 3 feet in the corner of the garage. But as your skills progress, you’ll need more space. It’s very important that the location you choose for your work area is secure, especially if you have young children around. Your work area will be filled with perils — things that can cause shocks, burns, and cuts, as well as things that under no circumstances should be ingested. Little hands are incredibly curious, and children are prone to put anything they don’t recognize in their mouths. So be sure to keep everything safely out of reach, ideally behind a locked door. FIGURE 3-1:My Mad-Scientist Lab really is a mad-scientist lab!