Make: Electronics by Charles Platt (classic books to read TXT) 📗

that you can apply negative voltage to either side of them. Electrolytics do have polarity, and won’t work unless you connect them the right way around.

The schematic symbol for a capacitor has two significant variants: with two straight lines (symbolizing the plates inside a capacitor), or with one straight line and one curved line, as shown in Figure 2-69. When you see a curved line, that side of the capacitor should be more negative than the other. The schematic symbol may also include a + sign. Unfortunately, some people don’t bother to draw a curved plate on a polarized capacitor, yet others draw a curved plate even on a nonpolarized capacitor.

Figure 2-69. The generic schematic for a capacitor is on the left. The version on the right indicates a polarized capacitor which requires its left plate to be “more positive” than its right plate. The plus sign is often omitted.

Figure 2-70. A tantalum capacitor was plugged into this breadboard, accidentally connected the wrong way around to a power source capable of delivering a lot of current. After a minute or so of this abuse, the capacitor rebelled by popping open and scattering small flaming pieces, which burned their way into the plastic of the breadboard. Lesson learned: observe polarity!

Capacitor Polarity

You must connect an electrolytic capacitor so that its longer wire is more positive than its shorter wire. The shell of the capacitor is usually marked with a negative sign near the shorter wire.

Some capacitors may behave badly if you don’t observe their polarity. One time I connected a tantalum capacitor to a circuit, using a power supply able to deliver a lot of current, and was staring at the circuit and wondering why it wasn’t working when the capacitor burst open and scattered little flaming fragments of itself in a 3-inch radius. I had forgotten that tantalum capacitors can be fussy about positive and negative connections. Figure 2-70 shows the aftermath.

Background

Michael Faraday and capacitors

The earliest capacitors consisted of two metal plates with a very small gap between them. The principle of the thing was simple:

If one plate was connected to a positive source, the positive charges attracted negative charges onto the other plate.

If one plate was connected to a negative source, the negative charges attracted positive charges onto the other plate.

Figures 2-67 and 2-68, shown previously, convey the basic idea.

The electrical storage capacity of a capacitor is known as its capacitance, and is measured in farads, named after Michael Faraday (Figure 2-71), another of the pantheon of electrical pioneers. He was an English chemist and physicist who lived from 1791 to 1867.

Although Faraday was relatively uneducated and had little knowledge of mathematics, he had an opportunity to read a wide variety of books while working for seven years as a bookbinder’s apprentice, and thus was able to educate himself. Also, he lived at a time when relatively simple experiments could reveal fundamental properties of electricity. Thus he made major discoveries including electromagnetic induction, which led to the development of electric motors. He also discovered that magnetism could affect rays of light.

His work earned him numerous honors, and his picture was printed on English 20-pound bank notes from 1991 through 2001.

Figure 2-71. Michael Faraday

Breadboarding the Circuit

I promised to free you in time from the frustrations of alligator clips, and that time has come. Please turn your attention to the block of plastic with lots of little holes in it that I asked you to buy. For reasons that I do not know, this is called a breadboard. When you plug components into the holes, hidden metal strips inside the breadboard connect the components for you, allowing you to set up a circuit, test it, and modify it very easily. Afterward you can pull the components off the breadboard and put them away for future experiments.

Without a doubt, breadboarding is the most convenient way to test something before you decide whether you want to keep it.

Almost all breadboards are designed to be compatible with integrated circuit chips (which we will be using in Chapter 4 of this book). The chip straddles an empty channel in the center of the breadboard with rows of little holes either side—usually five holes per row. You insert other components into these holes.

In addition, the breadboard should have columns of holes running down each side. These are used to distribute positive and negative power.

Take a look at Figures 2-72 and 2-73, which show the upper part of a typical breadboard seen from above, and the same breadboard seen as if with X-ray vision, showing the metal strips that are embedded behind the holes.

Figure 2-72. A typical breadboard. You can plug components into the holes to test a circuit very quickly.

Figure 2-73. This X-ray-vision view of the breadboard reveals the copper strips that are embedded in it. The strips conduct electricity from one component to another.

Important note: some breadboards divide each vertical column of holes, on the left and the right, into two separate upper and lower sections. Use your meter’s continuity testing feature to find out if your breadboard conducts power along its full length, and add jumper wires to link the upper and lower half of the breadboard if necessary.

Figure 2-74 shows how you can use the breadboard to replicate your oscillating relay circuit. To make this work, you need to apply the positive and negative power from your AC adapter. Because the wire from your AC adapter is almost certainly stranded, you’ll have difficulty pushing it into the little holes. A way around this is to set up a couple of pieces of bare 22-gauge wire, and use them as terminals to which you clip the wire from the adapter, as in Figure 2-75. (Yes, you still need just a couple of alligator clips for this purpose.) Alternatively, you can use a breadboard with power terminals built into it, which is more convenient.

Figure 2-74. If you place the components on your