Make: Electronics
Page 9
Set voltage
The minimum voltage that the relay needs to close its switch. This will be a bit less than the ideal coil voltage.
Operating current
The power consumption of the coil, usually in milliamps, when the relay is energized. Sometimes the power is expressed in milliwatts.
Switching capacity
The maximum amount of current that you can switch with contacts inside the relay. Usually this is for a “resistive load,” meaning a passive device such as light bulb. When you use a relay to switch on a motor, the motor takes a big initial surge of current before it gets up to speed. In this case, you should choose a relay rated for double the current that the motor draws when it is running.
Procedure
Turn the relay with its legs in the air and attach wires and LEDs as shown in Figure 2-59, with a 680Ω resistor (a 1K resistor will be OK if you don’t have the correct value). Also attach a pushbutton switch. (Your pushbutton switch may look different from the one shown, but as long as it is a SPST pushbutton with two contacts at the bottom, it will work the same way.) When you press the pushbutton, the relay will make the first LED go out and the second LED light up. When you release the pushbutton, the first LED lights up and the second one goes out.
Figure 2-59. As before, you can use patch cords, if you have them, instead of some of the wired connections shown here.
How It Works
Check the schematic in Figure 2-60 and compare it with Figure 2-59. Also see Figure 2-62, which shows how the pins outside the relay make connections inside the relay when its coil is energized, and when it is not energized.
Figure 2-60. Same circuit, shown in schematic form.
This is a DPDT relay, but we are only using one pole and ignoring the other. Why not buy a SPDT relay? Because I want the pins to be spaced the way they are when you will upgrade this circuit by transferring it onto a breadboard, which will happen very shortly.
On the schematic, I have shown the switch inside the relay in its relaxed state. When the coil is energized, the switch flips upward, which seems counterintuitive, but just happens to be the way that this particular relay is made.
When you’re sure you understand how the circuit works, it’s time to move on to the next step: making a small modification to get the relay to switch itself on and off, as we’ll do in Experiment 8.
Figure 2-61. The layout of the pins of the relay, superimposed on a grid of 1/10-inch squares.This is the type of relay that you will need in Experiment 8.
Figure 2-62. How the relay connects the pins, when it is not energized (left) and when it is energized (right).
Experiment 8: A Relay Oscillator
You will need:
AC adapter, breadboard, wire, wire cutters and strippers.
DPDT relay. Quantity: 1.
LEDs. Quantity: 2.
Pushbutton, SPST. Quantity: 1.
Alligator clips. Quantity: 8.
Resistor, approximately 680Ω. Quantity: 1.
Capacitor, electrolytic, 1,000 μF. Quantity: 1.
Look at the revised drawing in Figure 2-63 and the revised schematic in Figure 2-64 and compare them with the previous ones. Originally, there was a direct connection from the pushbutton to the coil. In the new version, the power gets to the coil by going, first, through the contacts of the relay.
Figure 2-63. A small revision to the previous circuit causes the relay to start oscillating when power is applied.
Figure 2-64. The oscillator circuit shown in schematic form.
Now, when you press the button, the contacts in their relaxed state feed power to the coil as well as to the lefthand LED. But as soon as the coil is energized, it opens the contacts. This interrupts the power to the coil—so the relay relaxes, and the contacts close again. They feed another pulse of power to the coil, which opens the contacts again, and the cycle repeats endlessly.
Because we’re using a very small relay, it switches on and off extremely fast. In fact, it oscillates perhaps 50 times per second (too fast for the LEDs to show what’s really happening). Make sure your circuit looks like the one in the diagram, and then press the pushbutton very briefly. You should hear the relay make a buzzing sound. If you have impaired hearing, touch the relay lightly with your finger, and you should feel the relay vibrating.
When you force a relay to oscillate like this, it’s liable to burn itself out or destroy its contacts. That’s why I asked you to press the pushbutton briefly. To make the circuit more practical, we need something to slow the relay down and prevent it from self-destructing. That necessary item is a capacitor.
Adding Capacitance
Add a 1,000 μF electrolytic capacitor in parallel with the coil of the relay as shown in the diagram in Figure 2-65 and the schematic in Figure 2-66. Check Figure 2-14 if you’re not sure what a capacitor looks like. The 1,000 μF value will be printed on the side of it, and I’ll explain what this means a little later.
Figure 2-65. Adding a capacitor makes the relay oscillate more slowly.
Figure 2-66. The capacitor appears at the bottom of this schematic diagram.
Make sure the capacitor’s short wire is connected to the negative side of the circuit; otherwise, it won’t work. In addition to the short wire, you should find a minus sign on the body of the capacitor, which is there to remind you which side is negative. Electrolytic capacitors are fussy about this.
When you press the button now, the relay should click slowly instead of buzzing. What’s happening here?
A capacitor is like a tiny rechargeable battery. It’s so small that it charges in a fraction of a second, before the relay has time to open its lower pair of contacts. Then, when the contacts are open, the capacitor acts like a battery, providing power to the relay. It keeps the coil of the relay energized for about one second. After the capacitor exhausts its power reserve, the relay relaxes and the process repeats.
Fundamentals
Farad basics
The Farad is an international unit to measure capacitance. Modern circuits usually require small capacitors. Consequently it is common to find capacitors measured in microfarads (one-millionth of a farad) and even picofarads (one-trillionth of a farad). Nanofarads are also used, more often in Europe than in the United States. See the following conversion table.
0.001 nanofarad
1 picofarad
1 pF
0.01 nanofarad
10 picofarads
10 pF
0.1 nanofarad
100 picofarads
100 pF
1 nanofarad
1,000 picofarads
1,000 pF
0.001 microfarad
1 nanofarad
1 nF
0.01 microfarad
10 nanofarads
10 nF
0.1 microfarad
100 nanofarads
100 nF
1 microfarad
1,000 nanofarads
1,000 nF
0.000001 Farad
1 microfarad
1 mF
0.00001 Farad
10 microfarads
&nbs
p; 10 mF
0.0001 Farad
100 microfarads
100 mF
0.001 Farad
1,000 microfarads
1,000 mF
(You may encounter capacitances greater than 1,000 microfarads, but they are uncommon.)
Fundamentals
Capacitor basics
DC current does not flow through a capacitor, but voltage can accumulate very quickly inside it, and remains after the power supply is disconnected. Figures 2-67 and 2-68 may help to give you an idea of what happens inside a capacitor when it is fully charged.
Getting Zapped by Capacitors
If a large capacitor is charged with a high voltage, it can retain that voltage for a long time. Because the circuits in this book use low voltages, you don’t have to be concerned about that danger here, but if you are reckless enough to open an old TV set and start digging around inside (which I do not recommend), you may have a nasty surprise. An undischarged capacitor can kill you as easily as if you stick your finger into an electrical outlet. Never touch a large capacitor unless you really know what you’re doing.
Figure 2-67. When DC voltage reaches a capacitor, no current flows, but the capacitor charges itself like a little battery. The positive and negative charges are equal and opposite.
Figure 2-68. You can imagine positive “charge particles” accumulating on one side of the capacitor and attracting negative “charge particles” to the opposite side.
In most modern electrolytic capacitors, the plates have been reduced to two strips of very thin, flexible, metallic film, often wrapped around each other, separated by an equally thin insulator. Disc ceramic capacitors typically consist of just a single disc of nonconductive material with metal painted on both sides and leads soldered on.
The two most common varieties of capacitors are ceramic (capable of storing a relatively small charge) and electrolytic (which can be much larger). Ceramics are often disc-shaped and yellow in color; electrolytics are often shaped like miniature tin cans and may be just about any color. Refer back to Figures 2-14 and 2-15 for some examples.
Fundamentals
Capacitor basics (continued)
Ceramic capacitors have no polarity, meaning 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 breadboard in the positions shown, they will create the same circuit that you built from wire and alligator clips in Experiment 8. Component values:
D1, D2: Light-emitting diodes
S1: DPDT relay
S2: SPST momentary switch
C1: Electrolytic capacitor, 1,000 µF
R1: Resistor, 680Ω minimum
Figure 2-75. If your breadboard doesn’t have screw terminals, insert two short pieces of solid-core wire with stripped ends and then attach the stranded wires from the adapter using alligator clips.
You’ll need some more 22-gauge wire, or some precut hookup wire, to supply the power to your components, which are plugged into the breadboard as shown in Figures 2-76 and 2-77. If you get all the connections right, the circuit should function the same way as before.
The geometry of the metal connecting strips in the breadboard often forces you to connect components in a roundabout way. The pushbutton, for instance, supplies power to the pole of the relay but cannot be connected directly opposite, because there isn’t room for it.