Cleanup and Recycling
We’ll use the batteries and the LED in the next experiment. The resistors can be reused in the future.
Experiment 4: Varying the Voltage
Potentiometers come in various shapes and sizes, but they all do the same thing: they allow you to vary voltage and current by varying resistance. This experiment will enable you to learn more about voltage, amperage, and the relationship between them. You’ll also learn how to read a manufacturer’s data sheet.
You will need the same batteries, battery carrier, alligator clips, and LED from the last experiment, plus:
Potentiometer, 2KΩ linear. Quantity: 2. (See Figure 1-46.) Full-sized potentiometers that look like this are becoming less common, as miniature versions are taking their place. I’d like you to use a large one, though, because it’s so much easier to work with.
One extra LED.
Multimeter.
Look Inside Your Potentiometer
The first thing I want you to do is find out how a potentiometer works. This means you’ll have to open it, which is why your shopping list required you to buy two of them, in case you can’t put the first one back together again.
Most potentiometers are held together with little metal tabs. You should be able to grab hold of the tabs with your wire cutters or pliers, and bend them up and outward. If you do this, the potentiometer should open up as shown in Figures 1-47 and 1-48.
Figure 1-46.
Figure 1-47.
Figure 1-48. To open the potentiometer, first pry up the four little metal tabs around the edge (you can see one sticking out at the left and another one sticking out at the right in Figure 1-47). Inside is a coil of wire around a flat plastic band, and a pair of springy contacts (the wiper), which conduct electricity to or from any point in the coil when you turn the shaft.
Depending whether you have a really cheap potentiometer or a slightly more high-class version, you may find a circular track of conductive plastic or a loop of coiled wire. Either way, the principle is the same. The wire or the plastic possesses some resistance (a total of 2K in this instance), and as you turn the shaft of the potentiometer, a wiper rubs against the resistance, giving you a shortcut to any point from the center terminal.
You can try to put it back together, but if it doesn’t work, use your backup potentiometer instead.
To test your potentiometer, set your meter to measure resistance (ohms) and touch the probes while turning the potentiometer shaft to and fro, as shown in Figure 1-49.
Figure 1-49. Measure the resistance between these two terminals of the potentiometer while you turn its shaft to and fro.
Dimming Your LED
Begin with the potentiometer turned all the way counterclockwise, otherwise you’ll burn out the LED before we even get started. (A very, very small number of potentiometers increase and decrease resistance in the opposite way to which I’m describing here, but as long as your potentiometer looks like the one in Figure 1-48 after you open it up, my description should be accurate.)
Now connect everything as shown in Figures 1-50 and 1-51, taking care that you don’t allow the metal parts of any of the alligator clips to touch each other. Now turn up the potentiometer very slowly. You’ll notice the LED glowing brighter, and brighter, and brighter—until, oops, it goes dark. You see how easy it is to destroy modern electronics? Throw away that LED. It will never glow again. Substitute a new LED, and we’ll be more careful this time.
Figure 1-50. The setup for Experiment 4. Rotating the shaft of the 2K potentiometer varies its resistance from 0 to 2,000Ω. This resistance protects the LED from the full 6 volts of the battery.
Figure 1-51. The LED in this photo is dark because I turned the potentiometer up just a little bit too far.
While the batteries are connected to the circuit, set your meter to measure volts DC as shown in Figures 1-52 through 1-54. Now touch the probes either side of the LED. Try to hold the probes in place while you turn the potentiometer up a little, and down a little. You should see the voltage pressure around the LED changing accordingly. We call this the potential difference between the two wires of the LED.
Figure 1-52.
Figure 1-53.
Figure 1-54. Each meter has a different way to measure volts DC. The manually adjusted meter (top) requires you to move a slider switch to “DC” and then choose the highest voltage you want to measure: In this case, the selected voltage is 20 (because 2 would be too low). Using the autoranging RadioShack meter, you set it to “V” and the meter will figure out which range to use.
If you were using a miniature old-fashioned lightbulb instead of an LED, you’d see the potential difference varying much more, because a lightbulb behaves like a “pure” resistor, whereas an LED self-adjusts to some extent, modifying its resistance as the voltage pressure changes.
Now touch the probes to the two terminals of the potentiometer that we’re using, so that you can measure the potential difference between them. The potentiometer and the LED share the total available voltage, so when the potential difference (the voltage drop) around the potentiometer goes up, the potential difference around the LED goes down, and vice versa. See Figures 1-55 through 1-57. A few things to keep in mind:
If you add the voltage drops across the devices in the circuit, the total is the same as the voltage supplied by the batteries.
You measure voltage relatively, between two points in a circuit.
Apply your meter like a stethoscope, without disturbing or breaking the connections in the circuit.
Figure 1-55. How to measure voltage in a simple circuit.
Figure 1-56. The meter shows how much voltage the LED takes.
Figure 1-57. The meter shows how much voltage the potentiometer takes.
Checking the Flow
Now I want you to make a different measurement. I want you to measure the flow, or current, in the circuit, using your meter set to mA (milliamps). Remember, to measure current:
You can only measure current when it passes through the meter.
You have to insert your meter into the circuit.
Too much current will blow the fuse inside your meter.
Make sure you set your meter to measure mA, not volts, before you try this. Some meters require you to move one of your leads to a different socket on the meter, to measure mA. See Figures 1-58 through 1-61.
Figure 1-58. Any meter will blow its internal fuse if you try to make it measure too high an amperage. In our circuit, this is not a risk as long as you keep the potentiometer in the middle of its range. Choose “mA” for milliamps and remember that the meter displays numbers that mean thousandths of an amp.
Figure 1-59.
Figure 1-60.
Figure 1-61. A manual meter such as the one here may require you to shift the red lead to a different socket, to measure milliamps. Most modern meters don’t require this until you are measuring higher currents.
Insert your meter into the circuit, as shown in Figure 1-62. Don’t turn the potentiometer more than halfway up. The resistance in the potentiometer will protect your meter, as well as the LED. If the meter gets too much current, you’ll find yourself replacing its internal fuse.
As you adjust the potentiometer up and down a little, you should find that the varying resistance in the circuit changes the flow of current—the amperage. This is why the LED burned out in the previous experiment: too much current made it hot, and the heat melts it inside, just like the fuse in the previous experiment. A higher resistance limits the flow of current, or amperage.
Now insert the meter in another part of the circuit, as shown in Figure 1-63. As you turn the potentiometer up and down, you should get exactly the same results as with the configuration in Figure 1-62. This is because the current is the same at all points in
a similar circuit. It has to be, because the flow of electrons has no place else to go.
It’s time now to nail this down with some numbers. Here’s one last thing to try. Set aside the LED and substitute a 1KΩ resistor, as shown in Figure 1-64. The total resistance in the circuit is now 1KΩ plus whatever the resistance the potentiometer provides, depending how you set it. (The meter also has some resistance, but it’s so low, we can ignore it.)
Figure 1-62. To measure amps, as illustrated here and in Figure 1-63, the current has to pass through the meter. When you increase the resistance, you restrict the current flow, and the lower flow makes the LED glow less brightly.
Figure 1-63.
Figure 1-64. If you substitute a resistor instead of the LED, you can confirm that the current flowing through the circuit varies with the total resistance in the circuit, if the voltage stays the same.
Turn the potentiometer all the way counterclockwise, and you have a total of 3K resistance in the circuit. Your meter should show about 2 mA flowing. Now turn the potentiometer halfway, and you have about 2K total resistance. You should see about 3 mA flowing. Turn the potentiometer all the way clockwise, so there’s a total of 1K, and you should see 6 mA flowing. You may notice that if we multiply the resistance by the amperage, we get 6 each time—which just happens to be the voltage being applied to the circuit. See the following table.
Total resistance
Current
Voltage
(KΩ)
(mA)
(Volts)
3
2
6
2
3
6
1
6
6
In fact, we could say:
voltage = kilohms × milliamps
But wait a minute: 1K is 1,000 ohms, and 1mA is 1/1,000 of an amp. Therefore, our formula should really look like this:
voltage = (ohms × 1,000) × (amps/1,000)
The two factors of 1,000 cancel out, so we get this:
volts = ohms × amps
This is known as Ohm’s Law. See the section, “Fundamentals: Ohm’s Law,” on the following page.
Fundamentals
Series and parallel
Before we go any further, you should know how resistance in a circuit increases when you put resistors in series or in parallel. Figures 1-65 through 1-67 illustrate this. Remember:
Resistors in series are oriented so that one follows the other.
Resistors in parallel are oriented side by side.
When you put two equal-valued resistors in series, you double the total resistance, because electricity has to pass through two barriers in succession.
When you put two equal-valued resistors in parallel, you divide the total resistance by two, because you’re giving the electricity two paths which it can take, instead of one.
In reality we don’t normally need to put resistors in parallel, but we often put other types of components in parallel. Lightbulbs in your house, for instance, are all wired that way. So, it’s useful to understand that resistance in a circuit goes down if you keep adding components in parallel.
Figure 1-65. One resistor takes the entire voltage, and according to Ohm’s Law, it draws v/R = 6/1,000 = 0.006 amps = 6mA of current.
Figure 1-66. When two resistors are in series, the electricity has to pass through one to reach the other, and therefore each of them takes half the voltage. Total resistance is now 2,000 ohms, and according to Ohm’s Law, the circuit draws v/R = 6/2,000 = 0.003 amps = 3mA of current.
Figure 1-67. When two resistors are in parallel, each is exposed to the full voltage, so each of them takes 6 volts. The electricity can now flow through both at once, so the total resistance of the circuit is half as much as before. According to Ohm’s Law, the circuit draws v/R = 6/500 = 0.012 amps = 12mA of current.
Using Ohm’s Law
Ohm’s Law is extremely useful. For example, it helps us to figure out whether a component can be used safely in a circuit. Instead of stressing the component until we burn it out, we can predict whether it will work.
For instance, the first time you turned the potentiometer, you didn’t really know how far you could go until the LED burned out. Wouldn’t it be useful to know precisely what resistance to put in series with an LED, to protect it adequately while providing as much light as possible?
Fundamentals
Ohm’s Law
For reasons I’ll explain in a moment, amps are normally abbreviated with the letter I. V stands for volts and R stands for resistance in ohms (because the omega symbol, Ω, is not easily generated from most keyboards). Using these symbols, you can write Ohm’s Law in three different ways:
V = I × R
I = V/R
R = V/I
Remember, V is a difference in voltage between two points in a simple circuit, R is the resistance in ohms between the same two points, and I is the current in amps flowing through the circuit between the two points.
Letter I is used because originally current was measured by its inductance, meaning the ability to induce magnetic effects. It would be much less confusing to use A for amps, but unfortunately it’s too late for that to happen.
How to Read a Data Sheet
Like most information, the answer to this question is available online.
Here’s how you find a manufacturer’s data sheet (Figure 1-68). First, find the component that you’re interested in from a mail-order source. Next, Google the part number and manufacturer’s name. Usually the data sheet will pop up as the first hit. A source such as Mouser.com makes it even easier by giving you a direct link to manufacturers’ data sheets for many products.
Figure 1-68. The beginning of a typical data sheet, which includes all relevant specifications for the product, freely available online.
Background
How much voltage does a wire consume?
Normally, we can ignore the resistance in electric wires, such as the little leads of wire that stick out of resistors, because it’s trivial. However, if you try to force large amounts of current through long lengths of thin wire, the resistance of the wire can become important.
How important? Once again, we can use Ohm’s Law to find out.
Suppose that a very long piece of wire has a resistance of 0.2Ω. And we want to run 15 amps through it. How much voltage will the wire steal from the circuit, because of its resistance?
Once again, you begin by writing down what you know:
R = 0.2
I = 15
We want to know V, the potential difference, for the wire, so we use the version of Ohm’s Law that places V on the left side:
V = I × R
Now plug in the values:
V = 15 × 0.2 = 3 volts
Three volts is not a big deal if you have a high-voltage power supply, but if you are using a 12-volt car battery, this length of wire will take one-quarter of the available voltage.
Now you know why the wiring in automobiles is relatively thick—to reduce its resistance well below 0.2Ω. See Figure 1-69.
Figure 1-69. When a 12-volt car battery runs some kind of electrical device through a long piece of thin wire, the resistance of the wire steals some of the voltage and dissipates it as heat.
Here’s an example. Suppose I want a red LED, such as the Vishay part TLHR5400, which has become such a common item that
I can buy them individually for 9 cents apiece. I click the link to the data sheet maintained by the manufacturer, Vishay Semiconductor. Almost immediately I have a PDF page on my screen. This data sheet is for TLHR, TLHG, and TLHY types of LED, which are red, green, and yellow respectively, as suggested by the R, G, and Y in the product codes. I scroll down and look at the “Optical and Electrical Characteristics” section. It tells me that under conditions of drawing a current of 20 mA, the LED will enjoy a “Typ,” meaning, typical, “forward voltage” of 2 volts. The “Max,” meaning maximum, is 3 volts.
Let’s look at one other data sheet, as not all of them are written the same way. I’ll choose a different LED, the Kingbright part WP7113SGC. Click on the link to the manufacturer’s site, and I find on the second page of the data sheet a typical forward voltage of 2.2, maximum 2.5, and a maximum forward current of 25 mA. I also find some additional information: a maximum reverse voltage of 5 and maximum reverse current of 10 uA (that’s microamps, which are 1,000 times smaller than milliamps). This tells us that you should avoid applying excessive voltage to the LED the wrong way around. If you exceed the reverse voltage, you risk burning out the LED. Always observe polarity!
Kingbright also warns us how much heat the LED can stand: 260° C (500° F) for a few seconds. This is useful information, as we’ll be putting aside our alligator clips and using hot molten solder to connect electrical parts in the near future. Because we have already destroyed a battery, a fuse, and an LED in just four experiments, maybe you won’t be surprised when I tell you that we will destroy at least a couple more components as we test their limits with a soldering iron.
Make: Electronics Page 4
