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.
Anyway, now we know what an LED wants, we can figure out how to supply it. If you have any difficulties dealing with decimals, check the Fundamentals section “Decimals,” on the next page, before continuing.
Background
The origins of wattage
James Watt (Figure 1-70) is known as the inventor of the steam engine. Born in 1736 in Scotland, he set up a small workshop in the University of Glasgow, where he struggled to perfect an efficient design for using steam to move a piston in a cylinder. Financial problems and the primitive state of the art of metal working delayed practical applications until 1776.
Despite difficulties in obtaining patents (which could only be granted by an act of parliament in those times), Watt and his business partner eventually made a lot of money from his innovations. Although he predated the pioneers in electricity, in 1889 (70 years after his death), his name was assigned to the basic unit of electric power that can be defined by multiplying amperes by volts. See the Fundamentals section, “Watt Basics,” on page 31.
Figure 1-70. James Watt’s development of steam power enabled the industrial revolution. After his death, he was honored by having his name applied to the basic unit of power in electricity.
How Big a Resistor Does an LED Need?
Suppose that we use the Vishay LED. Remember its requirements from the data sheet? Maximum of 3 volts, and a safe current of 20mA.
I’m going to limit it to 2.5 volts, to be on the safe side. We have 6 volts of battery power. Subtract 2.5 from 6 and we get 3.5. So we need a resistor that will take 3.5 volts from the circuit, leaving 2.5 for the LED.
The current flow is the same at all places in a simple circuit. If we want a maximum of 20mA to flow through the LED, the same amount of current will be flowing through the resistor.
Now we can write down what we know about the resistor in the circuit. Note that we have to convert all units to volts, amps, and ohms, so that 20mA should be written as 0.02 amps:
V = 3.5 (the potential drop across the resistor)
I = 0.02 (the current flowing through the resistor)
We want to know R, the resistance. So, we use the version of Ohm’s Law that puts R on the left side:
R= V/I
Now plug in the values:
R = 3.5/0.02
Run this through your pocket calculator if you find decimals confusing. The answer is:
R = 175Ω
It so happens that 175Ω isn’t a standard value. You may have to settle for 180 or 220Ω, but that’s close enough.
Evidently the 470Ω resistor that you used in Experiment 3 was a very conservative choice. I suggested it because I said originally that you could use any LED at all. I figured that no matter which one you picked, it should be safe with 470Ω to protect it.
Cleanup and Recycling
The dead LED can be thrown away. Everything else is reusable.
Fundamentals
Decimals
Legendary British politician Sir Winston Churchill is famous for complaining about “those damned dots.” He was referring to decimal points. Because Churchill was Chancellor of the Exchequer at the time, and thus in charge of all government expenditures, his difficulty with decimals was a bit of a problem. Still, he muddled through in time-honored British fashion, and so can you.
You can also use a pocket calculator—or follow two basic rules.
Doing multiplication: move the decimal points
Suppose you want to multiply 0.03 by 0.002:
1. Move the decimal points to the ends of both the numbers. In this case, you have to move the decimal points by a total of 5 places to get 3 and 2.
2. Do the multiplication of the whole numbers you have created and note the result. In this case, 3 x 2 = 6.
3. Move the decimal point back again by the same number of places you counted in step 1. In this case, you get 0.00006.
Doing division: cancel the zeros
Suppose you need to divide 0.006 by 0.0002:
1. Shift the decimal points to the right, in both the numbers, by the same number of steps, until both the numbers are greater than 1. In this case, shift the point four steps in each number, so you get 60 divided by 2.
2. Do the division. The result in this case is 30.
Theory
Doing the math on your tongue
I’m going to go back to the question I asked in the previous experiment: why didn’t your tongue get hot?
Now that you know Ohm’s Law, you can figure out the answer in numbers. Let’s suppose the battery delivered its rated 9 volts, and your tongue had a resistance of 50K, which is 50,000 ohms. Write down what you know:
V = 9
R = 50,000
We want to know
