Here’s what we’re going to do today:

- agree on what’s going to be on the final
- Quick lesson on percentages
- Resistor Codings
- Review homeworks

First, we will look at percentage problems. Then, we will apply percentages to a common application in the trades: *tolerance*. When you buy a box of nails that says it has 200 in the box, and it actually has 201 or 199 nails, the tolerance is ±1 nail. For resistors, tolerance is expressed as a percentage.

Calculating a resistor’s tolerance is another 3 step process:

**Translate the Band Colors into numbers.**Use this link to open a resistor band conversion pdf:

Band Colors

Notice that there are 4, 5 and 6 Band resistors. All of the resistors in the above graphic on this page are 4 Band resistors. The first resistor in the pdf is also a 4 Band. In a 4 Band, only the first two colors are the resistance values in ohms. 5 and 6 Band resistors have three numbers for resistance. So the first resistor in the pdf has a**Red**and**Green**, or 25.**Apply the Multiplier.**Now we “move the decimal” just like we did for metric unit conversions. I organize the 3 Steps with a

Worksheet

The next Band is called the Multiplier. Just like a metric prefix (kilo, milli) can move the decimal left or right, so to can a Resistor Band Multiplier. In our 4 Band resistor, the next color is**Orange**, which means our Multiplier is +3. So our resistance value is now:25 × 10^{+3}ohms. All a positive 3 means is that you are going to move the decimal 3 places to the right, making our resistance value 25,000 ohms.**Calculate the Tolerance.**Now that we know what the resistance is supposed to be, we can apply the Tolerance value to figure out the + and −, like we did for the extra or missing nail. In our example, the last color is**Gold**. Notice that White, Gold and Silver colors are outlined in black to distinguish them from the other colors. In an actual resistor, you won’t see black outlines. Gold means a tolerance of 5%. Here’s the calculation we have to do:

5% × 25,000 = 1,250

That means the actual resistance value is between 23,750 ohms and 26,250 ohms.

Let’s practice some more …