Appendix B. Reading Resistors and Capacitors

In order to use electronic parts, you need to be able to identify them, which can be a difficult task for a beginner. Most of the resistors that you find in a shop have a cylindrical body with two legs sticking out and have strange coloured markings all around them. When the first commercial resistors were made, there was no way to print numbers small enough to fit on their body, so clever engineers decided that they could just represent the values with stripes of coloured paint.

Today’s beginners have to figure out a way to interpret these signs. The key is quite simple: generally, there are four stripes, and each colour represents a number. One of rings is usually gold-coloured; this one represents the tolerance of that resistor. To read the stripes in order, hold the resistor so the gold (or silver in some cases) stripe is to the right. Then, read the colours and map them to the corresponding numbers. In the following table, you’ll find a translation between the colours and their numeric values.

For example, brown, black, orange, and gold markings mean 1 0 3 ±5 %. Easy, right? Not quite, because there is a twist: the third ring actually represents the number of zeros in the value. Therefore 1 0 3 is actually 1 0 followed by three zeros, so the end result is 10,000 ohms ±5 %.

Electronics geeks tend to shorten values by expressing them in kilo ohms (for thousands of ohms) and mega ohms (for millions of ohms), so a 10,000-ohm resistor is usually shortened to 10K ohm, while 10,000,000 becomes 10M ohm. Because engineers are fond of optimising everything, on some schematic diagrams you might find values expressed as 4k7, which means 4.7 kilo ohms, or 4700.

Sometimes you’ll run into resistors with a higher precision of 1 or 2%. These resistors add a fifth ring so that the value can be specified more precisely. It’s the same code, but with the first three rings representing the value and the fourth ring representing the number of zeros after the value. The fifth ring would be the tolerance: red for 2% and brown for 1%. For example, the 10K ohm example (brown, black, orange, and gold) would be brown, black, black, red, and brown, for a 1% resistor.

Capacitors are a bit easier: the barrel-shaped capacitors (electrolytic capacitors) generally have their values printed on them. A capacitor’s value is measured in farads (F), but most capacitors that you encounter will be measured in micro farads (μF). So if you see a capacitor labeled 100μF, it’s a 100 micro farad capacitor.

Many of the disc-shaped capacitors (ceramic capacitors) do not have their units listed, and use a three-digit numeric code indicating the number of pico farads (pF). There are 1,000,000pF in one μF. Similar to the resistor codes, you use the third number to determine the number of zeros to put after the first two, with one difference: if you see 0–5, that indicates the number of zeros. 6 and 7 are not used, and 8 and 9 are handled differently. If you see 8, multiply the number that the first two digits form by 0.01, and if you see 9, multiply it by 0.1.

So, a capacitor labeled 104 would be 100,000pF or 0.1μF. A capacitor labeled 229 would be 2.2pF.

As a reminder, here are the multipliers commonly used in electronics.