gaussmarkov: diy fx

Appearance

The fixed resistors commonly used in DIY stompboxes look like small cylinders with leads (or wires) coming out of each end. Such components are called axial leaded. Resistors can be soldered onto a circuit board oriented either way; the leads are interchangeable. The leads are bent so that they can stick into holes on the circuit board. As a result, this type of resistor is also called through-hole. These resistors have a fixed value and the color bands on the cylindrical case of a resistor tell this value. The case itself identifies the type of resistor. Carbon film (labelled B and C) and metal film (labelled D) resistors are common types in stompbox builds. Older guitar effects were usually made with carbon composition resistors (labelled A).

Symbols

In circuit diagrams (or schematics), there are two symbols for resistors. Occasionally one sees a keyboard representation using a string of slashes. Resistors are generally labelled with the capital letter R followed by a number (for example, R3). Usually, this label is a rough guide to where the resistor is located on a schematic or layout: you can expect to find R3 somewhere near R4.

On circuit board layouts, a resistor appears as an outline of its case and leads of the necessary length ending at solder pads. When a resistor is stood on end so that the leads go through adjacent holes, one sees something like the first image.

Values

Resistor values are called resistance, which is measured in units called ohms denoted by the symbol Ω, a capital Greek omega. Stompbox circuits use resistances as small as a few ohms and and as large as several million ohms. To abbreviate values, a resistor rated at 2.2 million ohms is often described as a 2.2MΩ or, more commonly, 2.2M resistor. M stands for mega. A 3,300 ohm resistance is usually denoted 3.3K (where K stands for kilo). Occasionally one also sees 100Ω written as 100R, rather than just 100. The decimal place is often replaced with the order of magnitude letter, as in

Fixed resistors are labelled with standard values grouped by decades in multiples of 10: 1–10, 10–100, 100–1000, and so on. Within a decade the values roughly follow a power series, where the number of values within a decade depends on the precision of the values. For example, ±5% resistors come in (approximate) multiples of 10^1/24≅ 1.1, chosen because it is slightly less than 10^5%=10^1/20 so that the ±5% intervals cover the entire decade interval. Across decades, these 24 standard values are multiplied or divided by powers of 10. The decade series 10, 11, 12, 13, 15, …, 91 becomes 100, 110, 120, 130, 150, …, 910 in the next decade, and so on.

Ratings

Resistors are also rated by power in watts. This is a value that should not be exceeded. In stompbox construction, people generally use 1/4 watt resistors, but one can also use resistors with power ratings larger than 1/4 watts. The next highest rating is 1/2 watts. Note that the cases of resistors get larger as the power rating goes up. This means that 1/2 watt resistors will often not fit as neatly on a published circuit layout as 1/4 watt resistors. Other than this, you will probably notice no difference. For an example of calculating power requirements, see the Power section of resistors: limiting current.

Further Information

For further information on decades and standard values, search the internet for standard EIA resistor decade values.

To learn about reading the color coded value markings on resistors, search the internet for resistor color codes.

To learn more about types of resistors, search the internet for “types of resistor”.

For an expert view on carbon comp resistors, see Using the Carbon Comp Resistor for Magic Mojo by R. G. Keen.

Acknowledgements

Thanks to idlechatterbox for suggesting improvements to this page!