OddMix.com - Technology Note - TN1208 - Karl Nagy
|Figure 1. Power Resistor 50 Watts Size|
|Figure 2. Variable Resistor Wirewound 100 Ohm|
For electronic circuits it is necessary to provide a variety of resistor values as required by the appropriate design values. At the same time, it is important to keep the number of individual resistor values that are need to be manufactured and stocked to a minimum.
For this reasons the EIA [Electronic Industries Association] recognized the necessity and has standardized on certain preferred values of resistors and also of some other electronic components. Like all mass produced components, resistors have a tolerance band. The present ISO [International Standards Organization] and IEC [International Electrotechnical Commission] and the older DIN and MIL standards use many different resistance series values.
The most often used of these set of resistors are the E24 series that is equivalent with the American EIA ±5% series. In that series the resistance range that is between 1.0 and 10 is divided into 24 steps. The way the steps within a decade are selected thus that each resistor value is ±5% away from, and it may overlaps with, it's nearest lower or higher neighbor.
|Steps in the E24 (R5) ±5% resistor decade|
The formula for any "Ex" resistance set is that the step distance between two consecutive resistance values are the nth root of ten, where "n" the series step number. Thus the E24 series has 24 standard steps within a resistance decade. The numerical value of the 24th root of ten equals 1.2589. That value is usually rounded to 1.25. The number 1.25 represents the step distance, or resistance ratio between two nearest resistance values within the series.
|Figure 3. Carbon Compound CC Resistor Set 1/8 - 2 Watts|
To further complicate the component inventory problem, each resistor family is available in many different power ratings - Figure 1 and 2. The most common power value for carbon compound [CC] resistors are the 1/10, 1/8, 1/4, 1/2, 1 and 2 Watts - Figure 3. In addition, the same power ratings are also available in many different types of resistor compositions.
|The E12 (R10) ±10% resistor decade - a subset of the E24 series|
Metal and carbon films are most often used as 1/10 Watt size and even smaller chip resistors employed as surface mount components. One of the oldest resistor types is the wire-wound. Newer types are the film resistors. Like the name implied, this original resistor was made by winding a length of wire onto a suitable bobbin as required. All radio and electrical, electronics circuits required some resistors. Initially, the radio amateurs designed and made resistors themselves, but soon an entirely new industry was built for the manufacture of electronic components.