The resistor is probably the most common and well known of all electrical components. Their uses are many, they are used to drop voltage, limit current, attenuate signals, act as heaters, act as fuses, furnish electrical loads and divide voltages.

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These uses are basic, for example, the voltage divider use is used in a variety of networks to divide voltages in specified increments of the applied voltage such as for analog to digital converters and digital to analog converters. They are used as matched pairs with relative accuracy much greater than their absolute accuracy. Matching is used in building voltage dividers and Wheatstone & Kelvin Bridges with extremely precision accuracy over a wide range of temperatures. This is done by matching the absolute value and the temperature coefficient of Resistance (TCR). This accuracy is limited only by the ability to accurately measure them and their stability.

There are numerous varieties of resistors. There are Precision Wirewound., NIST Standards, Power Wirewound, Fuse Resistors, Carbon Composition, Carbon Film, Metal Film, Foil, Filament Wound,, and Power Film Resistors. Each of these resistors has a useful purpose.

Resistors have numerous characteristics which determine their accuracy when used. Each will effect the accuracy to a greater or lesser extent depending on the application. Some of these characteristics are, Tolerance at DC, Temperature Coefficient of Resistance (TCR), Frequency Response, Voltage Coefficient, Noise, Stability with Time and Load, Temperature Rating, Power Rating, Physical Size, Mounting Characteristics, Thermocouple Effect, and Reliability.

I will go into further details on type of resistor, characteristics, and materials to manufacture them in future articles. Most of my experience has been in the design and manufacture of Bridges, Networks, Precision Wirewound, Metal Clad Power and Power Wirewound. These will be covered in greater detail.

These articles are intended to be general in nature. I would recommend that the appropriate manufacturer be consulted for specific characteristics on the resistors that they manufacture. Each manufacturer will have a specific group of characteristics in which they excel.

RESISTOR TERMS AND ABBREVIATIONS

Resistor Tolerance

Resistor Tolerance is expressed as the deviation from nominal value in percent and is measured at 25oC only with no appreciable load applied. When in use, it will change depending on the other conditions. For example, a 100 ohm resistor with a tolerance of 10 % can range in value from 90 ohms to 110 ohms and this will change as power is applied and the temperature varies.

Temperature Coefficient of Resistance

The Temperature Coefficient of Resistance (TCR) is expressed as the change in resistance in ppm (.0001 %) with each degree of change in temperature Celsius (Co). This change is not linear with the TCR the lowest at +25 oC and increasing as the temperature increases (or decreases). It can be either a bell shaped curve or an S shaped curve. It is treated as being linear unless very accurate measurements are needed, then a temperature correction chart is used. Normally a resistor with a TCR of 100 ppm will change 0.1 % over a 10 degree change and 1 % over a 100 degree change. The expression of ppm, one part in a million is similar to percent or 1 part in 100 (or percentile).

Frequency Response.

Frequency Response is the change in resistance with changes in frequency and is more difficult to measure. Where exact values are needed, these changes can be plotted but not very accurately, and normally in db change. These measurements can be made with a Boonton RX Meter which is designed for measuring low Q circuits.

Noise.

Noise levels are measured with very specialized equipment. It is extremely difficult to measure accurately and does not effect the value of the resistor but can have a devastating effect on low signals, digital amplifiers, high gain amplifiers, and other applications sensitive to noise. The best approach is to use resistor types with low or no noise in applications that are sensitive to noise.

The Voltage Coefficient is the change in resistance with applied voltage and is associated with Carbon Composition Resistors and Carbon Film Resistors. It is a function of value and the composition of the carbon mixture used in the manufacture of these resistors. This is entirely different and in addition to the effects of self heating when power is applied.

Thermocouple Effect.

The Thermocouple Effect is due to the Thermal emf generated by the change in the temperature at the junction of two dissimilar metals. Resistors with nickel leads as used in certain welded module applications will generate the highest thermal emf. The resistive element (the wire) of wirewound resistors is designed with a low thermal emf, but some of the wire used for high TCR resistors will have a much larger thermal emf. Wiki for Resistor

Stability.

The lower the load and the closer to +25 oC the resistor is maintained, the better the stability. Humidity will cause the insulation of the resistor to swell applying pressure (stress) to the resistive element causing a change. If severe enough, it can literally destroy the resistor.

Reliability.

Reliability is the degree of probability that a resistor (or any other device) will perform it’s desired function. There are two ways of defining Reliability. One is Mean Time Between Failures (MTBF) and the other is Failure Rate per 1,000 hours of operation. Both of these means of evaluating reliability must be determined with a specific group of tests and a definition of what is the end of life for a device, such as a maximum change in resistance or a catastrophic failure (short or open). Various statistical studies are used at arriving at these failure rates and large samples are tested at the maximum rated temperature with rated load for up to 10,000 hours (24 hrs per day for approximately 13 months).

A 100 ohm resistor with a tolerance of 10 % can range in value from 90 ohms to 110 ohms and this will change as power is applied and the temperature varies.

Normally a resistor with a TCR of 100 ppm will change 0.1 % over a 10 degree change and 1 % over a 100 degree change. Resistors with nickel leads as used in certain welded module applications will generate the highest thermal emf. The resistive element (the wire) of wirewound resistors is designed with a low thermal emf, but some of the wire used for high TCR resistors will have a much larger thermal emf.

Humidity will cause the insulation of the resistor to swell applying pressure (stress) to the resistive element causing a change.