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.
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. Matching is used in building voltage dividers and Wheatstone & Kelvin Bridges with extremely precision accuracy over a wide range of temperatures.
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. You can find all types of technical specifications for a resistor at http://www.riedon.com. Riedon is a manufacturer of resistors and carries many of these specifications and can even create custom resistors for special projects.
When used, resistors have numerous characteristics which determine their accuracy. 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 is expressed as the deviation from nominal value in percent and is measured at 25oC only with no appreciable load applied. It will change depending on the other conditions when in use. 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. You can find other information on resistors on the resistor wiki page.
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). 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.
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 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. When power is applied, this is entirely different and in addition to the effects of self heating.
The Thermocouple Effect is due to the Thermal emf generated by the change in the temperature at the junction of two dissimilar metals. This emf is due to the materials used in the leads or in the case of Wirewound Resistors the resistive element also. This can be minimized by keeping both leads at the same temperature. The thermal emf is the result of the difference in the temperature of one lead to the other lead. One lead will cause a positive emf and the opposite lead will generate a negative emf (or visa versa). When both leads are at the same temperature, the emf’s generated will cancel each other and the same is true where the resistive element joins the leads. 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.
Stability is the change in resistance with time at a specific load, humidity level, stress, and ambient temperature. 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. Changes in temperature alternately apply and relieve stresses on the resistive element thus causing changes in resistance. The wider the temperature changes and the more rapid these changes in are, the greater the change in resistance. If severe enough, it can literally destroy the resistor. Rapidly and continuously subjecting a device to it’s lowest and highest operating temperatures(called a Thermocycle Test) is considered a destructive test.
Reliability is the degree of probability that a resistor (or any other device) will perform it’s desired function. 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 (open or short).
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.