The Precision Wirewound is a highly accurate resistor with a very low TCR and can be accurate within .005%. A temperature coefficient of resistance (TCR) of as little a 3 part per million per degree Celsius (3ppm/oC) can be achieved. However these components are too expensive for general use and are normally used in highly accurate DC applications. The frequency response of this type is not good. When used in an rf application all Precision Wirewound Resistors will have a low Q
resonant frequency. The power handling capability is very small. These are generally used in highly accurate DC measuring equipment, and reference resistors for voltage regulators and decoding networks.
The accuracy is maintained at 25oC(degrees Celsius) and will change with temperature. The maximum value available is dependent upon physical size and is much lower than most other types of resistor. Their power rating is approximately 1/10 of a similar physical size in a carbon composition. They are rated for operation at +85oC or +125oC with maximum operating temperature not to exceed +145oC. This means that full rated power can be applied at +85 ( 125) oC with no degradation
in performance. It may be operated above +125 (85) oC if the load is reduced. The derating is linear, rated load at +125(85) oC and no load at +145oC. Life is generally rated for 10,000 hours at rated temperature and rated load. The allowable change in resistance under these conditions is 0.10%. Extended life can be achieved if operated at lower temperatures and reduced power levels. End of life requirements are generally defined by the manufacturer or in some case by user specification.
Some degradation in performance can be expected. In some cases, particularly if the tolerance is very low and the TC is low, the rated power is reduced to improve resistor stability through life. Precision Resistors regardless of type, are designed for maximum accuracy and not to carry power. The materials used in these resistors are highly stable heat treated materials that do change under extended heat and mechanical stress. The manufacturing processes are designed to remove any stresses
induced during manufacture. There is little detectable noise in this type of resistor. The stability and reliability of these resistors is very good and their accuracy can be enhanced by matching the absolute value and the temperature coefficient over their operating range to achieve very accurate voltage division.
The NIST (National Institute of Standards and Technology) Standard can be as accurate as .001% with roughly the same TCR as Precision Wirewound Resistors and are very stable. These are used as a standard in verifying the accuracy of resistive measuring devices. They are normally the Primary Standards of a company's test lab.
They are returned to the NIST for measurement and their accuracy is tracked through out the standards life to determine the Standard's stability. Most companies will have two sets of standards so that they can continue to measure while one set of standards are being measured by the NIST . They will alternate returning these NIST Standards to the NIST , one set one year and the other set the next year. For extremely accurate measurements, the Standard with the longest history
and the best stability will be used. If erratic readings are received from the NIST over a period of years, the Standard is retired. Also, if the reading has significantly changed since the last NIST reading, the standard is suspect and all measurements made using that standard must be checked. Normally, a standard will take about 3 years to stabilize and becomes more stable with time unless it has had excessive power applied or has been dropped. These
standards are generally stored in an
oil bath at +25oC. During measurement, a thermometer is placed in a cavity in the top of the Standard, called the oil well, and the temperature is recorded for each measurement so that the exact value can be determined. That is the value at +25oC plus or minus the change in value caused by the temperature coefficient. Each standard will have a temperature correction chart for exact values. Being stored in the oil bath prevents the Standard from being stressed by changes in room
temperature. These are highly precision devices and are expensive to buy and expensive to maintain, but they are the primary resistor reference for any test lab.
These resistors are furnished in a totally enclosed metal case and for values above 1 ohm, this enclosure is filled with mineral oil (other type of oil may contain additives that can cause corrosion in later life). The values below 1 ohm may be built in an enclosure that is perforated and these must be submersed in oil. If power is applied without it being submersed, the Standard will be ruined.
All NIST Type Standards are equipped with provisions for two, three, or four terminal measurements. The applied power is calculated and the temperature of the Standard is monitored during test. The lowest power level consistent with sufficient resolution to get the desired measurement is used (in the area of 0.01 watts) and any appreciable rise in temperature will dictate that the measurement should be suspended and the test set-up reviewed for ways to reduce the power level. These
Standards are rated for operation at room temperature only but their other characteristics are the same as Precision Wirewound Resistors.
POWER WIREWOUND RESISTORS
Power Wirewound Resistors are used when it is necessary to handle a lot of power. They will handle more power per unit volume than any other resistor. Some of these resistors are free wound similar to heater elements. These require some form of cooling in order to handle any appreciable amount of power. Some are cooled by fans and others are immersed in various types of liquid ranging from mineral oil to high density silicone liquids. Most are wound on some type of winding form. These
winding forms vary. Some examples are ceramic tubes, ceramic rods, heavily anodized aluminium, fibreglass mandrels, etc.
To achieve the maximum power rating in the smallest package size, the core on which the windings are made must have a material with high heat conductivity. It may be Steatite, Alumina, Beryllium Oxide, or in some cases hard anodized Aluminium. Theoretically, the anodized Aluminium core has a better heat conductivity than any other insulated material, with Beryllium Oxide being very close. There are specific problems with the anodized aluminium cores such as nicks in the coating,
abrasion during capping and controlling the anodized thickness. There are various shapes, oval, flat, cylindrical, and most shapes are designed to optimize heat dissipation. The more heat that can be radiated from the resistor, the more power that can be safely applied.
There is a group of these called "Chassis Mounted Resistors". These are generally cylindrical power resistors wound on a ceramic core moulded and pressed into an aluminium heat sink and usually with heat radiating fins. These are designed to be mounted to metal plates or a chassis to further conduct heat. This result in a rating approximately 5 times or more its normal rating.
These resistors come in a variety of accuracy's and TCRs. They can be custom made as a cross breed between a Precision Resistor and a Power Resistor; capable of handling more power than the standard Precision Wirewound but not as accurate. Practically speaking, tolerances of 1% and temperature coefficients of 20 ppm can be achieved on all except the parts that are coated with Vitreous Enamel and low values. The curing process for Vitreous (a type of glass) requires extremely
high heat and shrinks applying pressure to the winding. This particular group normally will run tolerances of 10% with a TCR of 100ppm/oC. Power Resistors come in a variety of ratings. Most are rated at +25oC and derated linearly to either +275oC or +350oC. Again if the ambient temperature of operation is +275oC, no power can be applied and at +125ooC 1/2 rated power can be applied.
These power rating are based on mounting the resistor in free air with the leads terminated at the recommended point. On axial lead components, this is 3/8 of an inch from the body. If they can be mounted closer, the resistor will run cooler or you can apply slightly more power and if mounted further out, you must reduce the power. CAUTION, if mounted directly over and in contact with a printed circuit board, the heat from the resistor can char the board if full power is
applied. I don't know of any PC Boards that are rated at +275oC.
Other means of increasing the amount of power you can apply
(a) bond the resistor to the chassis or other metal parts
(b) mount vertically to get the chimney effect (this is very helpful when using those wound ceramic tubes)
(c) terminate as close to the body as practical
(d) submerse in oil (CAUTION some types of resistor coating, particularly silicone based coatings will disintegrate when immersed in oil and heated). This will increase the rating as much as 5 times. or reduce the temperature rise of the resistor due to self heating.
The small power resistor can serve a two fold purpose, that is to fulfil it's purpose as a resistor and act as a heater in an enclosure. Some users have used them in crystal ovens to maintain the crystal at the desired temperature. It makes a reasonably cheap off the shelf heater that comes in a variety of wattage's , sizes and values.
One unique type of power resistor is the "Bathtub Boat Type". This consists of resistance wire wound on a fibreglass cord.. This is a continuously wound strip, cut into strips of the appropriate length with leads crimped. These resistive elements are placed in a ceramic shell (boat) and an highly filled cement is used to fasten these in the boat. The filler often used in the cement is a ceramic material with high heat conductivity. These are very inexpensive, no effort is made to
achieve tight tolerances, low TCRs, and the range of values is extremely limited. They are often found as surge resistors in TVs and other electronic /electrical equipment. Their main selling point is low cost. They are often sold with an enamel coating for a low power precision wirewound resistor that is even lower in cost.
One more item to consider, Power Wirewounds are made using alloys with melt temperatures ranging from +1200o C to +1500o C and may be operated cherry red without failure for short periods of time, however the resistance value and TCR will change significantly and the insulating material will severely degrade. The bathtub boat type can not be subjected to this type of overload, the fibreglass winding form will disintegrate.
Fuse Resistors serve a dual purpose, a resistor and a fuse. They are designed so that they will open with a large surge current. The fusing current is calculated based on the amount of energy required to melt the resistive material (the melt temperature plus the amount of energy required to vaporize the resistive material).
These resistors will normally run hotter than a normal precision or power resistor so that a momentary surge will bring the resistive element up to fusing temperature. Some designs create a hot spot inside the resistor to assist in this fusing. Calculations are made and samples are produced to verify the calculations. The major unknown is the heat transfer of the materials, which can be quite significant for pulse of long duration, and is very difficult to calculate. Mounting of
these devices is critical because it will effect the fusing current. These are quite often made to mount in fuse clips for more accurate fusing characteristics.
Carbon composition resistors were once the most common resistor on the market. They still have a very large market and prices are highly competitive. They are made from carbon rods cut in the appropriate length then moulded with leads attached. The mix of the carbon can be varied to change the resistivity for the desired values.
High values are much more readily available. Very low values are more difficult to achieve. A 5% tolerance is available. This is usually done by measuring and selecting values. Normal tolerances without measurement and selection is in the area of 20%.
The temperature coefficient of resistance is in the range of 1000 ppm/oC and is negative, that is when the temperature goes up the resistance goes down and when the temperature goes down, the resistance goes up. This is due to the carbon particles being relaxed (with increase in temperature) and being compressed (with the reduction in temperature).
These resistors also has a voltage coefficient. That is the resistance will change with applied voltage, the greater the voltage, the greater the change. In addition to a power rating, they also have a voltage rating. (The wirewound voltage rating is determined by the value and the wattage rating). The voltage rating of Carbon Composition Resistors is determined by physical size as well as the value and wattage rating.
One more item to consider is that due to their construction, they generate noise and this noise level varies with value and physical size. The power capability in relation to physical size is greater than Precision Wirewounds but less than Power Wirewounds.
CARBON FILM RESISTORS
Carbon Film Resistors have many of the same characteristics as carbon composition resistors. The material is similar therefore they have noise, a voltage coefficient, the TCR can be much lower because the formula can be varied to achieve this, the tolerance is much tighter due to the difference in manufacturing processes.
The Carbon Film Resistor is made by coating ceramic rods with a mixture of carbon materials. This material is applied to these rods in a variety of means, the one most familiar to me are dipping, rolling, printing , or spraying the rods in the appropriate solution. The thickness of the coating can be determined by the viscosity of the solution. This as well as the material composition will determine the ohms / square. Some of you may not be familiar with this term. It simply
means that if a material has a resistivity of 100 ohms / square, one square inch with have the same resistance as 1 square mm, or 1 square foot or 1 square yard or 1 square mile all equalling 100 ohms but the power handling capability is proportional to the size.
One batch of material can produce resistors in a wide range of values. These rods are cut to the length required for a specific size of resistor. These rods can then be spiral cut to a wide range of values. The original method of spiralling these was done with grinding wheels on a machine similar to a lathe. I am sure that later processes use lasers that are programmed to cut to specific values. The maximum ohmic value of this group is the highest in the discrete resistor
Tolerance of 1% can be achieved with out measuring and selecting. Tolerance of less than 1% can be achieved by measuring and selecting. You should use caution in getting tight tolerances in this type because the temperature coefficient, voltage coefficient and stability may mean that it is only good for that tolerance at the time it was installed. The TCR of carbon film resistors is in the neighbourhood of 100 to 200 ppm and is generally negative. Measuring and selecting can
yield even tighter TCRs.
The frequency response of this type of resistor is among the best, far better than Wirewounds, and much better that carbon composition. The wirewound resistors are inductive at lower frequencies and values and somewhat capacitive at higher frequencies regardless of value. Also wirewound resistors will have a resonant frequency. Carbon Composition Resistors will be predominately capacitive .
METAL FILM RESISTORS
Metal Film resistors are the best compromise of all resistors. They are not as accurate and have a higher temperature coefficient of resistance and are not as stable as Precision Wirewounds. They are more accurate, do not have a voltage coefficient, have a lower temperature coefficient than Carbon Film. TCRs of 50 to 100 ppm can be achieved.
They have a very low noise level when properly manufactured. In fact some of the screening processes measure the noise level to determine if there are problems in a particular batch of resistors.
Metal film resistors are manufactured by an evaporation/deposition process. That is the base metal is vaporized in a vacuum and deposited on a ceramic rod or wafer. Several attempts have been made to vaporize low TCR materials and deposit on these substrates, but to my knowledge, these attempts have not been successful. This is partially due to the different boiling points of the various base metals in these alloys (I use the word alloy not entirely accurately, for these
materials are not true alloys but amalgamations --- they do not bond to form a molecule as does a true alloy). The very low TCR resistive materials are heat treated to achieve the resistivity and low TCR. This is not compatible with an evaporation process.
The frequency characteristics of this type are excellent and better than Carbon Films. The one area that carbon films exceed metal films is the maximum values. Carbon films can achieve higher maximum values than any other group.
Foil resistors are similar in characteristics as metal films. Their main advantages are better stability than metal films and lower TCRs. They have excellent frequency response, low TCR, good stability, and very accurate. They are manufactured by rolling the same wire materials as used in precision wirewound resistors to make thin strips of foil. This foil is then bonded to a ceramic substrate and etched to produce the value required. They can be trimmed further by abrasive
processes, chemical machining or heat treating to achieve the desired tolerance. Their main disadvantage is the maximum value is less than Metal Film Resistors.
The accuracy is about the same as metal film resistors, the TCR and stability approaches Precision Wirewounds but somewhat less because the rolling process and the packaging process produce stresses in the foil. The resistive materials used in Precision Wirewound Resistors is very sensitive to stresses which result in instability and higher TCRs. Any stresses on these material will result in a change in the resistance value and TCR, the greater the stress, the larger the
change. This type can be used as strain gauges, strain being measured as a change in the resistance. When used as a strain gauge, the foil is bonded to a flexible substrate that can be mounted on a part where the stress is to be measured.
The Filament Resistors are similar to the Bathtub Boat Resistor except they are not packaged in a ceramic shell (boat). The individual resistive element with the leads already crimped is coated with an insulating material, generally a high temperature varnish. These are used in applications where tolerance, TCR, and stability are not important but the cost is the governing consideration. The cost on this type is slightly higher that carbon composition and the electrical
characteristics are better.
POWER FILM RESISTORS
Power film resistors are similar in manufacture to their respective metal film or carbon film resistors. They are manufactured and rated as power resistors, with the power rating being the most important characteristic. Power Film Resistors are available in higher maximum values than the Power Wirewound Resistors and have a very good frequency response. They are generally used in applications requiring good frequency response and/or higher maximum values. Generally for power
applications, the tolerance is wider, the temperature rating is changed so that under full load resistor will not exceed the maximum design temperature, and the physical sizes are larger, and in some cases, the core may be made from a higher heat conductive material and other means to help radiate heat.