RESISTOR

Resistor component of an electric circuit that resists the flow of direct or alternating electric current.An electric current is the movement of charged particles called electrons from one region to another.The amount of resistance to the flow of current that a resistor causes depends on the material it is made of as well as its size and shape. Some resistors obey Ohm's law, which states that the current density is directly proportional to the electrical field when the temperature is constant.The resistance of a material that follows Ohm's law is constant, or independent of voltage or current, and the relationship between current and voltage is linear. Modern electronic circuits depend on many devices that deviate from Ohm's law. In devices such as diodes, the current does not increase linearly with voltage and is different for two directions of current.Resistors are often made to have a specific value of resistance so that the characteristics of the circuit can be accurately calculated.Most resistors used in electric circuits are cylindrical items a few millimeters long with wires at both ends to connect them to the circuit.

What can a resistor do

Resistors can help divide voltages, and when combined with other elements can help convert voltages for a specific electrical design.Resistors can also be used to provide intense light or heat.For example, the heating element in a household cooking range is a resistor, as is the tungsten filament in a common incandescent lamp.

How to read the code

First find the tolerance band, it will typically be gold ( 5%) and sometimes silver (10%).
Starting from the other end, identify the first band - write down the number associated with that color; in this case Blue is 6.
Now 'read' the next color, here it is red so write down a '2' next to the six. (you should have '62' so far.)
Now read the third or 'multiplier' band and write down that number of zeros.
In this example it is two so we get '6200' or '6,200'. If the 'multiplier' band is Black (for zero) don't write any zeros down.
If the 'multiplier' band is Gold move the decimal point one to the left. If the 'multiplier' band is Silver move the decimal point two places to the left. If the resistor has one more band past the tolerance band it is a quality band.
Read the number as the '% Failure rate per 1000 hour' This is rated assuming full wattage being applied to the resistors. (To get better failure rates, resistors are typically specified to have twice the needed wattage dissipation that the circuit produces). Some resistors use this band for temco information. 1% resistors have three bands to read digits to the left of the multiplier. They have a different temperature coefficient in order to provide the 1% tolerance.
At 1% the temperature coefficient starts to become an important factor. at +/-200 ppm a change in temperature of 25 Deg C causes a value change of up to 1%

RESISTOR TYPES

PRECISION WIREWOUND

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.

NIST STANDARD

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 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.