Tuesday, 10 September 2013

Resistors -- Parameters, Types

Parameters of Resistors:
  • Rated Power: It is the maximum power up to which a resistor can withstand without burnout. For example consider a 100 ohm, 0.125 Watt resistor. The maximum current that can be allowed to flow through this resistor is 

So In a circuit, the current flowing through this 100 ohm resistor should not exceed the above said value. If current exceeds this, power dissipation will be greater than maximum power handling capability of resistor, which makes resistor to burn.
  • Tolerance: This parameter specifies the maximum and minimum allowable deviation from the desired resistor value. It is represented in %. For example consider 100 ohm, 5% tolerant resistor. This means that resistor value 100 ohm, need not be exactly 100 ohm but it can vary by 5% more or less. So it can be anywhere between 95 ohm and 105 ohm.
  • Temperature coefficient of resistor (TCR): This parameter gives change in resistor value with the temperature. This is specified in ppm/C (parts per million). TCR specification is generally at 25C. For example consider a 100 ohm resistor, 100 ppm TCR resistor. Then for a change of 100C temperature resistance value changes by 1%.
  • Voltage coefficient of resistor (VCR): This parameter gives change in resistor value with applied voltage across it. This is specified in ppm/V. For example consider a 100 ohm resistor, 100 ppm VCR resistor. Then for a change of 100V voltage across a resistor, resistor value changes by 1%.
  • Rated voltage: This is the maximum DC voltage or RMS AC voltage that can be applied across a resistor at rated temperature.

Types of resistors:

  • Metal Film resistor: Metal film resistor is made by sputtering resistive material over alumina substrate. Resistive material can be Ni-Cr Alloy, Tantalum nitride or lead oxide etc. The resistance value can be trimmed to a particular value by cutting spiral groove into the resistive material. This entire arrangement is coated in epoxy. These resistors exhibit good linearity.  Resistance values will have tolerances of 5%, 1%, to 0.5% and also TCR value can be as low as 25ppm/C. By using Laser trimming technique, still lower tolerance resistors are achievable. These types of resistors are widely used in military applications.
  • Carbon film resistor: Carbon film resistor is made by depositing carbon film over an insulating substrate such as ceramic rod. Helical cut is made into the carbon film to trim the resistor to a particular value. These resistors will have poor tolerances. These are widely used in commercial applications, because of their low cost.

  • Metal oxide film resistors: These resistors are made by coating a ceramic rod with material like tin-oxide. These resistors have higher power rating, voltage rating and operating temperature than metal film and carbon film resistors. But have poor tolerances at lower values and its TCR is also higher than metal film resistors. These are used in high temperature applications. 
  • Metal Glaze resistor: Suspension of glass and metal particles in organic solvent are used as resistive material in metal glaze resistors. Alumina rod will be dipped into that solution and will be withdrawn at a controlled rate. This alumina rod will be heated to 900 to 1100C and leads will be soldered on both ends. Different values of resistors can be trimmed to a value by helical cuttings in the resistive material formed on the alumina rod. These resistors can be used at extreme high temperatures and environmentally harsh environments. 

    Carbon composition resistors:
    Finely grind carbon mixed with ceramic powder is used as resistive material in carbon composition resistors. The ratio of carbon to ceramic powder determines the resistance value. These resistors have poor tolerance (5% to 20%). These resistors are used in high voltage pulse applications.
  • Wire wound resistors:
    Wire wound resistors are formed by winding a nichrome wire on an insulating substrate. These resistors can handle high power dissipation. These resistors are generally housed in ceramic outer case or aluminum housing. Aluminum housing resistors generally handle high power, by connecting aluminum housing to a heat sink.

  • Foil type resistors:
    Special metal alloy (manganese-ceranin) of several micrometers thickness is used as resistance element in foil resistor. This metal foil will be mounted on a thermally conductive material. These resistors have high stability and low TCR. These resistors are used in precision applications. These resistors are available in the range of several milliohms to several kilo ohms. Generally used for current measurement applications (Shunt).

  • Thin film resistors:Thin film resistors are fabricated by vaccum depositing resistive material on a ceramic substrate and will be etched through photolithography process. The resistive material is trimmed to a desired value, through laser trimming. The thickness of material deposited is 10nm to 2 um. These are resistors have good TCR and low tolerance than thick film resistors. So these are used in applications, which require precision resistors like measurement applications.
  • Thick film resistors:
                    Thick film resistors are fabricated by screen printing resistive material over ceramic substrate. The resistive material is trimmed to required value through laser trimming. The thickness of resistive material is 10 to 50um. These resistors have poor TCR and tolerance than thin film resistors. Cost of thick film resistors is less. 
    MELF resistors: MELF (Metal electrode leadless face) resistors consist of film of metal alloy deposited on ceramic substrate. Nickel caps are pressed on both sides of above mentioned structure. Then a helical groove is made by laser cutting to get desired resistance value. These resistors have high stability, accuracy, and pulse handling capability. These resistors allow high temperature operation. These are cylindrical surface mount devices. These are used high reliability electronics, harsh operating environment.

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