Passive Solid State Devices - Resistors PDF

LECTURE IV/I PASSIVE SOLID STATE DEVICES 1  VI/I PASSIVE SOLID STATE DEVICES 2.1 Passive solid state devices Passive solid state devices are – resistors – capacitors and – inductors...

LECTURE IV/I PASSIVE SOLID STATE DEVICES 1  VI/I PASSIVE SOLID STATE DEVICES 2.1 Passive solid state devices Passive solid state devices are – resistors – capacitors and – inductors 2  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS 2.1.1 RESISTORS : Resistors are used in circuits – to control the flow of current and, in the process, – produce dissipation of power. 3  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS For a conductor with – constant cross sectional area A and – length XL the resistance R (Ohms) or conductance G (Mhos) is R = ρXL/A = 1/G where ρ is the resistivity (ohm-meter) [ conductivity σ (mhos per meter) = 1/ρ ] 4 VI/I PASSIVE SOLID STATE DEVICES - RESISTORS A few types of A pack of resistors resistors 5  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS In an ideal resistor, the resistance remains constant regardless of the – applied voltage – current through the device or – environmental factors Real resistors present variation in electrical resistance with changing – temperature , other environmental factors as well as with – Aplied voltage and current through the device. 6  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS Real resistors also introduce some – inductance and – a small amount of capacitance, which change the dynamic behaviour of the resistor from the ideal. 7  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS To select a resistance following parameters must be defined : – resistance value, – tolerance, – power rating. – temperature coefficient – type (material and construction) 8  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS Common power ratings for standard resistors are 1/8 watt, 1/4 watt, 1/2 watt, 1 watt and 2 watts Resistors can be in – “through hole component” (THC) form with wire leads or – in “surface-mount” packages without wire leads. 9 VI/I PASSIVE SOLID STATE DEVICES - RESISTORS “Some through hole resistors” 10 VI/I PASSIVE SOLID STATE DEVICES – RESISTORS Resistors come in four basic types :  Carbon Composition (Molded Carbon)  Wirewound  Film  Thick and thin film resistors The only available resistor types until the 1960s, were wire wound and carbon composition. After 60’s there was a shift to film resistors. The majority of resistors today are film type. 11  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS 1. Carbon composition resistors : Carbon composition resistors were used in the 1960's and earlier, but are not so popular for general use now as other types have better specifications. Carbon Composite Resistor The resistive element is a slug made Slug  leads  mold  bake  seal from a mixture of - cheap, poor tolerance, rugged – powdered carbon and – ceramic. The mixture is held together by a resin. 12 After baking it has a solid form.  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS As the entire carbon composition body (not a layer of film) conducts the energy (higher thermal mass) carbon composition resistors have higher energy capability than other resistors. Carbon resistors can be made with a higher resistance than wirewound resistors and are considerably cheaper. However, the properties are inferior in terms of temperature coefficient, noise and voltage dependence. 13  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS Carbon composition resistors are : Good points : 1. Inexpensive, 2. Of low inductance and capacitance 3. Extremely durable and able to tolerate rough handling Bad points : 1. Not good for low noise applications, 2. Not very stable* (for example, the resistance value can change up to 5% over a shelf life of one year). 3. High TCR (Temperature Coefficient of Resistance) (+400 -1000ppm/ºC), 4. Has a positive voltage coefficient * (VCR) *Stability is evaluated as the change in resistance value for a specified life or use. 14  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS 2. Wirewound resistor : Wirewound resistors are commonly made by winding a metal wire around a ceramic, plastic, or fiberglass core. The winding is protected by a silicone or epoxy coating* or encased in a ceramic or metal tube. * epoxy coating is a protective material made from epoxy-based resin 15 VI/I PASSIVE SOLID STATE DEVICES - RESISTORS Wirewound resistor wind wire → leads → mold → seal 16 VI/I PASSIVE SOLID STATE DEVICES - RESISTORS There are two categories of Wirewound Resistors : Precision wirewound Power wirewound 17  VI/I PASSIVE SOLID STATE DEVICES - RESISTORS 2.1. Precision wirewound Range between 10Ω – 100kΩ (high resistances require thin and too long wires). Good Points are : highly accurate (