Resistor:

Introduction:

 

A resistor is a passive two-terminal electrical component used in circuits to implement electrical resistance. Resistors have a variety of purposes in electronic circuits, including lowering current flow, adjusting signal levels, dividing voltages, biassing active components, and terminating transmission lines. High-power resistors that can generate many watts of heat instead of electrical energy can be utilised as test loads for generators, power distribution systems, and motor controls.With temperature, time, or operating voltage changes, fixed resistors' resistances only slightly fluctuate. Variable resistors can be utilised as force sensors, heat sensors, light sensors, volume controls, lamp dimmers, humidity sensors, and chemical activity sensors.

 

Resistor

Resistors are widely used in electronic equipment and are common components of electrical networks and electronic circuits. Practical resistors can be made of a variety of materials and forms as discrete components. Additionally, integrated circuits incorporate resistors.

 

The resistance of a resistor determines its electrical purpose; typical commercial resistors are produced over a range of more than nine orders of magnitude. The nominal value of the resistance is within the component's stated manufacturing tolerance.

 

Resistor

Types of Resistors:

1) Fixed Resistors:-

A. Carbon composition:

      Carbon composition resistors (CCR) are made out of a solid cylindrical resistive element with wire leads inserted in it or coupled to metal end caps. Paint or plastic are used to protect the resistor's body. Carbon composition resistors from the early 20th century had uninsulated bodies, and the lead wires were soldered after being wrapped around the ends of the resistance element rod. The finished resistor was painted in order to assign colours to different values.

 

In carbon composition resistors, the resistive component is created by mixing finely powdered carbon with an insulating substance, typically ceramic. The combination is held together by a resin. The proportion of the fill material (the powdered ceramic) to the carbon affects resistance. Because carbon is a good conductor, lower resistances are produced at higher carbon concentrations. In the 1960s and earlier, carbon composition resistors were widely utilised, but they are no longer widely used because alternative varieties have better specifications for tolerance, voltage dependence, and stress.

 

When exposed to excessive voltages, carbon composition resistors experience value changes. Additionally, soldering heat causes a non-reversible change in resistance value if internal moisture content is considerable, such as from prolonged contact to a humid environment. Since carbon content resistors have poor long-term stability, they were factory sorted to, at most, a 5% tolerance. Due to their non-inductive nature, these resistors are advantageous for applications involving surge protection and voltage pulse reduction. Relative to the size of the component, carbon composition resistors have a higher capacity to withstand overload.

 

There are still carbon composition resistors available, but they are rather pricey. Fractions of an ohm to 22 megohms were the values. These resistors are no longer employed in the majority of applications because of their exorbitant cost. They are utilised in welding controllers and power supplies, nevertheless. They are often sought for to repair old-fashioned electronic devices where authenticity is important.

B. Carbon pile:

        A stack of carbon discs crushed between two metal contact plates makes up a carbon pile resistor. The resistance between the plates can be changed by varying the clamping pressure. When a changeable load is needed, such as when testing radio transmitters or car batteries, these resistors are utilised. For tiny motors up to a few hundred watts in domestic appliances (sewing machines, hand-held mixers), a carbon pile resistor can also be utilised as a speed control.

In automatic voltage regulators for generators, a carbon pile resistor can be used to regulate the field current and maintain a comparatively constant voltage. The carbon microphone also utilises this idea.

 

C. Lead arrangements:

         Typically, "leads" for through-hole components emerge "axially" from the body, that is, parallel to the component's longest axis. Others, however, have leads exiting their bodies "radially." Surface mount technology (SMT) may be used for other components, and high power resistors may have one of their leads built into the heat sink.

 

D. Printed carbon resistors:

         As part of the manufacturing process for printed circuit boards (PCBs), carbon composition resistors can be printed directly onto PCB substrates. This method can be applied to conventional fibreglass PCBs, though hybrid PCB modules tend to employ it more frequently. Typically, tolerances are fairly high and can reach 30%. Non-critical pull-up resistors would be an example of a typical use.

 

E. Carbon film:

        In order to generate a long, narrow resistive route, a helix is sliced into the carbon film before it is deposited on an insulating substrate to create carbon film resistors. Amorphous carbon has a resistivity that ranges from 500 to 800 m, which can produce a wide range of resistance values. Due to the exact distribution of pure graphite without binding, carbon film resistors have less noise compared to carbon composition resistors.Power ratings for carbon film resistors range from 0.125 W to 5 W at 70 °C. The range of available resistances is 1 ohm to 10 megaohm. The carbon film resistor may operate between 55 and 155 degrees Celsius. The maximum working voltage range for it is 200 to 600 volts. Applications demanding great pulse stability employ special carbon film resistors.

 

F. Metal film:

                The metal-film resistor is a popular axial-leaded resistor today. Resistances with Metal Electrode Leadless Face (MELF) frequently employ the same technology.

 

Nickel chromium (NiCr) is typically used to coat metal film resistors, however any of the cermet compounds mentioned above for thin film resistors may also be used. The material can be applied using methods other than sputtering, unlike thin film resistors (though this is one technique used). Similar to how carbon resistors are created, the resistance value is calculated by cutting a helix through the coating as opposed to etching. The end result is a temperature coefficient that is typically between 50 and 100 ppm/K and a respectable tolerance (0.5%, 1%, or 2%). Due to a low voltage coefficient, metal film resistors provide good noise characteristics and minimal non-linearity. They are advantageous because of their long-term stability.

 

G. Metal oxide film:

           In comparison to metal film, metal-oxide film resistors have a higher operating temperature, more stability, and greater dependability. They are utilised in tasks that need great levels of endurance.

 

2) Variable Resistor:

A. Adjustable resistors:

         The resistance of a resistor can be varied by routing the connected wires to various terminals using one or more set tapping points. In order to employ a larger or smaller portion of the resistance, certain wirewound power resistors contain a tapping point that slides along the resistance element.

 

The sliding resistance tap can be attached to a knob that an operator can reach when continuous modification of the resistance value during equipment operation is necessary. A rheostat is a device with two terminals that performs this function.

 

B. Potentiometers:

        A three-terminal resistor with a constantly changeable tapping point that is managed by the rotation of a shaft, a knob, or a linear slider is referred to as a potentiometer (colloquially, a pot).  A voltage divider that can be adjusted to generate a variable potential at the terminal attached to the tapping point is what gives a potentiometer its name. A potentiometer is frequently used for volume control in audio equipment.A basic low power potentiometer is made up of a flat resistance element (B) made of conductive plastic, metal film, or carbon composition, and a springy wiper contact (C) that moves across the surface. Resistance wire wound on a form with the wiper sliding axially along the coil is an alternative structure. Since the resistance changes as the wiper advances in increments equal to the resistance of one turn, these have a lesser resolution.

 

Precision applications require multiturn potentiometers with high resolution. These have wire-wound resistance elements that are commonly coiled on helical mandrels, and as the control is cranked, the wiper moves along a helical track, making constant contact with the wire. To increase resolution, certain wires have a conductive-plastic resistance coating. To span their whole range, these normally offer ten shaft spins. They can normally achieve three-digit precision and are typically equipped with dials that contain a graduated dial and a basic turns counter. They were widely employed in electronic analogue computers for establishing coefficients, and newer delayed-sweep oscilloscopes had one on the panel.

 

C. Resistance decade boxes:

        A resistance decade box, also known as a resistor substitution box, is a device that contains resistors in a variety of values and has one or more mechanical switches that enable any one of the box's several discrete resistances to be dialled in. The resistance is often accurate to high precision, ranging from field grade at 1% to laboratory/calibration grade accuracy of 20 parts per million. There are other inexpensive boxes with less accurate options. In laboratory, experimental, and development work, all types provide an easy way to choose and quickly change a resistance without having to attach resistors one at a time or even stock each value.The box's characteristics include the resistance range it offers, its highest resolution, and its accuracy. One box, for instance, provides resistances from 0 to 100 megohms, with a maximum resolution of 0.1 ohm and an accuracy of 0.1%.