Guide to

How to choose resistor types to use for a given circuit?

Resistor is a two terminal passive device that is used to introduce resistance in electrical circuits. Resistance is defined as property of a material/ resistor to oppose flow of current through it. It is a fundamental component of electronics, and is used in circuits to limit the flow of current. The higher the magnitude of the resistor, higher will be opposition to the flow of current. Practical resistors being discrete elements can be manufactured using different materials (i.e. carbon, Bakelite, Metal films etc.)  and in different forms, whoever resistors also implemented in integrated circuits using semiconductors.

Unit of resistance is Ohm (Ω), which is defined as volt/ ampere. Resistance of a resistor will be one ampere if a potential difference of one volt forces a current of one ampere through the circuit. Derived units of resistance like milli ohm, kilo ohm, mega ohm etc. are also very commonly used.

R=V/I

Where

R = Resistance (Ohm)

V = Potential Difference (Volt)

I = Current (Ampere)

Since resistor is a fundamental component of circuits and most commonly used two terminal devices, many types of resistor are available depending upon the material used for the manufacturing of resistor or either they are fixed type or variable, a large inventory of resistors is available in the market. Resistance of a resistor is purely dependent on its physical parameters and material properties.

R= ρ L/A

Where

ρ = resistivity of particular material (Ωm)

L = length (m)

A = Area (m2)

How a resistor will behave in a circuit depends on the conditions in which a particular resistor is employed. Applied voltage, current through resistor, frequency of operation and temperature all affect the performance of resistor, and the ability of the resistor to withstand these conditions will depend on the material and manufacturing technique used to manufacture the resistor. Performance of resistor depending upon material (wirewound, carbon film, metal film) used for its manufacturing or type of resistor is described below.

Wire wound resistors as the name suggest are manufactured by winding a wire that has controlled properties, generally nickel-chromium alloy on the ceramic tube. Different alloy wires are used to provide different ratings. Wirewound resistors are bulky and occupy more space and hence not suitable for applications where space is an issue. Winding of the wire wound resistor has both inductive and capacitive parameters, their effect becoming significant at higher frequencies so these types of resistor are not suitable for high frequency (greater than 50K Hz) circuits. As they can withstand high temperatures so can be used in applications where temperature can go up to 450 C.

Composition resistors are manufactured by mixing carbon particles with a binder. Finally, the resistor is enclosed in a protective packing and axial leads are provided for attaching the resistor into the circuit. These resistors are small and suitable for applications where there are space constraints. These resistors can produce a reasonable amount of noise due to the current passing from one carbon particle to another within the resistor, thus these types of resistor are not suitable where performance demands are high. As they are cheap and can withstand temperatures up to 130 C these resistors are widely used in general application circuits

Carbon film resistors are manufactured by deposition of carbon film on an insulating substrate. These resistors produce very little noise as compared to carbon composition resistors as they consist of a single piece of carbon film. These resistors can be used in applications where temperature is expected to between -55 to 150 C. For circuits requiring high stability, carbon film resistor is a good choice

Metal Film resistors are manufactured using nichrome or tin oxide in a molded phenolic case. Metal film resistors are reliable with excellent temperature characteristics. They are suitable for high grade applications. Since they are costly, they should not be used in budget constraint circuits.

Ceramic resistor or ceramic metal resistors are manufactured by using a mixture of metal and ceramic. These resistors are very accurate, stable and can withstand high temperatures. They are in rectangular shape and usually mounted on printed circuit boards.