There are 4 basic classifications of resistor: fixed, variable, linear and non-linear. A resistor is designed to have a known resistance value or “Those components and devices which are specially designed to have a certain amount of resistance and used to oppose or limit the electric current flowing through it are called resistors,” ElectricalTechnology.org noted.

When choosing what type of resistor to use for a given circuit there are a number of factors to be considered.

• Resistance

• Tolerance

• Rated Power Dissipation

• Package and mounting

• Voltage rating

• Material construction

• Parasitics (Inductance and Capacitance)

• Thermal range

• Noise

Resistance is “the fundamental selection criteria for a resistor,” as CircuitMaker.com notes. Though it should be noted the influence of the other factors above will impact the resistor selected, as multiple specific needs to address arise with each circuit. You need to determine what temperature ranges it will function under, what power needs, both power source and how it impacts the circuit.

The selection process should refer to the resistor color coding which provides elements to the resistor that help with your selection.

When choosing packaging and mounting the following questions should be considered.

 “Are you going to hand-solder a one-off? Are you prototyping or intend to do small batch assembly? Are you going to mass volume production? Is cost your primary concern? Do you need the resistor to withstand high voltages? Does it have to dissipate a lot of heat?”.

Given resistors control the flow of current, the impedance creates heat that has to be dissipated. This allows heat to build up and can cause circuit failure if the right power-rated resistor is not used.

Similarly, voltage rating becomes important depending on the size of the circuit. Smaller units mean contacts are closer together and if the right voltage rating of the resistor is not used it can cause arcing. High voltage systems require the use of larger or multiple resistors in some circuits to address the potential power dissipation needs.

Noise is also a factor. While resistors are generally simple passive devices they can be noisy. There are three main types of noise in resistors: shot, thermal and flicker.

In electronics shot noise originates from the discrete nature of electric charge. Thermal noise “is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltage. Thermal noise is present in all electrical circuits”. Flicker noise “occurs in almost all electronic devices and can show up with a variety of other effects, such as impurities in a conductive channel, generation and recombination noise in a transistor due to base current, and so on”.

When choosing based on noise factors it should be noted that the lower valued resistors have less noise than their higher valued alternatives.

Parasitics are the most difficult factors to measure, as manufacturers do not include the information about them in their products. As CircuitMakers explains “parasitic capacitance which is distributed between the terminals, as well as between the resistor surface and other structures in the design (like power or ground planes). When resistors are employed with heavy currents in power circuits, or at high frequencies as in RF circuits, the parasitics can have an impact”. 

These factors should be considered when selecting the right resistor.