A resistor is an electric device that is designed in such a way that it offers resistance to the flow of electric current in a circuit. Resistance to the flow of electricity is necessary for a number of reasons. First, in some circuits, there is need to offer resistance to the flow of electric current so that there is the heat that is generated by the electrical circuit. The heat could be used for heating purposes. Resistance may also be important in order to vary the intensity of electric currents into devices that operate well with varying electric currents.
Resistors can be custom made or they can be bought off the shelf. Off the shelf, resistors come already preconditioned based on the manufacturers' specifications. On the other hand, custom resistors are designed and made only when there is a demand for them. This may be necessary especially if the client is looking for something that cannot be obtained from the off the shelf resistors. Custom resistor manufacturing also is critical in cases where there is a need to cater for special features that may be too extreme to include in the normal resistors such as the amount of current and voltage admitted through a resistor.
The choice of a resistor for either of the above types usually depends on a number of factors. First, the resistance offered by the resistor is something that you should always take into account. Since the primary role of a resistor is to offer resistance to the flow of electricity, then any client should be interested in this aspect of any resistor they choose to buy. If for instance, you have a need for a stronger resistance, then a resistor with a higher resistor rating will be the one you choose. On the other hand, you will choose a resistor with a lower rating in the event that you need less resistance for your circuit.
Finally, you ought to also take into account the aspect of the resistance range that a resistor offers. Some resistors will fail beyond a certain power level. This is because they are wired to accept only a certain limit of power beyond which they fail. As a rule, therefore, by gauging the amount of power your circuit is likely to use, then you can select a resistor within the appropriate range of the power resistors you will be using.