A resistor will resist the flow of electricity equally in both directions. It is defined as a circuit element that presents a resistance to the flow of electric current. A current flowing through a resistance will create a voltage drop across that resistance in accordance with Ohm's law.
Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.
Resistors are made in a variety of ways because their particular characteristics and accuracy suit certain areas of application, such as High Stability, High Voltage, High Current etc, or are used as general purpose resistors where their characteristics are less of a problem. Some of these characteristics are; Temperature Coefficient, Voltage Coefficient, Noise, Frequency Response, Power and Temperature Rating, Physical Size, and Reliability.
Modern resistors can be classified into four broad groups;
- Compostion resistors.
- Film resistors.
- Wirewound resistors.
- Semiconductor resistors.
- non-inductive