Resistance values detailed above are a constant and do not change if the voltage or current-flow alters. But there are circuits that require resistors to change value with a change in temperate or light. This function may not be linear, hence the nameNONLINEAR RESISTORS.
There are several types of nonlinear resistors, but the most commonly used include : NTC resistors (figure a) (Negative Temperature Co-efficient) - their resistance lowers with temperature rise. PTC resistors (figure b) (Positive Temperature Co-efficient) - their resistance increases with the temperature rise. LDR resistors (figure c) (Light Dependent Resistors) - their resistance lowers with the increase in light. VDR resistors (Voltage dependent Resistors) - their resistance critically lowers as the voltage exceeds a certain value. Symbols representing these resistors are shown below.
Fig. 1.4: Nonlinear resistors - a. NTC, b. PTC, c. LDR
In amateur conditions where nonlinear resistor may not be available, it can be replaced with other components. For example, NTC resistor may be replaced with a transistor with a trimmer potentiometer, for adjusting the required resistance value. Automobile lightmay play the role of PTCresistor, while LDR resistor could be replaced with an open transistor. As an example, figure on the right shows the 2N3055, with its upper part removed, so that light may fall upon the crystal inside.