Nonlinear resistors
A nonlinear resistor is a circuit element that shows nonlinearity in opposition to the current or, whose V/I characteristic is not a straight line and exhibits a resistance that is nonlinear in nature. Various examples of nonlinear resistors are thermistors, varistors, diodes, and filaments of incandescent lamps. Thermistors and varistors are discussed in detail below.
Table of Contents
Thermistors:
A thermistor, a nonlinear resistor, is a heat-sensitive device usually made of a semiconductor material whose resistance changes very rapidly with the change in temperature. The important properties of the thermistors are cited below:
- The change in the resistance of a thermistor is very rapid and it alters with the change in temperature.
- Its temperature coefficient is very high.
- The temperature coefficient can be both positive as well as negative.
Construction:
Thermistors are made from metal oxides of iron, nickel, and cobalt along with binders. They are generally given the form of beads, discs, or rods during manufacturing. Two platinum leads are commonly attached at the two ends for electrical connections. The arrangement is enclosed in a very small bulb made up of glass or epoxy and is sealed.
The figure shows the resistance/temperature characteristic of a typical thermistor with a negative temperature coefficient. The resistance decreases progressively from 4000 Ω to 3 Ω as its temperature varies from – 50°C to +150°C.
Applications
A thermistor with a negative temperature coefficient of resistance may be used to safeguard against current surges in a circuit where this could be harmful e.g. in a circuit of an old radio where the heaters of the radio valves are in series.
A thermistor T is included in the circuit. When the supply voltage is switched on, the thermistor has an initial high resistance because of low temperature. Therefore, the high resistance makes sure that the current is clamped to a low value. As it warms up, with the rise in temperature, the resistance of the thermistor drops considerably, and thus the flow of current through the heater increases.
A thermistor with a negative temperature coefficient can be used to issue an alarm for excessive temperature of the winding of motors, transformers, and generators. Whenever the temperature of the winding is lower and the thermistor is comparatively cool, it offers high resistance to the flow of current in this state.
Therefore, only a minimal current flows through the thermistor and the relay coil, which is insufficient to actuate the contacts. When the temperature rises in the windings, making the thermistor hot, its resistance to the flowing current becomes low. This low resistance makes a large current flow in the relay coil possible and the closure of the contacts is achieved. This completes the circuit as shown in the figure and the lamp or buzzer activates.
Varistor (Thyrite):
A varistor is a nonlinear resistor in which the resistance inversely follows the voltage which means the resistance falls with the rise in the potential difference. Therefore, a varistor is a voltage-dependent resistor. Varistors are basically made from the powder of silicon carbide and other metal oxides and are manufactured in a disc shape. The Voltage vs current characteristic of a typical varistor is shown.
The curve represents the current on the Y-axis, which increases drastically with the increase in the voltage in the X-axis. Thus, when the voltage increases from 1.8 kV to 10 kV, the current rapidly increases from 1 mA to 100 A. Varistors are always placed in parallel in critical components because the high transient voltages can cause damage to them. Under normal conditions, the varistor offers a high-resistance state and consequently draws very little current. On the occurrence of a voltage surge, the varistor shifts to its low-resistance state conducting a relatively large amount of current and dissipating much of the surge energy as heat. Thus, the connected equipment in parallel is saved from damage.
Applications
- Nonlinear resistors are used in telecommunication equipment to protect against surges.
- In the renewable energy industry, varistors are used to protect solar inverters and wind turbine electronics from various surges.
- Nonlinear resistors as varistors are also used in domestic appliances like refrigerators, and air conditioners to protect against electric transients.
- Varistors are used in the filtering circuits to suppress electrical noise.
- They are used in the capacitor protection against severe voltage spikes which can potentially cause damage to the capacitors.
