Cermets are a hybrid mixture of ceramic and metal. These are made by screening a mixture of glass and metal oxide on to a ceramic substrate. It is then fired at a temperature high enough to melt the glass. These resistors are immune to humidity and have better temperature coefficient than carbon composition.
Range of values 50 ohms to 5 mega ohms
Power rating 0.5 W to 5 W
Voltage rating 200 to 500 volts
Tolerances 10% for 50 ohms to 5 mega ohms
MULTI TURN POTENTIOMETERS:
A metal ceramic resistance track is placed in dust proof plastic case. The ends, leads and wiper connections are brought out for mounting on a plastic coated base. The wiper is made up of gold plated bronze. It travels along the resistance track via stainless steel lead screw.
Law of operation linear
Value 10 to 500 kilo-ohms
Power rating 1 W
Temperature coefficient of resistance 200ppm/0C
Life expectancy 500 cycles
These are potentiometer used for adjusting resistance value to the correct required value. These are of two types basically.
(1) Single turn
(2) Multi turn
Carbon composition, cermets and wire would elements are used in preset potentiometers (trimmers) with a variety of package sizes and power ratings.
In metal film type resistors, the resistance element consists of a suitably shaped insulating core (ceramic or plastic) on which the film of metal or metal oxide is deposited.
NON-LINEAR RESISTORS: Those resistors which do not obey ohm's law are called non-linear resistors. They can be divided into three categories:
(2) Voltage dependent resistor
(3) Light dependent resistor
Thermistors: A thermistor is a temperature dependent resistor. There are two kinds, classified according to the sign of their temperature coefficients.
(a) PTC THERMISTOR: A positive temperature coefficient (PTC) thermistor is a resistor with a positive temperature coefficient. When the temperature rises, the resistance of PTC increases.
These are manufactured from barium titanate, titanium dioxide, powdered barium carbonate and strontium carbonate. These are mixed and calcined at 12000C. After pulverizing the resultant solid, pellets are pressed and fired in a controlled atmosphere. Electrical properties are dependent upon material used and thermal conditions. Electrodes are screened and fired on the pellets leads are then attached with very high temperature solder.
APPLICATIONS: PTCs are often found in television in series with the demagnetizing coil. There they are used to provide a short-duration current burst through the coil when the TV is turned on. One specialized version of a PTC is the poly switch which acts as a self-repairing fuse.
NTC THERMISTORS: A NEGETIVE temperature coefficient (NTC) thermistor is also a temperature dependent resistor, but with a negative temperature coefficient. When the temperatures rises, then the resistance of NTC drops.
MANUFACTURING: These are made up of semiconductors or ceramic materials. Various mixtures of oxides of manganese, Nickel, cobalt, titanium, copper-iron and uranium are sintered. Electrical characteristics are controlled by varying both the type of oxide and physical shape of thermistor. Standard configurations of this are beads, glass, probes, discs, rods, and washers.
APPLICATIONS: NTC Thermistors are often used in simple temperature detectors and measuring instruments. A thermistor and resistor in series is used for time delay and surge suppression.