Types of Resistors

Carbon Composition
It is a combination of carbon particles and a binding resin with different proportions for providing
desired resistance. Attached to the ends of the resistive element are metal caps which have axial
leads of tinned copper wire for soldering the resistor into a circuit. The resistor is enclosed in a
plastic case to prevent the entry of moisture and other harmful elements from outside. Billions of
carbon composition resistors are used in the electronic industry every year. They are available in
power ratings of lI8, lI4, 1/2, I and 2 W, in voltage ratings of 250,350 and 500 V. They have low
failure rates when properly used.
Such resistors have a tendency to produce electric noise due to the current passing from one
carbon particle to another. This noise appears in the form of a hiss in a loudspeaker connected to a
hi-fi system and can overcome very weak signals. That is why carbon composition resistors are used
where performance requirements are not demanding and where low cost in the main consideration.
Hence, they are extensively used in entertainment electronics although better resistors are used in
critical circuits.
(b) Deposited Carbon
Deposited carbon resistors consist of ceramic rods which have a carbon film deposited on them.
They are made by placing a ceramic rod in a methane-filled flask and heating it until, by a gascracking
process, a carbon film is deposited on them. A helix-grinding process forms the resistive
path. As compared to carbon composition resistors, these resistors offer a major improvement in
lower current noise and in closer tolerance. These resistors are being replaced by metal film and
metal glaze resistors. '
(c) High-Voltage Ink Film
These resistors consist of a ceramic base on which a special resistive ink is laid down in a helical
band. These resistors are capable of withstanding high voltages and find extensive use in cathoderay
circuits, in radar and in medical electronics. Their resistances range from I ill to 100,000Mil
with voltage range upto 1000 kV.
(d) Metal Film
Metal film resistors are made by depositing vaporized metal in vacuum on a ceramic-core rod.
The resistive path is helix-ground as in the case of deposited carbon resistors. Metal film resistors
have excellent tolerance and temperature coefficient and are extrememly reliable. Hence, they are
very suitable for numerous high grade applications as in low-level stages of certain instruments
although they are much more costlier.
(e) Metal Glaze
A metal glaze resistor consists of a metal glass mixture which is appied as a thick film to a
ceramic substrate and then fired to form a film. The value of resistance depends on the amount of
metal in the mixture. With helix-grinding, the resistance can be made to vary from I Q to many
megohms.
Another category of metal glaze resistors consists of a tinned oxide film on a glass substrate.
if) Wire-wound
Wire-wound resistors are different from all other types in the sense that no film or resistive
coating is used in their construction. They consist of a ceramic-core wound with a drawn wire
having accurately-contralled characteristics. Different wire alloys are used for providing different
resistance ranges. These resistors have highest stability and highest power rating.
Because of their bulk, high-power ratings and high cost, they are not suitable for low-cost or
high-density, limited-space applications. The completed wire-wound resistor is coated with an insulating
material such as baked enamel.
(g) Cermet (Ceramic Metal)
The cermet resistors are made by firing certain metals blended with ceramics on a ceramic
substrate. The value of resistance depends on the type of mix and its thickness. These resistors have
very accurate resistance values and show high stability even under extreme temperatures. Usually,
they are produced as small rectangles having leads for being attached to printed circuit boards (PCB).