Elenco Electronic Component Kit - Manual (Page 3)

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RESISTORS
RESISTORS, How are they made?
There are many different types of resistors used in
electronics. Each type is made from different
materials. Resistors are also made to handle
different amounts of electrical power. Some
resistors may change their value when voltages are
placed across them. These are called voltage
dependent resistors or
nonlinear resistors. Most
resistors are designed to change their value when
the temperature of the resistor changes. Some
resistors are also made with a control attached that
allows the user to mechanically change the
resistance. These are called variable resistors or
potentiometers. Figure 2 shows physical shapes of
some different types of resistors.
The first commercial resistors made were formed by
wrapping a resistive wire around a non-conducting
rod (see Figure 3). The rod was usually made of
some form of ceramic that had the desired heat
properties since the wires could become quite hot
during use. End caps with leads attached were then
placed over the ends of the rod making contact to
the resistive wire, usually a nickel chromium alloy.
The value of wirewound resistors remain fairly flat
with increasing temperature, but change greatly with
frequency. It is also difficult to precisely control the
value of the resistor during construction so they
must be measured and sorted after they are built.
By grinding carbon into a fine powder and mixing it
with resin, a material can be made with different
resistive values. Conductive leads are placed on
each end of a cylinder of this material and the unit is
then heated or cured in an oven. The body of the
resistor is then painted with an insulating paint to
prevent it from shorting if touched by another
component. The finished resistors are then
measured and sorted by value (Figure 4). If these
resistors are overloaded by a circuit, their resistance
will permanently decrease. It is important that the
power rating of the carbon composition resistor is
not exceeded.
Figure 2
Carbon Film
Variable
Carbon Composition
THE WIREWOUND RESISTOR
Figure 3
THE CARBON COMPOSITION RESISTOR
Ceramic Rod
Wire
End Cap Protective Coating
Figure 4
Insulating Paint Carbon & Resin
Mixture
Conductive Wire