Figure 20. A larger wire wound
pot that is panel mountable and quite
sturdy, appearing on the front panel
of industrial instrumentation.
components, and are physically
smaller and capable of dissipating
less power than the larger wire wound
pot Figure 20 demonstrated.
Some trimmers are quite precise
and offer very good resolution. Figure
22 shows a cut-away view of a multi-turn trimmer. Note the wiper (piece
of metal serving as a contact) that
slides along the wire wound column.
It advances as you turn it by its end
slot with a screwdriver. This varies its
resistance. Since the wiper is metal, it
offers very little resistance, and current
seeks the path of least resistance.
Therefore, the current does not go
through the entire winding of wire,
but diverts through the wiper. Varying
its position along this wire column
also varies its effective resistance. There are also cermet
trimmers if the inductance inherent in a wire wound
trimmer is potentially detrimental to your application.
Figure 21. A variety of small
trimmer variable resistors.
Figure 22. A rectangular multiple-turn trimmer
showing a cut-away view.
Cermet is a durable substance able to withstand high
temperatures, formed by bonding ceramic particles with
metal. A cermet is therefore comprised of ceramic (cer) and
metallic (met) materials. A cermet has the optimal properties
of both a ceramic — such as high temperature resistance and
hardness — and a metal, with the ability to undergo plastic
Figure 23. A plot of a logarithmic versus a linear
pot response to rotation versus its resistance.
Linear versus Logarithm or
“Audio” Taper Pots
If you rotate a 10 turn pot’s shaft five turns, you
would expect one-half of the maximum resistance. This is
true if the pot has a linear taper. However, there are special pots and trimmers (but mostly pots) that have their
resistance progressing at a logarithmic rate. That is why we
sometimes call them audio taper pots. Notice that a linear
pot’s resistance progresses in a straight line — thus linear
Psychoacoustics is the science of human perception of
sounds. Fundamental to this is measuring the “intensity”
range of audible sounds. This is enormous. Therefore, you
measure sound pressure level logarithmically. This is precisely
why linear taper pots are so useful in sound applications. As
you may recall, logarithms are useful for expressing numbers
over a very large range, such as dB (decibels).
cermet trimmers (Figure 21). These cermet trimmer pots
are mostly printed circuit board mountable or SMD
A logarithmic pot shows a distinctly parabolic func-