lektor January & February 2021 39is, of course, the volume control of an ampli-
fier (although in the present digital era these
are now often digital encoders). The there
are the trimpots that are generally inside the
device (usually on the circuit board) and are
intended for once-only adjustment using a
small screwdriver or special trimming tool.Figure 2 shows various examples of (trim)
pots. On the right you see the slide potenti-
ometer, or fader, that is often used in mixing
panels. In the middle are two ordinary poten-
tiometers with a mono version at the top and
a stereo version at the bottom. Such a stereo
potentiometer is really two mono potentiom-
eters on a common shaft.In the rotary potentiometer a sliding contact
moves back and forth along a carbon track.
Such potentiometers often start to ‘crackle’
after some time due to age and contamination.
This is extremely undesirable, particularly with
audio amplifiers. If a particular potentiome-
ter is not hermetically sealed then a special
spray cleaner can give temporary relief (with
the emphasis on temporary). Otherwise you
can try turning it vigorously from one end to
the other a few times, but this too will likely
only be a temporary solution.Cermet potentiometers are less sensitive to
these ageing symptoms (and are therefore
also more expensive). Figure 2 at the top in
the middle provides two examples (mono on
the right and stereo on the left). Finally, at the
top left of Figure 2, there are two closed-vari-
ant trimpots that are a little less sensitive to
dirt and dust ingress.are sometimes difficult to distinguish. In
case there is any doubt, again, reach for
the ohmmeter.
Resistors are (for obvious reasons) not
produced in every conceivable value. Instead
they are manufactured in standardised
E-series. Here the ratio of two consecutive
values within a decade is more or less the
same. The E-values are in turn produced
in different series of decades (for example
2.2 Ω, 22 Ω, 220 Ω, 2.2 kΩ, 22 kΩ, 220 kΩ and
2.2 MΩ). Table 2 shows the most common E
series values for E3, E6, E12 and E24, where
the number indicates the number of values
per decade. For precision applications there
are the E48, E96 and E192 series, i.e. a total
of seven standardised E series.
By the way, there is also an R-series, but the
probability that you will get involved with
them in electronics is very low. In electrical
engineering you will come across R-series only
in relation to fuses or miniature circuit breakers.
If you need an extremely precise and, at the
same time, awkward value that does not occur
in any of the E series, then it can be achieved
with an appropriately selected parallel and/
or series connection of E24 resistors. Or you
could use a variable resistor...
Variable resistors
Potentiometers, also known simply as a ‘pot’,
and trimpots are resistors whose resistance
value can be changed. Potentiometers have
a shaft that protrudes through a hole in the
enclosure of a device that can be operated
by the user. The most well-known example
Figure 2. Potentiometers and trimpots.Table 1: Colour code for resistors
Value Multiplier Tolerance
4 bands 1 st band 2 nd band – 3 rd band 4 th band
5 bands 1 st band 2 nd band 3 rd band 4 th band 5 th band
black – 0 0 1
brown 1 1 1 10 ±1%
red 2 2 2 100 ±2%
orange 3 3 3 1 k
yellow 4 4 4 10 k
green 5 5 5 100 k ±0,5%
blue 6 6 6 1 M
violet 7 7 7 10 M
grey 8 8 8 100 M
white 9 9 9 1 G
gold – – – 0.1 ±5%
silver – – – 0.01 ±10%Table 2: Standard resistor values
E3 E6 E12 E24
1 1 1 1
1.1
1.2 1.2
1.3
1.5 1.5 1.5
1.6
1.8 1.8
2.0
2.2 2.2 2.2 2.2
2.4
2.7 2.7
3.0
3.3 3.3 3.3
3.6
3.9 3.9
4.3
4.7 4.7 4.7 4.7
5.1
5.6 5.6
6.2
6.8 6.8 6.8
7.5
8.2 8.2
9.1