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Potentiometer:
There are many instances where only a portion of an output voltage
from a signal source is needed. If we allowed the full output voltage from
a home CD player to be driven into the input of an amplifier, the
amplifier would play at or near full power at all times. This would become
quite annoying in a very short period of time. To reduce the overall
volume, we need to allow only a fraction of the full signal through to the
amplifier. To control the level of the signal, we use a potentiometer. A
potentiometer (also know as a 'pot') is a modified resistor.
Potentiometers can be used to allow a change in the resistance in a
circuit or as a variable voltage divider (in the case of a volume
control). If you have a rotary volume control on your TV or radio, it is
(more than likely) a potentiometer being used as a variable voltage
divider.* A
potentiometer generally has 3 terminals. 2 of the terminals are connected
to the opposite ends of a resistive element. The 3rd terminal (usually, is
physically in-between the other 2 terminals) is called the wiper. The
wiper is a contact (actually, generally many very small contacts) that
slides along the resistive element. The diagram below shows the schematic
symbol for a pot.
*If your volume
control clicks and steps the volume up or down with each click, it's
probably a rotary encoder (a switch), not a potentiometer.
The following diagram shows how the schematic symbol relates to the
drawing of the potentiometer.
Voltage
Dividers
A Potentiometer as a Variable Voltage Divider:
We covered voltage dividers on the resistors
page. A pot connected as it is in the following diagram will act to divide
the voltage like the 2 individual resistors. In the diagram below, you can
see that the linear taper‡
potentiometer is in the middle of its range of travel. You can also see
that 12 volts is applied to terminals 'A' and 'B' are connected to the 12
volt battery. This means that the output†
from the slider will be 6 volts.
‡ This means that the resistance of the
resistive element increases in direct proportion to the distance traveled
along the resistive element. In the middle of travel, the resistance from
the sliding terminal to either of the other terminals is half of the total
resistance.
†The output is simply the
voltage at the point where the wiper contacts the resistive element.
Resistance Taper:
In the previous paragraph, I mentioned that the potentiometer had a
'linear' taper. These are general purpose potentiometers and may be used
for controlling DC voltage (as it did in the diagram) or to control the
levels of the individual bands on an equalizer. For volume controls you
need to use a potentiometer with a 'log' (short for logarithmic) taper.
This is because the human auditory system works logarithmically. If we'd
use a linear taper pot for a volume control, the low end of the volume
control's range would be 'touchy'. What I mean to say is that the volume
would seem to increase really fast at the bottom end of the VC's range and
would require a lot of travel at the upper end of the VC's range. As you
can see by the graph below, the resistance of the linear pot (red line) at
50% of its travel is 50% of its resistance. The log pot's resistance is
far less than 50% at 50% of its travel. You have to go to about 85% of its
travel to reach the midpoint of its resistance (500 ohms for a 1000 ohm
pot).
In the next diagram, you can see that the slider has been moved toward
the positive terminal. This means that the output voltage will be higher
than 6 volts. In this case the output voltage is 7.5 volts DC. If the
slider was moved closer to the negative terminal, the voltage would be
lower.
Output with AC:
If the pot was connected to an AC signal generator (i.e. sine wave
generator) instead of a battery, the voltage on the output terminal would
be ~63% of the input voltage from the generator. If the input signal was
12 volts AC, the output would be 7.5 volts AC. The following diagram shows
the input and the output voltages graphically with both AC and DC input
voltages. It is based on the same slider position as was used in the
previous diagram.
Rotary Potentiometers:
The following diagram shows a rotary potentiometer at 50% and ~63% of
its range of travel. These positions would correspond to the positions of
the slider pot in the previous diagram. This is a single turn
potentiometer which means that it can travel its entire range within 1
complete revolution if its shaft (knob). Actually, a single turn pot will
only travel through about 270º of a complete revolution. There are other
potentiometers that have some sort of mechanism to increase the number of
times the shaft must be turned to travel through its entire range of
motion. This is generally done with gears but I've seen it done in a
planetary gear configuration using ball bearings. The multi-turn
potentiometers are used to make it easier to precisely set the output
level.
Power Ratings:
Potentiometers have power ratings just as resistors do. When the
potentiometer are used for preamp level audio and such, power dissipation
isn't really a problem. If you are using the potentiometer to control a
signal with significant current flow, you'd have to calculate the power
dissipation across its resistive element and use a potentiometer of a
sufficient power rating. Refer to the Ohm's Law
page for the appropriate formula.
In the picture below, at the top is a multisection potentiometer used
in older head units. Below, various styles of printed circuit board
potentiometers.
The diagram below shows how to wire a volume control to be inserted
into the preamp line. If noise is a problem when you touch the metal
shaft, ground the metal part of the pot. A potentiometer specifically
designed for use as a volume control (a pot with a 'logarithmic' taper)
will give the most linear volume control. The Radio Shack pt# 271-1732 is
a good choice.
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