Download Variable Resistors (“Pots”)

Electric Circuits
Tech 101.
Variable Resistors (Pots)
Supplement
Prepared by Mike Crompton. (Rev. 10 March 2009)
Variable Resistors (“Pots”)
As the name implies variable resistors or “Pots” as they are more commonly known,
allow resistance to be changed or varied, usually from 0Ω to some finite value
determined by the size of the ‘pot’. For example a 1kΩ pot can vary it’s resistance from
0Ω to 1000Ω (1kΩ ). Some pots are designed to be mounted on printed circuit boards
(P.C.B.s) and have three pins that would be placed in holes in the board and then soldered
in position. Others require 10 turns of a shaft to change the resistance from minimum to
maximum, (for some strange reason they are called ’10 turn pots’)! The pots used in most
applications are ¼ to ½ Watt and can vary from as little as 50Ω up to 1 or 2MΩ. Pots
made from resistance wire (wire wound pots) have higher power handling capabilities but
usually fairly low resistance values.
However the most common type are designed to be mounted on some type of chassis and
then have a knob attached to the shaft. These pots are the familiar volume, tone,
brightness and contrast controls on consumer goods, the controls to adjust the voltage and
current levels on the lab power supplies, or any of dozens of other controls that require
some form of variation.
1/2 Turn
They are most often constructed from a semi
circular track of carbon around which a ‘wiper’
travels picking off the required amount of
resistance between 2 of three contacts, as shown
in Fig.1 at right. With the wiper in the position
shown the resistance between pins 1 and 2 will be
approximately 1/3 of the total resistance and
between pins 2 and 3 it will be approx 2/3 of the
total. Regardless of where the wiper is, the
resistance between pins 1 and 3 will never vary
and will always be maximum. Turning the shaft
in a counter clockwise (CCW) direction will
reduce the pin 1 to pin 2 resistance and increase
the pin 2 to pin 3 resistance. When turned fully
CCW pin 1 to 2 will be zero Ohms and pin 2 to 3
will be at maximum. Turning it in the clockwise
(CW) direction will have the opposite effect. (Pin
1 to 3 will always be the same maximum
resistance).
Wiper
Outer
Case
Shaft
Carbon
Track
Fully
CCW
Fully
CW
1
3
2
Terminals
1
Fig.1
2
3
Theoretical or
circuit diagram
representation
The resistance will vary at a linear rate as the shaft is turned if the pot is a ‘linear pot’.
This means that at ¼ of a turn, going clockwise from the maximum CCW position, the
resistance from pin 1 to 2 will be ¼ of the total, at ½ a turn ½ of the total, at ¾ of a turn ¾
of the total and of course a full turn will produce the full resistance. The more common
‘log pot’ (sometimes referred to as a ‘tapered pot’), when at a ¼ turn will only produce
about 10% of total resistance, ½ turn about 15%, ¾ turn about 50% and the remaining
50% will be in the last ¼ turn.
2
Pots are usually connected in one of two configurations, as a
‘potentiometer’ in which the pot (that’s where it got it’s name) is used
to produce a varying voltage (see Fig.2). It is in fact an infinitely
variable voltage divider. The voltage between pin 2 and ground can
vary from maximum (+10V in this case) to 0V dependant on the
position of the wiper. In the diagram at right with the wiper at the top
of it’s travel the voltage at pin 2 will be +10V w.r.t. ground. Pin 2 will
be at 0V when the wiper is at the bottom of it’s travel. The current
through the pot will remain constant regardless of where the wiper is
(providing there is nothing connected to pin 2 other than a voltmeter).
Connected as a ‘rheostat’
3
3
+10V
+10V
the pot will vary the
2
circuit
current.
(See
Fig.3a
Fig.3a). Note that when
Pin1
2
the wiper is at the top of
shorted
to Pin2
it’s travel (Fig.3b) there
Pin1
and to
is actually zero resistance
shorted
Pin3
in circuit. Pin 1 is not
to Pin2
only shorted to pin 2 as it
1
1
is supposed to be, but
Fig.3b
also to pin3. The current
under these conditions would tend towards infinity and would burn
out the pot or some other part of the circuit. To prevent this there
must always be a series current limiting resistor in circuit with a rheostat.
3
+10V
2
Fig. 2
1
+10V
Current
limiting
resistor
2
3
1
Fig.4
The size of the series limiting resistor will depend upon the maximum amount of current
desired. To use Fig.4 as an example, if the pot was 10kΩ, VSUPPLY is 10V and the
maximum allowed current is to be 100mA, then using Ohm’s Law:
R = V/I
therefore
R = 10V / 0.1A = 100Ω
Even with the pot wiper at the top of it’s travel, giving 0Ω resistance, there is still the
100Ω of the limiting resistor in circuit restricting the current to 100mA. With the wiper at
the bottom of it’s travel the current will be:
I = V/ R
therefore
I = 10V / (10,000Ω+100 Ω) = 0.00099A = 0.99mA
3