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Transcript
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