| Potentiometer as a rheostatPARTS AND MATERIALS  
                      
                      6 volt battery 
                      Potentiometer, single turn, 5 kΩ, linear 
                      taper (Radio Shack catalog # 271-1714) 
                      Small "hobby" motor, permanent-magnet type 
                      (Radio Shack catalog # 273-223 or equivalent)  For this experiment, you will need a 
                    relatively low-value potentiometer, certainly not more than 
                    5 kΩ.    CROSS-REFERENCES  Lessons In Electric Circuits, Volume 
                    1, chapter 2: "Ohm's Law"    LEARNING OBJECTIVES  
                      
                      Rheostat use 
                      Wiring a potentiometer as a rheostat 
                      Simple motor speed control 
                      Use of voltmeter over ammeter to verify a 
                      continuous circuit  
 
 SCHEMATIC DIAGRAM  
                      
 
 ILLUSTRATION  
                      
 
 INSTRUCTIONS  Potentiometers find their most sophisticated 
                    application as voltage dividers, where shaft position 
                    determines a specific voltage division ratio. However, there 
                    are applications where we don't necessarily need a variable 
                    voltage divider, but merely a variable resistor: a 
                    two-terminal device. Technically, a variable resistor is 
                    known as a rheostat, but potentiometers can be made 
                    to function as rheostats quite easily.  In its simplest configuration, a 
                    potentiometer may be used as a rheostat by simply using the 
                    wiper terminal and one of the other terminals, the third 
                    terminal left unconnected and unused:  
                        
 
 
                      Moving the potentiometer control in the 
                    direction that brings the wiper closest to the other used 
                    terminal results in a lower resistance. The direction of 
                    motion required to increase or decrease resistance may be 
                    changed by using a different set of terminals:  
                      Be careful, though, that you don't use the 
                    two outer terminals, as this will result in no change in 
                    resistance as the potentiometer shaft is turned. In 
                    other words, it will no longer function as a variable 
                    resistance:  
                      Build the circuit as shown in the schematic 
                    and illustration, using just two terminals on the 
                    potentiometer, and see how motor speed may be controlled by 
                    adjusting shaft position. Experiment with different terminal 
                    connections on the potentiometer, noting the changes in 
                    motor speed control. If your potentiometer has a high 
                    resistance (as measured between the two outer terminals), 
                    the motor might not move at all until the wiper is brought 
                    very close to the connected outer terminal.  As you can see, motor speed may be made 
                    variable using a series-connected rheostat to change total 
                    circuit resistance and limit total current. This simple 
                    method of motor speed control, however, is inefficient, as 
                    it results in substantial amounts of power being dissipated 
                    (wasted) by the rheostat. A much more efficient means of 
                    motor control relies on fast "pulsing" of power to the 
                    motor, using a high-speed switching device such as a 
                    transistor. A similar method of power control is used in 
                    household light "dimmer" switches. Unfortunately, these 
                    techniques are much too sophisticated to explore at this 
                    point in the experiments.  When a potentiometer is used as a rheostat, 
                    the "unused" terminal is often connected to the wiper 
                    terminal, like this:  
                      At first, this seems rather pointless, as it 
                    has no impact on resistance control. You may verify this 
                    fact for yourself by inserting another wire in your circuit 
                    and comparing motor behavior before and after the change:
                     
                      If the potentiometer is in good working 
                    order, this additional wire makes no difference whatsoever. 
                    However, if the wiper ever loses contact with the resistive 
                    strip inside the potentiometer, this connection ensures the 
                    circuit does not completely open: that there will still be a 
                    resistive path for current through the motor. In some 
                    applications, this may be an important. Old potentiometers 
                    tend to suffer from intermittent losses of contact between 
                    the wiper and the resistive strip, and if a circuit cannot 
                    tolerate the complete loss of continuity (infinite 
                    resistance) created by this condition, that "extra" wire 
                    provides a measure of protection by maintaining circuit 
                    continuity.  You may simulate such a wiper contact 
                    "failure" by disconnecting the potentiometer's middle 
                    terminal from the terminal strip, measuring voltage across 
                    the motor to ensure there is still power getting to it, 
                    however small:  
                      It would have been valid to measure circuit 
                    current instead of motor voltage to verify a completed 
                    circuit, but this is a safer method because it does not 
                    involve breaking the circuit to insert an ammeter in series. 
                    Whenever an ammeter is used, there is risk of causing a 
                    short circuit by connecting it across a substantial voltage 
                    source, possibly resulting in instrument damage or personal 
                    injury. Voltmeters lack this inherent safety risk, and so 
                    whenever a voltage measurement may be made instead of a 
                    current measurement to verify the same thing, it is the 
                    wiser choice.  |