Variable Resistors in DVD Drives

• There are some DVD drives that have a variable resistor in the laser lens circuit. These are used to adjust the voltage at the laser. If the drive is fairly old, the laser can become weak and won't read more compressed data from a DVD movie disc but can still read the less compressed CD music discs. In this situation you can adjust the variable resistor in the circuit to increase the voltage feeding the laser and therefore increase its intensity.
  
  

Variable Resistors for Volume Control

• The volume control nobs on most electronic equipment are generally variable resistors. The variable resistor can adjust the volume of a device by changing the resistance to the transistor's amplified output and therefore control the volume. By adjusting the potentiometer, you are either increasing or decreasing the resistance in the circuit, allowing more or less amplification. Using the formula above you can see that a decrease in resistance causes an increase in voltage, and oppositely then, an increase in resistance will cause a decrease in the voltage.
  
  

Variable Resistor Types

• Variable resistors come in a couple of styles. There is the larger potentiometer that comes in either a rotating knob or slider controlled wiper style. There are also very small potentiometers that are controlled with a small screw in the center that can be adjusted with a screwdriver. All of the types do the same job with varying degrees of resistance adjustment.
  
  

Other Types Of Variable Resistors

• There are several other types of variable resistors used for different ways of controlling a circuit. There are photo-resistors that will change resistance with the amount of light that shines on it. Metal oxide varistors are used to prevent power surges from destroying equipment, by reducing the resistance when a high voltage is applied, allowing them to absorb the power surge protecting the equipment. Thermistors are designed to change resistance values with temperature changes, and can be used as a temperature gauge. Humistors are designed to change resistance values with humidity changes.