Voltage Regulator

Basics:

  • In electronics we use linear (analogue) voltage regulators:
    • The fact that they are analogue may mean that the voltage input may vary indefinitely.
    • Their resistance varies according to the voltage in the input terminal thus resulting in a steady output.

Uses:

  • Used to produce a stable output voltage from an unstable input voltage.
    • They are designed to maintain a constant output voltage level.
      • This makes them very useful in computers and such like.
  • They are often used to regulate the voltage output of batteries:
    • Batteries when fully charged supply more voltage then when they are almost depleted.

Pinout:

  • Ground pin (this is the adjust pin on adjustable voltage regulators which are useful if you need a nonstandard voltage like 7V). Adjustable voltage regulators are not explored in these notes.
  • Input pin.
  • Output pin.
  • They also generally have a heat sink because there is voltage across them and this generates a lot of heat energy.

How Do They Work?

  • Electronic voltage regulators all have set voltage output levels and can only decrease the input voltage.
    • They also only work with DC current.
      • Speculation.
  • The hole at the top of the component is for the attachment of a heat sink.
  • Adjustable voltage regulators generate an output voltage that is between the input voltage and the voltage at the ‘adjust’ terminal:
    • The input voltage must be higher than the ‘adjust’ voltage because of the way in which the ‘adjust’ voltage is generated (see below).
    • This ‘adjust’ terminal is equivalent to the ground terminal of fixed regulators.
    • By adjusting the voltage at this terminal it is possible to adjust the voltage output of the regulator:
      • It is possible to do this by connecting a potential divider between the output and the ground and then connecting the central output rail of the potential divider to the ‘adjust’ rail of the regulator.

Circuit:

  • Most voltage regulator circuits have their ground pins connected to the 0V rail, and their input and output pins connected to the inputs and outputs respectively. Parallel to the voltage regulator connected between the input rail and the ground rail, and the output rail and the ground rail are usually some electrolytic capacitors (around 10 micro Farads):
    • The values of the capacitors do not have to be 10 micro Farads.
      • A slight variation is fine.
    • These capacitors ensure the stability of the output.
      • They do not always have to be there but with them there it is guaranteed that there will be a very stable output voltage.
  • Linear voltage regulators:
    • Are not very efficient.
      • The power wasted as heat energy can be found using the formula P=IV
        • V in this case is equal to Vin – Vout and is the voltage across the voltage regulator.
      • This is the reason why it is best to have an input voltage that is relatively close to the output voltage.
        • Remember also that:
          • Efficiency

            • Thus when Vin approaches the value of Vout the efficiency becomes better.
    • They have a drop out voltage.
      • This is the minimum voltage that must be fed to the regulator in order to guarantee that the output voltage is at the level that the regulator was made to supply:
        • Most of the time the input voltage must be at least a volt or two above the output voltage of the regulator in question.
        • This is why one must be careful that the output voltage of a battery, even when it is almost depleted, is at least one or two volts higher than the output voltage expected from the regulator. Assuming, of course, that the regulator in question is being used to regulate the voltage from a battery.
          • If this is not possible then it would be wise to use the slightly more expensive low drop out voltage regulators.
        • If the voltage input is lower than the drop out voltage then the voltage output would be lower than the specified output voltage.
  • They are often used to protect sensitive circuits.

 

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