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Integrated Circuit Voltage Regulators

In the previous section, a basic linear voltage regulator configuration was introduced. Several types of linear regulator are available in integrated circuit (IC) form. Generally, linear regulators are three-terminal devices providing positive or negative output voltages which can be either fixed or adjustable. Typical linear IC regulators are introduced in these last experiments.

Fixed Positive Linear Voltage Regulators:

Many types of IC regulator are available. However, the 7800 series of IC regulators is representative of three-terminal devices which provide a fixed positive output voltage. The following diagram shows the main standard terminals of an IC regulator: input, output, and ground. IC regulators have a designation number which determines the output voltage. For example, 7805 designates a +5.0 V regulator. The table further below presents some sample designations of part numbers for other commonly available output voltages. The 7900 series designates fixed negative outputs. These devices are available with a 2% or 4% output tolerance.

fixed-positive-linear-voltage-regulator.jpg

Type number  Output voltage 
 7805  + 5.0 V
 7808   + 8.0 V
 7812   + 12.0 V
 7818   + 18.0 V
 7905  - 5.0 V
 7906  - 6.0 V
 7915  - 15.0 V

As you can see in the diagram above, capacitors, although not always necessary, are sometimes used in the input and output. The output capacitor acts as a line filter to improve transients. The input capacitor prevents unwanted oscillations, especially when the regulator is some distance away from the power supply filter, in which case the line could have a significant inductance. Typical metal and plastic packages for linear regulators are shown next.

metal-plastic-packages-linear-regulators.jpg

When used with an adequate heat sink, the 7800 can produce the following output excess currents:

  • 7800: 1 A
  • 78L00: 100 mA
  • 78M00: 500 mA
  • 78T00: 3 A

Adjustable Linear Voltage Regulators:

Other kinds of voltage regulators, including the LM317 for instance, provide an adjustable output voltage. They usually also need input and output capacitors for the reasons described above; an input, output, and adjustment terminal are provided in this case. As shown in the diagram, the external fixed and variable resistors, R1 and R2 respectively, permit the output voltage's adjustment. For the LM317, the output voltage Vout can be varied from 1.2 to 37 V, depending on R1 and R2 values; an output current of 1.5 A is available for loads.

The LM317's operation is known as "floating" due to the fact that the adjustment terminal is connected to ground, but it floats the voltage across R2, which allows the voltage at the output to be much higher than that of a fixed-voltage regulator. Adjustable linear voltage regulators with a negative output are also available. An example is the LM337.

adjustable-linear-voltage-regulators.jpg

 

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