Linear Power Supply: linear regulated supply

Linear regulated power supplies are able to offer exceedingly low levels of output noise and good stabilisation, but at the expense of size and efficiency..

Linear Power Supply Circuits Primer & Tutorial Includes:
Linear power supply     Shunt regulator     Series regulator     Current limiter     7805, 7812, & 78** series regulators    

See also: Power supply electronics overview     Over-voltage protection     PSU specs     Digital Power    

Linear power supplies are widely used because of the advantages they offer in terms of overall performance.

Linear regulated power supplies are often used in exacting situations where the regulation and removal of noise is paramount.

While linear power supplies may not be as efficient as other types of power supply technology, they offer the best performance and are therefore used in many applications where noise is of great importance. Often audio amplifiers and many other items of electronic equipment use linear power supplies to obtain the best performance.

Typical variable linear power supply for bench laboratory use
Typical variable linear power supply for bench laboratory use

Linear power supply basics

Linear regulated power supplies gain their name from the fact that they use linear, i.e. non-switching techniques to regulate the voltage output from the power supply. The term linear power supply implies that the power supply is regulated to provide the correct voltage at the output. Sometimes the sensing of the voltage may be accomplished at the output terminals, or on some occasions it may be achieved directly at the load.

In terms of the overall make-up of a linear power supply, it can be split into several blocks as detailed below.

The main elements of the linear power supply are:

  • Input transformer:   As many regulated power supplies take their source power from an AC mains input, it is common for linear power supplies to have a step down or occasionally a step up transformer. This also serves to isolate the power supply from the mains input for safety.
    Transformer circuit symbol
  • Rectifier:   As the input from an AC supply is alternating, this needs to be converted to a DC format. Various forms of rectifier circuit are available.

    Note on Diode Rectifier Circuits:

    Diode rectifier circuits are used in many areas from mains power supplies to radio frequency demodulation. The diode rectifier circuits use the capability of the diode to only pass current in one direction. There are several varieties from half wave to full wave, bridge rectifiers, peak detectors and more.

    Read more about Diode Rectifier Circuits

    Even for DC powered regulators, a rectifier may be placed at the input to guard against inverse connection of the supply.
  • Smoothing:   Once rectified from an AC signal, the DC needs to be smoothed to remove the varying voltage level. Large reservoir capacitors are used for this.

    Action of a smoothing capacitor on the rectified waveform in a power supply
    Smoothing action of a reservoir capacitor
  • Linear regulator:   Once a smoothed supply is available, this can then be applied to the linear regulator. This will provide a properly regulated output.

Linear power supply regulators

There are two main types of linear power supply:

  • Shunt regulator:   The shunt regulator is less widely used as the main element within a voltage regulator. For this form of linear power supply, a variable element is placed across the load. There is a source resistor placed in series with the input, and the shunt regulator is varied to ensure that the voltage across the load remains constant. Read more about the Shunt regulator
  • Series regulator:   This is the most widely used format for a linear power supply. As the name implies a series element is placed in the circuit, and its resistance varied via the control electronics to ensure that the correct output voltage is generated for the current taken.

Both of these types of linear regulator used in power supplies has its uses and can be used in different situations.

Linear power supply advantages / disadvantages

The use of any technology is often a careful balance of several advantages and disadvantages. This is true for linear power supplies which offer some distinct advantages, but also have their drawbacks.

Linear PSU advantages

  • Established technology:   Linear power supplies have been in widespread use for many years and their technology is well established and understood.
  • Low noise:   The use of the linear technology without any switching element means that noise is kept to a minimum and the annoying spikes found in switching power supplies are now found.

Linear PSU disadvantages

  • Efficiency:   In view of the fact that a linear power supply uses linear technology, it is not particularly efficient. Efficiencies of around 50% are not uncommon, and under some conditions they may offer much lower levels.
  • Heat dissipation:   The use of a series or parallel (less common) regulating element means that significant amounts of heat are dissipated and this needs to be removed.
  • Size:   The use of linear technology means that the size of a linear power supply tends to be larger than other forms of power supply.

Despite the disadvantages, linear regulated power supply technology is still widely used, although it is more widely used where low noise and good regulation are needed. One typical application is for audio amplifiers where the linear supply is able to provide optimum performance for powering all the stages of the amplifier.

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