Linear Power Supplies

 
 
 

linear power supply showing transformer rectifier and regulation circuitry   Their are two main types of linear power supplies: Regulated and unregulated. Both types have the following in common.
Transformer: to provide the change in the AC supply voltage to create an output voltage close to the final desired DC voltage.
Rectifier: A device to convert the AC voltage to DC.
Smoothing: Techniques for eliminating the voltage variations left over from the AC supply after rectification.
Analogue Components:  The name linear derives from the fact that the components that they employ analogue circuitry to achieve the required output voltage rather than digital switching circuitry. Specifically they use analogue devices such as transistors operating in the linear "active" region of their volt/current operating range.

Unregulated Power Supplies
unregulated power supply showing transformer and rectifier with large smooting capacitor    Unregulated power supplies simply employ the techniques described above and nothing further.  For the very basic unregulated power supply their may be no smoothing whatsoever. Typically these units have relatively large ripple voltages corresponding to the half cycle of the original AC waveform. This large variation makes them highly unsuitable for anything other than crude power supplies for lights and motors etc.. For example using one to power an amplifier would result in large amounts of noise appearing on the speaker outputs as "mains hum". Their advantage is that they are very cheap to manufacture and are relatively robust. The other main disadvantage of using unregulated is that when the load changes, i.e. variations in the demand for current at the required voltage, the voltage will vary. This can be highly undesirable especially for sensitive circuitry. For the hobbyist it is sometimes beneficial to purchase one of these units and add additional circuitry to improve the output quality in accordance with project requirements.
Regulated Power Supplies
regulated linear power supply showing transformer and regulation circuit    Regulated power supplies start by having a "front end" consisting of an unregulated power supply followed by circuitry for regulating the output voltage variations.  They usually employ good smoothing techniques before feeding the regulator part of the circuit, which, at the very least, would be a large capacitor. A regulated power supply monitors the load conditions. i.e. their is feedback corresponding to the current being taken by the load. This is used to nullify any resulting variations in the output voltage. With these techniques the resulting ripple variations in the output voltage can be made very small.  One disadvantage in this technique is that the regulator circuitry is having to cope with voltage and current variations which inevitably means that there will be a resulting voltage drop across the device. With the device operating in it's active or "linear" region this corresponds to a power loss. Efficiency is therefore a consideration here. It is usually necessary to have a heat sink on these units with some form of cooling fan in with the higher power ratings. In the next section we will see how this efficiency is improved with the use of switching techniques which ensure that the regulation device spends very little time in it's linear region and therefore does not consume as much power.
 

   
                   
     
                   
 

.      .

 
 

Copyright pc-control.co.uk 2008