Power Supplies Tutorial No.3 (Regulated Linear Power Supply)


In this article we build on the understanding of the unregulated power supply and now look at a regulated power supply.

Regulated Linear Power Supply

The regulated power supply contains the same components as the unregulated supply however we now insert a regulator and additional capacitance to obtain an output voltage that it much more stable and immune to supply voltage and load variations.

Likely places you will find these power supplies would be lab bench power supplies. Although newer switch-mode supplies are making their way onto lab benches as well. Lets just say “old school” lab power supplies.











So we have already looked at all the components up to the actual regulator in the Unregulated Power Supply article. If you haven’t looked at this article yet, now would be the perfect time. Just as with the unregulated supply the waveform across the Filter Capacitor, whilst being a relatively stable DC voltage, contains a large ripple component and will vary quite substantially with input and load variations.

Voltage Regulator

To smooth this voltage to an acceptable level we introduce a new component, the Voltage Regulator. These days the voltage regulator is generally an IC however a regulator can be built from discrete components. In some cases where particularly high output currents are desired it is quite normal to have additional series pass transistors and/or discrete components.

The voltage regulator operates on the basis of controlling the conduction of a series pass power transistor based on the comparison of a feedback signal from the output and a stable voltage reference. The example shown uses a Zener diode to form a stable internal voltage reference (Vz). A portion of the output voltage is selected using the R1 R2 voltage divider (V2) and these are both applied to the inputs of the Operation Amplifier. The output of the Op Amp is used to control the Power Transistor and therefore regulate the output voltage. We will cover Op Amps in detail in a later tutorial but for the moment just accept this as fact.

The regulator circuit relies on the fact that Vs will always be higher than the desired Vout. With this being the case Vout is controlled by continuously adjusting the amount of voltage dropped across itself to ensure Vout is always correct.

The circuitry inside common 3 terminal regulators 78xx and 79xx is more complicated than the simple circuit we have looked at here in that it also contains shutdown circuitry and the voltage reference and control circuitry is much more complicated however the same principles are at play.

Output Capacitor

The last component in our regulated power supply circuit is the Output Capacitor. The primary purpose of this capacitor is to ensure the regulator operates as planned. If the regulator was perfect the capacitor would not be required however we don’t live in a perfect world and without the capacitor, oscillations would appear on the output. The capacitor doesn’t allow these oscillations to occur.

In the next article we will take a look at the process of designing a power supply and look at the considerations that need to be thought about as part of the design process.

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