Power Supplies Tutorial No.2 (Unregulated Linear Power Supply)

29.04.18

We will start with the simplest design and work upward in complexity from that as one builds on the other.

Unregulated Linear Power Supply

Unregulated power supplies contain a step-down transformer, rectifier, filter capacitor, and a bleeder resistor. They are cheap to make due to the minimal number of components however the main disadvantage is the output voltage is not constant. It will vary with differences in the AC input voltage and with differing load conditions. There can also be a substantial amount of ripply on the DC output depending on the load conditions. 

Likely places you will find these power supplies would be cheap and nasty DC plug-packs

Lets look at the individual components:

Transformer:

The input transformer converts the incoming mains AC supply voltage to the required level for the power supply. The output of the transformer is also isolated from the Mains AC supply. If the transformer is converting to a higher voltage it is said to be a step up transformer and in the case where converting to a lower voltage it is a step down transformer

Transformers consist of two types of windings, primary and secondary. The input voltage is applied to the primary winding and the converted voltages appear on the secondary windings. There may be multiple primary windings on a transformer to allow 120 and 240 Volt operation by connecting the windings in series for 240V and parallel for 120V operation. There may also be multiple secondary windings,or different tap points on a winding to provide a variety of converted voltages.

Transformers operate on the principle of Faraday’s laws of electromagnetic induction. If an varying voltage (in out case AC Mains) is applied to the primary winding then a varying magnetic field is generated. As the secondary winding is wound on the same core the varying magnetic field causes a varying electrical current to be produced in the secondary windings. The amount of step up or step down in voltage is proportional to the ratio of primary to secondary windings on the core.

Note the AC voltages for transformers are specified as RMS voltage, you need to be aware that the actual peak voltages on the secondary of the transformer will be equal to the product of RMS voltages and square root of two (Vpeak = Vrms x 1.414). BE CAREFUL.

 

Rectifier:

There are two methods of rectification, half wave and full wave.

 
Full wave converts both the negative and positive excursions of the AC Sign wave to DC where half wave only converts either the positive or negative excursion of the AC to DC. Full wave rectification is therefore much more efficient so we will only be considering it here.

 

 

 

 

Full wave rectification can be achieved using 4 diodes connected in a bridge format, or in the case of a centre tapped secondary winding with 2 diodes. Lets consider the bridge rectifier circuit.

During the positive half cycle, current flows from the AC Supply through D2 (forward biased), through the load, through D1 (forward biased) and back to the AC supply, shown by the red path. With positive being at the bottom terminal of the load.

During the negative half cycle, current flows from the AC supply through D3 (forward biased), through the load, through D4 (forward biased) and back to the AC supply, shown by the red path. Again with positive being at the bottom terminal of the load.

Thus the full wave bridge rectifier provides a DC voltage albeit comprising peak ripple of the AC input less the two forward bias diode voltages.

Filter Capacitor

While the full wave rectified voltage requires smoothing to provide a stable voltage to be suitable as a power supply. This is accomplished using a filter capacitor.

The capacitor is charged by the full wave rectified voltage and between peaks the load draws current from the capacitor. This provides a smoothing effect as can be seen form the diagram. Filter capacitors values in Linear supplies need to be selected, taking into account the load current to ensure the voltage does not reduce too much between charge cycles.

 

 

 

 

 

And now back to our Unregulated Linear Power Supply circuit.

The bleeder resistor is used to ensure that any charge on the filter capacitor is bleed away after the power is removed from the supply.

And there ends the first Power supply circuit. As can be seen, while we have certainly produced a DC voltage that can be put to good use, the amount of ripple and the potential for the voltage to vary with input and load changes could be problematic for some circuits.

Next we will be looking at taking the unregulated supply and stabilising that output voltage.

 

 

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