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Mains Transformer Power Supplies

Updated on February 17, 2013

Mains Transformers

Mains transformer based power supplies have in many instances been replaced by switch mode power supplies. However they are still essential for powering sensitive radio, audio and instrumentation equipment where the electromagnetic smog produced by a switch mode power supply would render the equipment practically inoperable.

There are two basic types of mains transformer in use. The first type uses a laminated silicon iron core. The second more expensive type uses a toroidal core which has the advantage of creating only a small external magnetic field. It is very important to mount a toroidal transformer correctly keeping it away from metalwork and using plastic rather than metal mounting bolts.

Mains transformers are not rated by watts but by a somewhat similar volt-amp rating (AV) for each secondary winding.
This is simply the maximum current that can be drawn at the specified secondary winding voltage.

The regulation percentage of a particular transformer is very important. It is defined as the difference between the no load voltage and full load voltage of a secondary winding divided by the no load voltage. It is typically around 20% for small transformers and 5% for transformers above 200VA.

The AC voltage of a secondary winding is given in RMS terms, this allows you to calculate the power dissipated into a resistive load using Ohms law. The actual peak voltage is 1.41 times the RMS voltage.


Rectifier Configurations

There are a large number of diode rectifier configurations available allowing voltage doubling and negative supply rail options as well as the standard DC output. These additional options can only provide a limited amount of current output but are nevertheless very useful for supplying dual rail operational amplifiers and audio amplifier circuits than need high supply line headroom.

It is a very good idea to parallel each diode with a 10nF or 100nF ceramic capacitor to reduce the amount of harmonics produced by the rectification process and also to prevent rectification of any stray radio frequency signals that could be picked up by sensitive receiver equipment as (tunable) hum.

Figure 1

The basic full-wave bridge (Greatz) rectifier circuit.
The basic full-wave bridge (Greatz) rectifier circuit.

Figure 2

The bi-phase half-wave rectifier circuit.  If the two transformer secondary windings were rated at 12V each the output voltage would be 1.4*12=16.8V
The bi-phase half-wave rectifier circuit. If the two transformer secondary windings were rated at 12V each the output voltage would be 1.4*12=16.8V

Figure 3

The simple voltage doubler circuit.
The simple voltage doubler circuit.

Figure 4

Extra negative voltage.
Extra negative voltage.

Figure 5

Extra double positive voltage.
Extra double positive voltage.

Figure 6

Full-wave voltage doubler.
Full-wave voltage doubler.

Figure 7

Greatz circuit and extra full-wave voltage doubler.
Greatz circuit and extra full-wave voltage doubler.

Figure 8

Extra negative voltage full-wave.
Extra negative voltage full-wave.

Figure 9

Half-wave negative and positive supply.  An advantage of this circuit is that load variations on the positive supply do not affect the negative supply for example.
Half-wave negative and positive supply. An advantage of this circuit is that load variations on the positive supply do not affect the negative supply for example.

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