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Symmetric Power Supply is the power supply that uses a transformer with dual voltage. In this power supply will generate 3 phase voltage is positive (), negative (-) and CT (0). CT = Center Tap).
A transformer that reads 12V CT 12, meaning that as they pass rectifier (diode) will produce a voltage 12, 0,-12V and 0 (CT) serves as a ground (neutral phase). If there is no writing can be measured using the AVO meter digital with measuring range AC position. 

Black probe in the middle and the red probe to look left and right voltage tap on the secondary. If same value between tap the left and right, this is called symmetric transformers.
However transformers can also produce high voltage 24V, that is by connecting the 12V left and right in to rectifier diode without functioning 0 (CT).

In Figure 1, show how to count transformer voltage value 12V &12V.
Place a Black Probe in the center tap; Place a Red probe to left-tap, see scale needle the AVO meter.  Next, move red probe to right-tap, see scale needle the AVO meter again. If the same of value then called a symmetric voltage.

Figure 1
Power Supply Circuit - Symmetric Power Supply

And in Figure 2, show how to make a 24V voltage.
further development please try it yourself.

Figure 2 Symmetric Power Supply

In Figure 3, show schematic diagram, how to connect the transformer to the rectifier diode will produce a voltage 12V and-12V.
Electrolit capacitors and capacitor serves to dissipate electric corrugation of the magnetic induction in the transformer or eliminate the hum.

Schematic Diagram Symmetric Power Supply

Beware, do not touch parts of the primary transformer because it is dangerous.
When do the splicing / soldering unplug the power cord from the outlet of your home.

May be useful.