There have many ways to get negative and positive allowance strain with only applies one transformer secondary reels. this planning is one of its the realization.
This circuit applies second bridge rectifier, D1...D4, where rectifier holds mutually transformer capacity passed C1 and C2. Because of the strain yielded isolated by DC to transformer. Where circuit unconnected with rectifier D5....D8, hence positive terminal C3 earns unconnected directly to line 0V to give symmetrical power supply allowance.
Because C3 loaded from higher level impedance than C4, because of erection of condenser C1 and C2, hence this condenser must have higher level value compared to C4. Otherwise, hence impedance in and wrinkling strain from negativity allowance would many differing from his its positive allowance.
Condenser working stress at least having to equal to transformer strain peak value. with component values like the one passed to circuit diagram, transformer strain 15V can ration finite of 0,1A with shrink strain 1V. condenser values can be turned into bigger with the same factor to less shrink strain. election of bridge rectifier must have enough work reach to overcome maximum transformer top strain and payload current.
