This below circuit is very useful for our mini projects. To do mini projects we often need power supply of 3V, 6V, 12V etc. But a general 3-0-3 step down transformer can achieve us those 3 voltages easily. Now there’s a question that we can get 3volts from 3-0-3 transformer by 2 diodes (conventional full wave rectifier with 50% load which works on center tap transformers), and we cal also get 6Volts by fixing a bridge rectifier to both 3,3 ends, which have potential difference of 6volts.Now how can we make 12volts?
Well, an easy solution is to use an coupler here. We use a simplified voltage coupler using two diodes and two capacitors (aka Greinacher voltage doubler circuit). It works on the following principle. When Ac has one direction one capacitor is charged, and the other charged when Ac is in opposite direction, and we get both capacitors charged. We take output from both capacitors in series, so we get double voltage. The circuit below, if we input the 6V Ac from both end of transformer output coil, we get 12Volts DC output from this circuit.