 Upon pressing the doorbell button, power is connected through the rectifier diode or series of rectifier diodes called a full wave rectifier, which allows the current to flow in only one direction, into the ballast capacitor.The ballast capacitor charges at a rate far greater than the rest of the circuit needs to complete a given song.Once the button is released, the capacitor retains the charge and maintains power for a short duration to the rest of the circuit.