The input of the transformer is for 230V AC current and has two 21V AC current outputs. The outputs are connected in parallel and then they go in to the rectifying bridge. The rectifying bridge transforms the AC current from the input into DC current on the output. The positive and negative output of the rectifier is connected to the positive and negative of the 20000µF capacitor that smooth-ens the current. From the capacitor you can now connect to your drivers DC input.