Will using a transformer to convert 230V to 12V DC consume more energy than using a few capacitors and resistors without a transformer?
no, the transformer is much more efficient
HOWEVER- a transformer only changes the voltage of AC current- it does not change it to DC. You will still need diodes or rectifiers. You can also use a motor/generator- a 230v AC motor turns a 12 v DC generator.
An AC powered unregulated power supply usually uses a transformer to convert the voltage from the wall outlet (mains) to a different, nowadays usually lower, voltage. If it is used to produce DC, a rectifier is used to convert alternating voltage to a pulsating direct voltage, followed by a filter, comprising one or more capacitors, resistors, and sometimes inductors, to filter out (smooth) most of the pulsation.
A transformer is a device to convert high voltage AC to low voltage AC & vice-versa. It works on the principle of induction. Since induction occurs only in an AC supply, a transformer cannot work on DC.. So, it is not possible to convert DC supply to AC using transformer. You would need an inverter to convert DC to AC.
No. A transformer doesn't source or sink energy, or convert energy from one form to another. The ideal transformer merely changes the parameters of an electrical current, with no effect on energy levels. A non-ideal (real-life) transformer decreases the energy in the electrical circuit, because its losses rob some of the energy and turn it to heat.
You cannot convert a single-phase transformer into a three-phase transformer. However, you can connect three individual (but identically specified) single-phase transformers to create a three-phase 'transformer bank'. The primary and secondary connections of the transformer bank can be connected as required: e.g. delta-star, etc. If necessary (e.g. in an emergency), two single-phase transformers could also be used to create a three-phase transformer bank, using an 'open' (e.g. 'open delta') connection -but their combined rating will…
By using a rectifier. A transformer cannot by itself be used to convert AC to DC, and one cannot use a transformer with DC (say a battery). A transformer requires AC so that it can induce current into the secondary windings, which then steps down (or steps up) the output AC voltage. The output AC can then be rectified into DC using diodes (or a bridge rectifier).
It is kind of transformers called center-tap transformer its main function is to transform primary voltage to 2 secondary voltages or vice versa. In your statement the transformer will convert the input voltage to 2 voltage each is 9v or the opposite it will convert the 2 input voltages with each 9v to a certain value depending on the turns ratio of the transformer.