A circuit diagram consisting of diode and transformer?

Answer 1

This is probably a half-wave rectifier.

Rectifiers are used to convert AC to "DC". A half-wave rectifier can be constructed with a transformer and a diode connected between the positive terminal of transformer secondary and the load. This doesn't provide DC power to the load, but instead blocks the negative portions of the AC signal and only allows current to flow into the load while the AC voltage is positive. A capacitor is typically connected across the load to smooth out the voltage, making the input to the load more DC-like. The capacitor charges up when the AC voltage is positive and the diode is conducting, then discharges through the load when the diode is not conducting. This is still not DC power since there is a certain amount of ripple, or variation in the load voltage. Selection of the capacitor value is important to reduce ripple to an acceptable level. The transformer is simply used to change the AC voltage to whatever is desired.

The half-wave rectifier is a very simple circuit. Other kinds rectifiers, for instance a full-wave rectifier, offer better performance.