Final answer:
A diode placed in series with an AC power source and a resistor forms a half-wave rectifier circuit, which converts AC to pulsating DC.
Step-by-step explanation:
When a diode is placed in series with an AC power source and a resistor, the result is a half-wave rectifier circuit. A half-wave rectifier circuit allows current to pass through only during positive or negative half cycles of the AC waveform, effectively converting AC voltage to pulsating DC voltage.
With only one diode in the circuit, it can't invert the other half of the AC waveform, thus not becoming a full-wave rectifier which requires additional diodes. Other types of circuits like voltage regulator circuits and power supply circuits can include components such as additional diodes, capacitors, and voltage regulators to stabilize and adjust the output.