Final answer:
A diode rectifier allows current flow in only one direction when it is forward biased. It is a crucial component in the conversion of AC from a secondary coil into DC. Therefore, the answer is 1. One.
Step-by-step explanation:
A diode rectifier is used to convert alternating current (AC) from a secondary coil into direct current (DC). Diodes are semiconductor devices that permit current flow in only one direction, when they are forward biased - meaning the anode is more positive than the cathode. Therefore, the correct answer to how many directions a diode rectifier allows current flow is 1. One.
Direct current (DC) has a constant voltage and flows in one direction, while alternating current (AC) periodically reverses direction. Diodes, being crucial components of rectifiers, play an essential role in the conversion process by allowing the current to flow only during the positive halves (or negative halves, in the case of a different type of rectifier) of the AC wave, effectively 'rectifying' it into DC.