Diodes allow current to flow in only one direction. They have a small, constant voltage drop of about 0.7V when conducting electricity forward. Diodes do not conduct in reverse unless the reverse voltage exceeds a maximum breakdown value. There are two main types: signal diodes for small currents below 100mA, and rectifier diodes for larger currents used in power supplies to convert alternating current to direct current. Zener diodes are designed to conduct reliably in reverse at a specific breakdown voltage, making them useful for maintaining a fixed voltage in circuits.