Selenium rectifiers are a type of rectifying device that was commonly used in electronics and electrical systems before the advent of modern semiconductor diodes. They were used to convert alternating current (AC) to direct current (DC) by exploiting the rectifying properties of selenium, a chemical element with semiconducting properties.
Here's how selenium rectifiers work:
Semiconductor Properties of Selenium: Selenium is a unique material that exhibits semiconductor behavior. This means its electrical conductivity can be controlled and altered under certain conditions. When selenium is exposed to light, heat, or an electric field, its conductivity changes.
Rectification: In a rectifier circuit, the main purpose is to convert AC to DC. AC voltage alternates in polarity, while DC voltage remains constant in polarity. To achieve rectification using selenium, a stack of selenium wafers is used. Each wafer is coated with conductive and insulating layers.
Half-Wave Rectification: Selenium rectifiers were commonly used for half-wave rectification. In a half-wave rectifier, only one half of the AC input waveform is allowed to pass through to the output, while the other half is blocked. This is achieved by controlling the conductivity of the selenium wafers.
Forward and Reverse Biasing: In a selenium rectifier, the selenium wafers are alternately connected to the AC input. When the selenium wafers are forward-biased, their conductivity increases, allowing current to flow in one direction (during one half of the AC cycle). When the selenium wafers are reverse-biased, their conductivity decreases, effectively blocking current flow in the opposite direction (during the other half of the AC cycle).
Limitations and Disadvantages:
Inefficiency: Selenium rectifiers had relatively low efficiency compared to modern semiconductor diodes. They produced significant heat during operation due to their relatively high internal resistance.
Voltage Drop: Selenium rectifiers had a higher voltage drop compared to modern diodes, which meant that a significant portion of the input voltage was lost as heat.
Limited Current Capacity: Selenium rectifiers had limited current-carrying capacity, making them unsuitable for high-power applications.
Reliability: Selenium rectifiers were less reliable and had a shorter lifespan compared to modern diodes.
Replacement by Semiconductor Diodes: The limitations of selenium rectifiers eventually led to their replacement by more efficient and reliable semiconductor diodes, such as silicon diodes. Semiconductor diodes have lower voltage drops, higher efficiency, greater current-carrying capabilities, and longer lifespans.
In summary, selenium rectifiers were an early type of rectifying device that utilized the semiconductor properties of selenium to convert AC to DC. However, due to their inefficiency, voltage drop, and limited capacity, they were replaced by more advanced semiconductor diodes in most modern electronic and electrical systems.