How Do Half-Wave Selenium Diodes Work?

Alternating current (AC) electricity changes directions 60 times per second. To change it to direct current (DC) you need to pass it through a rectifier, which can be half-wave or full-wave. Some rectifiers employ selenium diodes.

Selenium Diode Construction

One electrical contact has a 50 to 60 micrometer layer of halogen-doped selenium deposited on a one micrometer coating of nickel or bismuth on a steel or aluminum plate. This is covered with a film of cadmium alloy which makes up the other electrical contact. Annealing crystallizes the selenium layer to make the diode.

Half-Wave Rectifier

As AC electricity changes directions, it goes from zero volts to a positive cycle, back to zero and then through a negative cycle. A half-wave rectifier using a selenium diode passes electricity during only one half of the cycle, and blocks the electricity during the other half.

Stacking

Stacking selenium diodes on top of each other makes them suitable for higher voltage and current applications. Although lower-costing silicon diodes have mostly replaced selenium diodes, they still work well in special applications such as arc welders and electrostatic air cleaners.

References

• QSL.net: Radio Amateur Exam
• USGS: Selenium