Explain photoconductors, photovoltaic cells, optoisolators, solid-state relays and optical shaft encoders.
Syllabus: Electro-optical technology: photoconductivity; photovoltaic devices; optical sensors and encoders; optically isolated switching
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
In a photoconductive device, light frees charge carriers in the material, so its resistance decreases and more current flows. They are made from a crystalline semiconductor.
RememberMore light: lower resistance. Material: crystalline semiconductor.
The photovoltaic effect is the conversion of light to electrical energy. Photons carry the light, and electrons absorb the energy. Power-generating cells are made of silicon. A fully illuminated silicon cell gives about 0.5 V open-circuit. Efficiency is the fraction of the light converted to electricity.
RememberPhotovoltaic: light to electricity, silicon, about 0.5 V per cell. Electrons absorb the energy.
The common optoisolator (optocoupler) is an LED and a phototransistor in one package. Light crosses an insulating gap, so a control circuit switches another circuit with no electrical connection between them. That isolation is why it is used with solid-state circuits that switch 120 VAC. A solid-state relay uses semiconductors to do the job of an electromechanical relay, often with an optoisolator on its input.
RememberOptoisolator: LED plus phototransistor, electrical isolation. Solid-state relay: semiconductors replace the mechanical relay.
A wheel with slots or marks turns between a light source and a detector. Each slot interrupts the light, giving a pulse train, so counting pulses detects rotation.
RememberEncoder: patterned wheel interrupting a light beam.