E6F: Electro-Optical Technology

E6F covers devices and systems that convert between light and electrical energy, or that use light to transfer electrical signals with galvanic isolation. Topics include photovoltaic cells (how they work, efficiency definition, material, and open-circuit voltage), photoconductivity and the resistance change when light strikes semiconductor materials, optoisolators/optocouplers, optical shaft encoders, solid-state relays, and the practical reason for using optical isolation when controlling AC power circuits.

The Extra exam draws one question from E6F. Questions test specific knowledge of how each electro-optical device works, the definitions of efficiency and photovoltaic effect, and the role of optical isolation in safe AC circuit control.

Photovoltaic Cells

A photovoltaic (PV) cell converts light energy directly into electrical energy. When photons strike the semiconductor material, they transfer their energy to electrons, freeing them to flow as current. It is electrons — not protons, photons, or holes directly — that absorb the energy from incident light in a PV cell.

Photovoltaic Effect

The photovoltaic effect is the conversion of light to electrical energy. When photons with sufficient energy strike a PN junction, they create electron-hole pairs. The built-in electric field at the junction separates these pairs, driving electrons toward the N-side and holes toward the P-side, generating a voltage and current without any external bias.

PV Cell Efficiency

The efficiency of a photovoltaic cell is defined as the relative fraction of light that is converted to current. It is not the open-circuit voltage divided by short-circuit current, and it is not expressed in lumens — it is simply the ratio of electrical power output to the optical power input, expressed as a percentage.

PV Cell Material and Voltage

The most common material used in power-generating photovoltaic cells is silicon. Silicon's band gap (approximately 1.1 eV) is well matched to the solar spectrum, and silicon is abundant and well-understood from decades of semiconductor manufacturing.

A fully illuminated silicon photovoltaic cell produces an open-circuit voltage of approximately 0.5 volts. This is determined by the silicon band gap and the junction characteristics under illumination. Practical solar panels connect many cells in series to achieve useful voltages.

Photoconductivity

Photoconductive materials change their electrical resistance when exposed to light. When light strikes a photoconductive material, photons free additional charge carriers in the semiconductor, increasing the number of current-carrying electrons and holes. This results in decreased resistance — more carriers means lower resistivity.

The most common materials used to create photoconductive devices are crystalline semiconductors. Crystalline semiconductor materials (silicon, germanium, gallium arsenide, cadmium sulfide) have well-defined band gaps that match specific wavelength ranges of light, allowing selective sensitivity to particular parts of the spectrum.

Optoisolators and Optocouplers

An optoisolator (also called an optocoupler) transfers a signal from one circuit to another using light, without any direct electrical connection between the two circuits. The most common configuration is an LED and a phototransistor packaged together:

1. The input signal drives an LED, converting electrical current to light
2. The light crosses a small gap inside the sealed package
3. A phototransistor on the output side detects the light and produces a corresponding output current

The isolation voltage of a typical optoisolator may be 1,000 to 5,000 volts, making it safe to use a 3.3 V or 5 V logic signal to control a 120 VAC load without any direct electrical path between the two circuits.

Optical Encoders and Solid-State Relays

Optical Shaft Encoder

An optical shaft encoder is a device that detects rotation by interrupting a light source with a patterned wheel. A disc with evenly spaced slots or holes is attached to a rotating shaft. As the shaft turns, the disc alternately blocks and passes a light beam between a light source and a photodetector. The resulting pulse train can be counted to determine rotation angle, direction, and speed. Optical encoders are used in antenna rotators, CNC equipment, and motor control systems.

Solid-State Relay

A solid-state relay (SSR) is a device that uses semiconductors to implement the functions of an electromechanical relay. Instead of mechanical contacts that open and close, an SSR uses transistors, SCRs, or triacs to switch current. Solid-state relays typically include optoisolator input stages for control-to-load isolation, have no moving parts (longer life, silent operation), and can switch much faster than mechanical relays.

E6F Practice Questions