Two ways light affects a semiconductor
What is the photovoltaic effect?
The conversion of light to electrical energy.
What absorbs the energy from light falling on a photovoltaic cell?
Electrons.
A photon transfers its energy to an electron, freeing it to move. In a photovoltaic cell the junction’s field sweeps those carriers apart, producing a voltage — the cell is a source.
What happens to photoconductive material when light shines on it?
Resistance decreases.
A photoconductive material is not a source. Light frees carriers, so it conducts better — it is a light-controlled resistor. It needs an external supply to do anything.
The distinction to hold: photovoltaic generates, photoconductive varies resistance.
Which of these materials is most commonly used to create photoconductive devices?
Crystalline semiconductor
Photovoltaic cells
What is the most common material used in power-generating photovoltaic cells?
Silicon.
What is the approximate open-circuit voltage produced by a fully illuminated silicon photovoltaic cell?
0.5 volts.
The General figure, restated. Half a volt per cell, hence roughly 36 cells in series for a nominal 12 V panel.
What is the efficiency of a photovoltaic cell?
The relative fraction of light that is converted to current.
Typically 15 to 22% for silicon panels you can buy. Worth knowing because it sets the panel area for a given power, and because a specification quoted without it tells you nothing.
Optoisolators
What is the most common configuration of an optoisolator or optocoupler?
An LED and a phototransistor.
An LED and a phototransistor in one lightproof package, facing each other. Signal in becomes light becomes signal out — with no electrical connection between the two sides.
Why are optoisolators often used in conjunction with solid-state circuits that control 120 VAC circuits?
Optoisolators provide an electrical isolation between a control circuit and the circuit being switched
That isolation is the whole product. Your 5 V logic controls a mains-voltage circuit with no conductive path between them, so a fault on the mains side cannot reach the logic — or you.
What is a solid-state relay?
A device that uses semiconductors to implement the functions of an electromechanical relay.
A relay with no moving parts: an optoisolator driving a triac or MOSFET. Silent, fast, with no contacts to arc or wear — and with a small on-state voltage drop and leakage that a mechanical relay does not have.
Shaft encoders
Which of the following describes an optical shaft encoder?
A device that detects rotation by interrupting a light source with a patterned wheel.
A slotted disc between an LED and a detector. Rotation produces pulses; two detectors offset from each other tell you the direction as well as the amount.
It is what is under the tuning knob of every modern radio. There is no potentiometer and no mechanical link to frequency — the knob generates pulses, and firmware decides what they mean. Which is why the same knob can tune, select menus, and set filter width depending on mode.
Check yourself
- A device’s resistance falls when you shine a torch on it. Photovoltaic or photoconductive?
- You want your microcontroller to switch a mains-powered fan. What component, and why?
- How does a radio know which way you turned the tuning knob?
Answers
- Photoconductive — it varies resistance. A photovoltaic device would generate a voltage.
- A solid-state relay, which contains an optoisolator — giving electrical isolation between the low-voltage control circuit and the mains.
- An optical shaft encoder with two offset detectors: the order in which the two pulse trains change gives the direction.