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Photosensor Contents Photosensor Contents 1 Opto-isolator 1 1.1 History ................................................. 1 1.2 Operation ............................................... 1 1.3 Electric isolation ............................................ 1 1.4 Types of opto-isolators ........................................ 2 1.4.1 Resistive opto-isolators .................................... 2 1.4.2 Photodiode opto-isolators ................................... 3 1.4.3 Phototransistor opto-isolators ................................. 4 1.4.4 Bidirectional opto-isolators .................................. 4 1.5 Types of configurations ........................................ 4 1.6 Alternatives .............................................. 4 1.7 Notes ................................................. 5 1.8 References ............................................... 6 1.9 Sources ................................................ 7 1.10 External links ............................................. 8 2 Photodetector 9 2.1 Types ................................................. 9 2.2 Frequency range ........................................... 9 2.3 See also ................................................ 9 2.4 References .............................................. 9 2.5 External links ............................................. 10 3 Optoelectronics 11 3.1 See also ................................................ 11 3.2 References ............................................... 11 3.3 External links ............................................. 11 4 Photodiode 12 4.1 Principle of operation ......................................... 12 4.1.1 Photovoltaic mode ...................................... 13 4.1.2 Photoconductive mode .................................... 13 4.1.3 Other modes of operation ................................... 13 i ii CONTENTS 4.2 Materials ................................................ 13 4.2.1 Unwanted photodiode effects ................................. 13 4.3 Features ................................................ 14 4.4 Applications .............................................. 14 4.4.1 Comparison with photomultipliers .............................. 15 4.5 Photodiode array ........................................... 15 4.6 See also ................................................ 15 4.7 References ............................................... 16 4.8 External links ............................................. 16 4.9 Text and image sources, contributors, and licenses .......................... 17 4.9.1 Text .............................................. 17 4.9.2 Images ............................................ 17 4.9.3 Content license ........................................ 18 Chapter 1 Opto-isolator This article is about the electronic component. For the a boom in optoelectronics, and by the end of the optical component, see optical isolator. 1970s the industry developed all principal types of opto- In electronics, an opto-isolator, also called an opto- isolators. The majority of opto-isolators on the market use bipolar silicon phototransistor sensors.[5] They attain medium data transfer speed, sufficient for applications like electroencephalography.[6] The fastest opto-isolators 1 3 use PIN diodes in photoconductive mode. 1.2 Operation 2 4 An opto-isolator contains a source (emitter) of light, al- most always a near infrared light-emitting diode (LED), that converts electrical input signal into light, a closed op- [7] Schematic diagram of an opto-isolator showing source of light tical channel (also called dielectrical channel ), and a (LED) on the left, dielectric barrier in the center, and sensor (pho- photosensor, which detects incoming light and either gen- totransistor) on the right.[note 1] erates electric energy directly, or modulates electric cur- rent flowing from an external power supply. The sensor coupler, photocoupler, or optical isolator, is a com- can be a photoresistor, a photodiode, a phototransistor, ponent that transfers electrical signals between two iso- a silicon-controlled rectifier (SCR) or a triac. Because lated circuits by using light.[1] Opto-isolators prevent high LEDs can sense light in addition to emitting it, construc- voltages from affecting the system receiving the signal.[2] tion of symmetrical, bidirectional opto-isolators is possi- Commercially available opto-isolators withstand input- ble. An optocoupled solid state relay contains a photo- to-output voltages up to 10 kV[3] and voltage transients diode opto-isolator which drives a power switch, usually with speeds up to 10 kV/μs.[4] a complementary pair of MOSFETs.A slotted optical switch contains a source of light and a sensor, but its op- A common type of opto-isolator consists of an LED and a tical channel is open, allowing modulation of light by ex- phototransistor in the same opaque package. Other types ternal objects obstructing the path of light or reflecting of source-sensor combinations include LED-photodiode, light into the sensor. LED-LASCR, and lamp-photoresistor pairs. Usually opto-isolators transfer digital (on-off) signals, but some techniques allow them to be used with analog signals. 1.3 Electric isolation 1.1 History Electronic equipment and signal and power transmis- sion lines can be subjected to voltage surges induced by The value of optically coupling a solid state light emitter lightning, electrostatic discharge, radio frequency trans- to a semiconductor detector for the purpose of electri- missions, switching pulses (spikes) and perturbations in cal isolation was recognized in 1963 by Akmenkalns,et power supply.[8] Remote lightning strikes can induce al. (US patent 3,417,249). Photoresistor-based opto- surges up to 10 kV, one thousand times more than the isolators were introduced in 1968. They are the slow- voltage limits of many electronic components.[9] A cir- est, but also the most linear isolators and still retain cuit can also incorporate high voltages by design, in which a niche market in audio and music industry. Com- case it needs safe, reliable means of interfacing its high- mercialization of LED technology in 1968–1970 caused voltage components with low-voltage ones.[10] 1 2 CHAPTER 1. OPTO-ISOLATOR wires.[15] The physical layout of an opto-isolator depends primar- ily on the desired isolation voltage. Devices rated for less than a few kV have planar (or sandwich) construction.[16] The sensor die is mounted directly on the lead frame of its package (usually, a six-pin or a four-pin dual in-line package).[7] The sensor is covered with a sheet of glass or clear plastic, which is topped with the LED die.[7] The LED beam fires downward. To minimize losses of light, the useful absorption spectrum of the sensor must match the output spectrum of the LED, which almost invariably lies in the near infrared.[17] The optical channel is made as thin as possible for a desired breakdown voltage.[16] For example, to be rated for short-term voltages of 3.75 kV and transients of 1 kV/μs, the clear polyimide sheet in the Avago ASSR-300 series is only 0.08 mm thick.[18] Breakdown voltages of planar assemblies depend on the thickness of the transparent sheet[16] and the configura- tion of bonding wires that connect the dies with exter- nal pins.[7] Real in-circuit isolation voltage is further re- duced by creepage over the PCB and the surface of the Planar (top) and silicone dome (bottom) layouts - cross-section through a standard dual in-line package. Relative sizes of LED package. Safe design rules require a minimal clearance (red) and sensor (green) are exaggerated.[note 2] of 25 mm/kV for bare metal conductors or 8.3 mm/kV for coated conductors.[19] Opto-isolators rated for 2.5 to 6 kV employ a different [20] The main function of an opto-isolator is to block such layout called silicone dome. Here, the LED and sen- high voltages and voltage transients, so that a surge in one sor dies are placed on the opposite sides of the package; [20] part of the system will not disrupt or destroy the other the LED fires into the sensor horizontally. The LED, parts.[2][11] Historically, this function was delegated to the sensor and the gap between them are encapsulated isolation transformers, which use inductive coupling be- in a blob, or dome, of transparent silicone. The dome tween galvanically isolated input and output sides. Trans- acts as a reflector, retaining all stray light and reflecting formers and opto-isolators are the only two classes of it onto the surface of the sensor, minimizing losses in a [20] electronic devices that offer reinforced protection — they relatively long optical channel. In double mold designs protect both the equipment and the human user operating the space between the silicone blob (“inner mold”) and this equipment.[12] They contain a single physical isola- the outer shell (“outer mold”) is filled with dark dielectric tion barrier, but provide protection equivalent to double compound with a matched coefficient of thermal expan- [21] isolation.[12] Safety, testing and approval of opto-couplers sion. are regulated by national and international standards: IEC 60747-5-2, EN (CENELEC) 60747-5-2, UL 1577, CSA Component Acceptance Notice #5, etc.[13] Opto-isolator 1.4 Types of opto-isolators specifications published by manufacturers always follow at least one of these regulatory frameworks. 1.4.1 Resistive opto-isolators An opto-isolator connects input and output sides with a beam of light modulated by input current. It transforms useful input signal into light, sends it across the dielectric Main article: Resistive
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