光電耦合元件

An opto-isolator integrated circuit. The "MB 111", manufactured by RFT ("Rundfunk- und Fernmelde-Technik"), contains an infrared LED and silicon photodiode with an integrated amplifier stage.

[编辑] 簡介

光電耦合元件(Opto-isolator,或optical coupler，縮寫為OC），光耦合器是能夠跨越隔離層傳輸訊號的一種裝置，主要由光發射器和光偵測器組成。兩種元件通常會整合到同一個封裝，但它們之間除了光束之外不會有任何電氣或實體連接。光發射器大都是發光二極體，光偵測器的種類比較多，但多半由光電二極體或光電晶體擔任。

本文講述的是the electronic component。關於本條目名稱之其他意思，詳見「光電耦合元件 (消歧义)」。

In electronics, an opto-isolator (or optical isolator, optocoupler, photocoupler, or photoMOS) is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated — since the signal goes from an electrical signal to an optical signal back to an electrical signal, electrical contact along the path is broken.

[编辑] 結構

Schematic diagram with an LED and phototransistor

A common implementation involves a LED and a phototransistor, separated so that light may travel across a barrier but electrical current may not. When an electrical signal is applied to the input of the opto-isolator, its LED lights, its light sensor then activates, and a corresponding electrical signal is generated at the output. Unlike a transformer, the opto-isolator allows for DC coupling and generally provides significant protection from serious overvoltage conditions in one circuit affecting the other.

With a photodiode as the detector, the output current is proportional to the amount of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode.

In photovoltaic mode, the diode acts like a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity.

In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light.

An opto-isolator can also be constructed using a small incandescent lamp in place of the LED; such a device, because the lamp has a much slower response time than an LED, will filter out noise or half-wave power in the input signal. In so doing, it will also filter out any audio- or higher-frequency signals in the input. It has the further disadvantage, of course, (an overwhelming disadvantage in most applications) that incandescent lamps have finite life spans. Thus, such an unconventional device is of extremely limited usefulness, suitable only for applications such as science projects.

The optical path may be air or a dielectric waveguide. The transmitting and receiving elements of an optical isolator may be contained within a single compact module, for mounting, for example, on a circuit board; in this case, the module is often called an optoisolator or opto-isolator. The photosensor may be a photocell, phototransistor, or an optically triggered SCR or Triac. Occasionally, this device will in turn operate a power relay or contactor.

[编辑] 應用

Several types of opto-couplers. The top left and far right detect the presence of an object in between them. They are interruptible. The middle one detects reflections from objects in front of it. The two on the bottom left are both opto-isolators.

A simple circuit with an opto-isolator. When switch S₁ is closed, LED D₁ lights, which triggers phototransistor Q₁, which pulls the output pin low. This circuit, thus, acts as a NOT gate.

Among other applications, opto-isolators can help cut down on ground loops and block voltage spikes.

• One of the requirements of the MIDI (Musical Instrument Digital Interface) standard is that input connections be opto-isolated.
• They are used to isolate low-current control or signal circuitry from transients generated or transmitted by power supply and high-current control circuits. The latter are used within motor and machine control function blocks. For example, a computer modem is built with a small circuit driven from the power supplied by the telephone line; the telephone signal is passed from that circuit to the main modem circuit (which is powered by the computer's own power supply) via an opto-isolator.
• The classical ball computer mouse is a common application, using infrared emitter LEDs and phototransistors to form optocouplers. The ball of the mouse turns a pair of optical encoder wheels. These wheels periodically block the optocouplers and thereby translate the motion of the mouse into a sequence of pulses. These pulses are then used to record the motion. The principle of operation does not require infrared light, but the infrared sensor is less sensitive to interference from common flickering visible light sources such as fluorescent lamps and CRT displays.

[编辑] 參看

• Optoelectronics
• Galvanic isolation

[编辑] 外部聯結

• Optocouplers: When and how to use them (PDF)

[编辑] 製造商

• Avago Technologies
• Fairchild Semiconductor
• Macron International Group
• Panasonic Electric Works Corporation of America
• Vishay