Opto-isolator

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(Redirected from Opto-coupler)

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In electronics, an opto-isolator (or optical isolator, optocoupler or photocoupler) 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.

A common implementation involves an LED and a light sensor, 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.

[edit] Application

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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.
Opto-isolators are used to protect hospital patients from accidental electric shock. Patients with IV's in their bodies are particularly susceptible, sometimes succumbing to 'carpet shock.'
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.
The classical computer mouse is a common application, using infrared emitter led's and phototransistors to form optocouplers. They are used to translate the mousewheel velocity into digital motion information. The principle of operation does not require to utilize infrared light, though this frequency range is somehow resistant against interference with visible light.