Opto-isolators

The operation principle of an opto-isolator is to convert an electrical signal to an optical signal, transmit that light signal and to convert it back into an electrical signal. Of course, one chooses an emitter which well matches the spectral response of the receiver. Typically, one uses an infrared light-emitting diode (LED, usually GaAs-based) and a silicon phototransistor. Those are mounted in a distance of a few millimeters, for example, within an opaque package.

There are also isolators with other kinds of receivers, for example photoresistors (which are largely outdated) or p–i–n photodiodes, or with Darlington phototransistors for a particularly high current transfer your ratio (see below). Early devices used an incandescent lamp as transmitter.

There are even devices containing a triac; the output then remains switched on even when the input signal disappears, until the output current is interrupted in other ways.

Simple isolators contain only the LED and a phototransistor, for example (see Figure 1), and no additional electronics. Others contain a complete receiver circuit, which requires some constant operation voltage and supplies an output voltage signal. Similarly, some devices contain a driver circuit which receives an input voltage signal.

A special form is the solid-state relay, which is explicitly used for switching a load, somewhat similar to a conventional relay based on a magnetically actuated mechanical switch; the receiver may contain one or two MOSFETs (a special kind of transistors).

Of course, the electrical integration must be done according to the type and specifications of the particular isolator used. There are substantial differences between different isolators. For example, devices with photoresistors can be used with arbitrary polarity, while those with photodiodes or phototransistors can not.

There are also bidirectional opto-isolators. In principle, one may realize such a device based on two LEDs, where each one reacts to light of the other one. However, much better performance is achieved by using one opto-isolator for each direction, which may be integrated into a common package.

The optical coupling between transmitter and receiver can be done in different ways. For example, one may just mount the transmitter close to the receiver to avoid substantial losses of light. For improved isolation, one can insert some dielectric material between them. One may also use a reflecting silicone dome or a kind of multimode waveguide. Particularly for high isolation voltages, one may prefer to use a relatively long waveguide, perhaps even a long optical fiber.

Essential parameters of an opto-isolator are:

• One requires a certain input current to the LED. This is not a fixed value, however: it depends on how much photocurrent is required on the receiver side, particularly for the common devices based on an LED and a phototransistor: the maximum photocurrent is proportional to the optical power, which itself is proportional to the LED drive current. Therefore, apart from a maximum allowed input current manufacturers specify a current transfer ratio, which is the ratio of photocurrent to the drive current. The current transfer ratio depends on the efficiency of the emitter, the efficiency of the light transfer to the receiver, and on the responsivity of the receiver. Opto-isolators differ very much in that respect; while some have values well below 1%, others (with a phototransistor and efficient light coupling) can have ratios above 5 (= 500%).
• Such a device has a certain specified isolation voltage, i.e., a maximum allowed voltage between transmitter and receiver, which is often a couple of kilovolts. For operation with higher voltages, an electrical breakdown may occur, which would typically destroy the device.
• The operation bandwidth can easily be in the megahertz region, sometimes even above 1 GHz. The fastest response is obtained with p–i–n photodiodes. Phototransistors are typically slower than photodiodes.
• Typically, the relation between photocurrent and input drive current is fairly linear over a substantial range. (To exploit that linearity, one may need a driver circuit which provides an output current proportional to its input voltage.) However, some devices are meant to be for digital signals, where linearity is no relevant aspect.

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