Optical Isolators

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1. Schematic Symbol

A commonly used schematic symbol for an optical isolator is shown below:

opto isolator schematic symbol
Figure 1. A commonly used symbol for a standard optical isolator.

2. The Current-Transfer Ratio

The current-transfer ratio of a photo-transistor based optical isolator tells you how well it amplifies an input signal to an output signal.

For optical isolators with a single phototransistor driver, the current-transfer ratio is normally in the 20-120% range.

3. Phototriac Outputs

Some optical isolators have phototriac outputs instead of the normal phototransistor output. A phototriac is a triac whose gate is controlled by the incoming light source, essentially the same as what a phototransistor is to a transistor.

phototriac based optical isolator internal schematic
Figure 2. The internal schematic of an optical isolator with a phototriac output driver rather than a phototransistor. Image from http://www.ti.com/.

The outputs of phototriac optical isolators usually go on to drive power TRIACs which switch a mains powered load.

4. Component Packages

To achieve good isolation, different-net pins must be kept well away from each other to prevent arcing. This prevents many small-pitch packages from being used for optical isolators.

The two most common packages used for optical isolation components are the through-hole DIP package and the surface-mount SOIC package.



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