Optical Isolators

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1. Overview


  • Transistor output optocouplers.

2. Transistor Output Optocouplers

2.1. 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.

Many opto-isolators use a infrared emitting diode optically coupled to a photo transistor, all within a single enclosed component.

AC capable opto-couplers usually have two LEDs in anti-parallel (e.g. the HCPL-354 by Avago).

2.2. The Current-Transfer Ratio

The current-transfer ratio (CTR) of a photo-transistor based optical isolator tells you how well it amplifies an input signal to an output signal. It is the ratio of the collector current \(I_C\) at the output side relative to the forward current \(I_f\) going through the LED on the input side, and then expressed as a percentage:

\[CTR = \frac{I_C}{I_f} \cross 100%\]

For optical isolators with a single phototransistor driver, the current-transfer ratio is normally in the 20-400% range. Sometimes within a family there are multitude of parts with different transfer ratios, specified by slightly different part numbers. The EL3H7-G series from Everlight is one such example.

The current transfer ratio is specified at a particular forward current I_f through the diode and particular voltage across the output collector-emitter V_{CE}.

3. Phototriac Output Optocouplers

Phototriac output optocouplers (a.k.a. triac couplers) are like phototransistor optocouplers, except they have a traic instead of a transistor on the output. The triac’s gate is controlled by the incoming light source. Phototriac optocouplers are used to switch on and off an AC load in an isolated manner.

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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