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Audiohm Optocouplers: Principles of Operation
When the LED is "off", the photoresistor resistance is high (R "off" - "dark" state). Depending on the component's specification, this can be in the region of 10's of Kilohms to Megaohms.
A current is applied to the LED and it emits light. The optical coupling material guides the light optimally onto the photoresistor. The light falling on the photoresistor causes the resistance to reduce, after a period of time (usually ms) the photocell reaches its R"on" resistance.
When the supply to the LED is removed and the LED stops emitting light, the photoresistor reacts to the removal of the light by starting to return to its "dark" state. It increases in resistance until (after a determined period of time) it reaches its R "off" resistance.
You can see from the graph that the time taken for the photocell to reduce in resistance in the LED "on" state and increase in resistance in the LED "off" state is not equal. This is a phenomenon typical of a photoresistor.
Silonex manufactures optocouplers with different "R on" and "R off" resistance values and "T on" and "T off" speed ratios for the various different applications in audio which demand fast or slow sweep speed and high or low resistance.
The performance characteristics of the optocouplers are all directly linked. On to off speed, dynamic range, and on and off resistance values are all traded off against each other, the perfect device doesn't exist and excellent application performance is almost always a compromise on one of the parameters, however the audible performance gain is really worth it.
Modes of operation
Analog Mode
In analog mode, the optocoupler performs the function of a "current controlled variable resistor". LEDs emit light directly proportional to the supply current and the photoresistor changes its resistance in proportion to the light level. This is a major benefit for audio designers who are using FETs or VCAs. When used in analog mode the photoresistor has considerably less distortion than a FET and is more cost effective than a VCA (voltage controlled amplifier).
Digital Mode
When used in digital mode, the optocoupler is performing as a switch. Optocouplers have some unique properties in terms of switching or controlling audio signals and other audio components. Other electronic switching devices may perform better as switches but introduce adverse and unwanted distortion into the circuit. Audiohm optocouplers are specifically designed and manufactured to have considerably less distortion than any other switching method.
