Vactrol

Resistive opto-isolator (RO), also called photoresistive opto-isolator, vactrol (after a genericized trademark introduced by Vactec, Inc. in the 1960s), analog opto-isolator or lamp-coupled photocell, is an optoelectronic device consisting of a source and detector of light, which are optically coupled and electrically isolated from each other. The light source is usually a light-emitting diode (LED), a miniature incandescent lamp, or sometimes a neon lamp, whereas the detector is a semiconductor-based photoresistor made of cadmium selenide (CdSe) or cadmium sulfide (CdS). The source and detector are coupled through a transparent glue or through the air.

Electrically, RO is a resistance controlled by the current flowing through the light source. In the dark state, the resistance typically exceeds a few MOhm; when illuminated, it decreases as the inverse of the light intensity. In contrast to the photodiode and phototransistor, the photoresistor can operate in both the AC and DC circuits and have a voltage of several hundred volts across it. The harmonic distortions of the output current by the RO are typically within 0.1% at voltages below 0.5 V.

RO is the first and the slowest opto-isolator: its switching time exceeds 1 ms, and for the lamp-based models can reach hundreds of milliseconds.Parasitic capacitance limits the frequency range of the photoresistor by ultrasonic frequencies. Cadmium-based photoresistors exhibit a "memory effect": their resistance depends on the illumination history; it also drifts during the illumination and stabilizes within hours, or even weeks for high-sensitivity models. Heating induces irreversible degradation of ROs, whereas cooling to below −25 °C dramatically increases the response time. Therefore, ROs were mostly replaced in the 1970s by the faster and more stable photodiodes and photoresistors. ROs are still used in some sound equipment, guitar amplifiers and analog synthesizers owing to their good electrical isolation, low signal distortion and ease of circuit design.

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