page_title: Resistive opto-isolator
section_title: Degradation
hierarchical_section_title: Resistive opto-isolator / Physical properties / Degradation
caption_attribution_description: English: Russian OEP series opto-isolators. These are living fossils made in 1992: incandescent lamp light source, cadmium sulphide sensors. Slow and inefficient, they make superb volume controls, guitar filters and synth VCFs (as long as you don't need fast control response). Yellow tint on tops of the caps is lacquer stain, not rust. Manufacturer: Izmeritel Chernivtsi, Ukraine Русский: Оптопары серий ОЭП. Выпущенные в 1992 году, это самые древние типы оптопар - источником работают лампы накаливания, приёмником - сульфид-кадмиевые резисторы. "Широко используются в узких кругах" в качестве управляемых напряжением регуляторов громкости, гитарных и синтезаторных фильтрах и пр. Завод Измеритель Черновцы, Украина.
context_page_description: Resistive opto-isolator, also called photoresistive opto-isolator, vactrol, 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, a miniature incandescent lamp, or sometimes a neon lamp, whereas the detector is a semiconductor-based photoresistor made of cadmium selenide or cadmium sulfide. 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 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.
context_section_description: Irreversible degradation of a photoresistor can be induced by exceeding its maximum specified voltage even for a short period. For high-resistivity devices, this voltage is determined by the leakage currents flowing on the semiconductor surface and varies between 100 and 300 V for. For low-resistivity models, the voltage limit is lower and originates from the Joule heating.
  The service life of a RO is determined by the lifetime of the light source and the acceptable drift of the parameters of the photoresistor. A typical LED can operate for 10,000 hours, after which its parameters slightly degrade. Its lifetime can be prolonged by limiting the controlling current to half of the maximum value. ROs based on incandescent lamps typically fail after about 20,000 hours, due to the burnout of the spiral, and are more prone to overheating.
  Degradation of the photoresistor is gradual and irreversible. If the operating temperature does not exceed the limit (typically 75 °C or less) then for each year of continuous operation, the dark resistance falls by 10%; at higher temperature such changes can occur within minutes. The maximum power dissipated in the photoresistor is usually specified for 25 °C and decreases by 2% for every °C of heating.
  Cooling below −25 °C sharply increases the response time of a photoresistor. These changes are reversible unless the cooling induces cracking in the plastic components. Soviet ROs packed in metal cases could withstand even at −60 °C, but at these temperatures their response time reached 4 seconds.
caption: Such metal-case Soviet ROs could be operated between −60 and 55 °C
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