page_title: Resistive opto-isolator
section_title: Transfer characteristic
hierarchical_section_title: Resistive opto-isolator / Physical properties / Transfer characteristic
caption_attribution_description: English: Idealized transfer function of a LED-LDR optoisolator ("vactrol"). Green band approximated fluctuations due to memory effect at room temperature. Wider red band approximates effects of thermal drift, changes in optical coupling due to thermal shifts in LED emission band, etc. As a general rule, all irregularities decrease with the increase in illuminance. Adapted from drawings in PerkinElmer (2001). Photoconductive Cells and Analog Optoisolators (Vactrols®), pp. 34 (response curve) 10, 12 (thermal drift), and idealized model in Silonex (2007). Audiohm Optocouplers: Audio Characteritics.
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: The transfer characteristic of RO is usually represented as the electrical resistance of the photoresistor as a function of the current through the light source; it is a convolution of three main factors: the dependence of light source intensity on its current, optical coupling and spectral matching between the light source and photoresistor, and the photoresponse of the photoresistor. The first dependence is almost linear and temperature independent for LEDs in the practical range of the controlling current. On the contrary, for incandescent lamps the light-current curve is not linear, and the emission spectrum varies with temperature, and thus with the input current. As to the light detector, its properties depend on temperature, voltage and the history of utilization (memory effect). Therefore, the transfer characteristic takes a range of values.
  The equivalent circuit of the photoresistor consists of three components:
  RD – dark resistance, which is determined by the semiconductor and can range from a few MOhm to hundreds GOhm;
  RRL – residual resistance of illuminated but non-loaded photoresistor, typically between 100 Ohm and 10 kOhm;
  RI – ideal photoresistance, which is inversely proportional to the light intensity.
  Because of the large value of RD, the total resistance is mainly determined by RI. The dynamic range of the photoresistor with respect to illumination is equal to the ratio of the critical illumination Φcr to the sensitivity threshold Φₜₕ.
  RD and RI, but not RRL, decrease with increasing voltage that results in signal distortions. At low illumination levels, the resistance of cadmium-based ROs increases by about 1% upon heating by 1 °C. At higher light intensities, the thermal coefficient of resistance can change its values and even its sign.
caption: Idealized transfer function of an LED-based RO, i.e., dependence of the RO resistance on the LED current. Green band approximates fluctuations in resistance caused by memory effect at room temperature. Red band approximates effects of thermal drift and changes in optical coupling.[45]
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