page_title: Resistive opto-isolator
section_title: Simple voltage dividers
hierarchical_section_title: Resistive opto-isolator / Applications / Simple voltage dividers
caption_attribution_description: Three basic voltage divider circuits employing vactrols (photoresitor or photocell optocouplers): Series Shunt Series-shunt
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: In the simplest output-limiting circuits, the RO is placed in the top (series connection) or lower (shunt) arm of the voltage divider. The series connection provides a greater controlling range (−80 dB) at DC and low frequencies. The operation is complicated by the nonlinearity of the resistance vs. the controlling current. The narrowing of the dynamic range due to the parasitic capacitance is significant at frequencies as low as hundreds of Hz. The reaction is significantly faster to the increase than decrease in the controlling current.
  The shunt connection results in smoother transfer characteristics and lower signal distortions, but also in a lower modulation range (−60 dB). This limitation is lifted by connecting two shunt dividers in series, which keeps the transfer characteristic smooth. The best combination of a smooth transfer characteristic, low distortion, wide range of adjustment, and nearly equal rates of increase and decrease of the transmission coefficient is achieved in a series-parallel circuit composed of two ROs and a series resistor. The frequency response of such circuit is similar to that of the series connection.
caption: Series, shunt and series-shunt voltage dividers.
  Series-shunt divider requires two drive signals (IC UP and IC DOWN).
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