Tuesday, October 5, 2010

Understanding and testing opto-isolators

An opto-isolator also called an opto-coupler or photo-coupler allows a signal to pass from one circuit to another but allows the two circuits to remain electrically isolated. The most common opto-isolator which comes in IC package consists of an LED which shines onto the base of a photo-transistor (usually an npn transistor) and allows current to flow from collector to emitter until the LED is turned off. When a signal is applied to the LED it then shines light that is varied in brightness with the same amplitude as the input signal, this light lands upon the photo-transistor (the resistance of the collector/emitter junction now changes with the varying light) which passes the signal onto the next circuit.

Basic opto-isolator symbol.
If you do a lot of work on switch mode power supplies than you have surely seen the opto-isolators used in the feed back section of the power supply.

Opto-isolators in LCD TV SMPS.

If you want to read more about opto-isolators including my 3 methods for testing them than dowload my free article "Understanding and Testing Opto-isolators" by clicking here.

Included in the article is a schematic and parts list for a simple opto-isolator tester.

Don't forget to visit http://www.preher-tech.com and also if you have any electronics related questions you can email me john@preher-tech.com for assistance.

Recommended Repair Guides:

LCD TV Repair Guide

LCD TV Repair Case Histories

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