OPTO-COUPLERS are also called OPTO-ISOLATORS or PHOTO-COUPLERS.
An opto-coupler is an optical link and it connects two circuits via this link.
The optical link is contained within a chip. A Light Emitting Diode inside the chip shines on a photo-diode, photo-transistor or other photo device.
When the photo device sees illumination, the resistance between its terminals reduces. This reduced resistance can activate another circuit. See this in operation in the animation below.
An opto-coupler is designed to withstand a very high voltage (500v to 10kv) between its input and output. This is called ISOLATION. It can also be called INSULATION, but the term isolation is used more frequently.
The other main advantage of an opto-coupler is the ability to have the input and output at different potentials. This is also called isolation but it means the two circuits can have different "earth rails" or 0v rails.
All opto-couplers have the same feature - a circuit activates another circuit via an optical link - passing information from one circuit to another.
There are a number of differences between opto-couplers. Some have a transistor, triac or Schmitt Trigger output, some have high isolation and others have a high speed of operation. Some will transmit audio and digital information, other will only transmit digital information.
Transistor versions can be used to transmit digital or audio information.
When deciding on an opto-coupler, here are the 5 steps:
1. Start with isolation voltage. Do you need 500v isolation or 2kv?
2. Check type of output. Do you want to pass audio or digital information?
3. Check speed of operation. Do you need high speed?
4. Check availability
5. Check price
Nearly all opto-couplers have the following features:
Between 1mA and 15mA is needed to illuminate the LED.
The transistor in a photo-transistor type is capable of withstanding 20v - 70v.
Photo-thyristor types are capable of withstanding 250v - 400v.
For high-speed operation a NAND gate version is available.
HOW AN OPTO-COUPLER WORKS:
(this is a theoretical circuit - see below for a practical circuit)
between the collector and emitter of the
The voltage on the output REDUCES
The animation above shows an opto-coupler in operation.
The output transistor will pass 30mA to 100mA.
When the photo-transistor in the opto-coupler above is
not detecting illumination, it has a high resistance between pins 3 and
4. We can say the output is LOW (this is not technically correct, but
will be ok for this discussion).
A complete range of opto-couplers is available from: ISOCOM Ltd OPTOCOUPLERS.
See their complete list with parameters for each device.
Click: pinouts to link to all optocouplers having that pinout!
Here are a few common optocouplers:
The diagrams below show the range of layouts for opto-couplers: