Opto-isolators or Optocouplers

The best part about the transformers are 'electrical isolation' than just providing Higher /Lower Voltage difference.

We know how transformers work by mutual linkage of Magnetic Flux between the primary and secondary coils. 

But there is also another way of doing it. 

The input and the output can also be isolated by the means of light. Yes and that can be done by Opto-isolaters. Let's see how...

Opto-Isolator/ Opto-coupler/ Photo-coupler is an electronic component that interconnects two separate electrical circuits through a light sensitive optical interface.

An Opto-coupler consists of a LED and a light sensitive receiver which can be a single photo-diode, photo-transistor, photo-resistor, photo-SCR, or a photo-TRIAC. 

The Construction is shown below:

HOW IT WORKS

Assume a photo-transistor device as shown. Current from the source signal passes through the input LED which emits an infra-red light whose intensity is proportional to the electrical signal. This emitted light falls upon the base of the photo-transistor, causing it to switch-ON and conduct in a similar way to a normal bipolar transistor.  The base connection of the photo-transistor can be left open for maximum sensitivity or connected to ground via a suitable external resistor to control the switching sensitivity making it more stable. When the current flowing through the LED is interrupted, the infra-red emitted light is cut-off, causing the photo-transistor to cease conducting. The photo-transistor can be used to switch current in the output circuit.  The spectral response of the LED and the photo-sensitive device are closely matched being separated by a transparent medium such as glass, plastic or air. Since there is no direct electrical connection between the input and output of an optocoupler, electrical isolation up to 10kV is achieved.

 Optocouplers are available in four general types : " each one having an infra-red LED source but with different photo-sensitive devices. "

1. Photo-transistor
2. Photo-darlington
3. Photo-SCR
4. Photo-triac  

           APPLICATIONS

1. To switch a range of other larger electronic devices such as transistors and triacs providing the required electrical isolation between a lower voltage control signal and the higher voltage or current output signal.

2.  Common applications for optocouplers include microprocessor input/output switching.

3. DC and AC power control

4.  PC communications, signal isolation and power supply regulation which suffer from current ground loops, etc. The electrical signal being transmitted can be either analogue (linear) or digital (pulses)

            MERITS

     ·         Opto isolators transfer data at incredibly high speeds.

·         They are relatively small, inexpensive to produce, and effective at limiting voltage levels across multiple circuits.

·         Opto isolators can make electrical devices safer, less prone to damage, and more efficient by removing all interference, including electrical interference that lightning strikes create, power surges, alternating pulses, and electrostatic discharge.

 DEMERITS

     ·            Opto isolators are becoming outdated due to smaller devices such as microminiature transformers and spin valves that can provide similar results on a much smaller scale. 

·         High humidity can negatively affect optoisolators as it causes electricity to arc. This results in the electronic isolation being destroyed or interfered with significantly. A drastic change in barometric pressure and air pollution can cause similar issues. This makes it necessary for optoisolators to be kept in a climate-controlled area when possible.