When to use an optocoupler?

Discussion in 'General Electronics Chat' started by Xavier Pacheco Paulino, Apr 20, 2017.

  1. Xavier Pacheco Paulino

    Thread Starter New Member

    Oct 21, 2015
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    Hi,

    I want to control a 12V/0.15A motor by using a simple transistor connected to the output of a MCU. I know that it can perfectly work like that. While looking into some example circuits, I saw that some of them use an optocoupler. My question is: When is it strictly needed?
     
  2. Reloadron

    Well-Known Member

    Jan 15, 2015
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    Hard to say when it is strictly needed. Maybe look at the reasoning for using one? Motors, depending on their type tend to create noise on power supply lines and when using devices like a u-controller or similar they do not fare well with noise on their power lines. They like to reset at random and get weird. So, often good design practices call for using different power supplies, one for motor(s) and another for sensitive electronics. Enter the opto-coupler which allows control with isolation. I guess it is a matter of when good design habits and practice dictate their use. That is my story and I am sticking to it! :)

    Ron
     
  3. crutschow

    Expert

    Mar 14, 2008
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    With a good single point ground for the two supplies (if you have them) an opto isolator is usually not needed.
    If the ground scheme isn't good then one might be needed.

    If you only have a single power supply for everything then an opto isolator likely won't help much.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Two reasons for a opto isolation is either where galvanic isolation is needed as between two separate supply sources for each system, the other where there is no galvanic isolation but voltage transition is required as in between control from a PC parallel port and a mains switched AC motor, both are referenced to earth ground.
    Max.
     
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Only two reasons for using an opto-coupler,

    1) different supplies used.
    2) protection for the microcontroller or plc from unwanted inputs from the external world.

    If you are using a safe low voltage isolated supply, then just use a simple transistor or relay.
     
  6. DNA Robotics

    Member

    Jun 13, 2014
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    Parallel port breakout boards use opto-isolators to protect the computer from home brew interfacing.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As mentioned in post #4, many assume that using an opto isolation on a PP output they have galvanic isolation from say a AC fed spindle motor, whether using a SSR, a relay or any opto isolator, due to the fact the PP is referenced to earth ground as is the AC motor by way of the grounded neutral, no galvanic isolation exists, just voltage transition.
    Max.
     
  8. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Its probably the cheapest isolation between high & low voltage circuits. There are other methods for more specialised applications.

    There are dozens of types - all start with a basic IRLED on the input side. Most common is the basic BJT output side, but you can get Darlington output and MOSFET (including bidirectional). Most incarnations of 4-layer device are covered; like triac, SCR and trigger diode. You can also get them with logic gate type output - more in the territory of solid state switches, you can get MOSFET types that handle more current than a typical opto.

    Every once in a while you find an application where an opto coupler is *convenient* even if isolation isn't needed.
     
  9. Reloadron

    Well-Known Member

    Jan 15, 2015
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    I had all but forgotten but several years ago I recall an application using a linear type opto-coupler which was pretty slick. I want to think it had to do with monitoring the cells in a battery powered automobile but really can't recall. I just recall linear opto-couplers and thinking to the effect of that was pretty cool. I had pretty much forgotten that application.

    Ron
     
  10. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Properly proportional opto couplers are the subject of various application notes. It actually uses 2 opto couplers. The 2 IRLEDS are in series and driven by an op amp and the signal to be isolated. One of the output transistors is part of the usual signal path, the other is in the op amp feedback loop.

    It compensates for the non linearity of the IRLED so that the output is a meaningful representation of the input.
     
  11. Reloadron

    Well-Known Member

    Jan 15, 2015
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    Here is a good example. I had all but forgotton about those little guys till your mention of the types.

    Ron
     
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