Connect with us

What happens when an Optocoupler or RF isolator fails?

Discussion in 'General Electronics Discussion' started by WSBoaBoa, Jan 9, 2013.

Scroll to continue with content
  1. WSBoaBoa

    WSBoaBoa

    3
    0
    Jan 9, 2013
    I am trying to find some way of preventing two circuits from being affected from one another but I need a device that can withstand extremely powerful surges with extremely short ramp up times. I have read up on Optocouplers and RF Isolators but I need to understand when the rating is exceeded on the input side and the device fails does it just stop functioning or does the current flow into the secondary circuit?

    I need a solution for this and was thinking about using fiber optics instead so that the length could withstand any amount of current but finding a digital solution prepackaged like an optocoupler or RF isolator would be far simpler for my need.
     
  2. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    And optocoupler is just and LED and a phototransistor. If the LED is blown out, the output will be permanently off.

    Bob
     
  3. WSBoaBoa

    WSBoaBoa

    3
    0
    Jan 9, 2013
    Thanks for the reply. Yes, optoisolators use a small insulating film or dielectric but just wanted to make sure when a high voltage spikes come in that it only blows the source side and has no effect on the other circuit no matter the power applied.

    I do hear Optocouplers are fairly slow when dealing with digital signals which is why I was also wondering about RF Isolators.
     
  4. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    The optoisolator will have a voltage isolation figure. They are very high.

    Bob
     
  5. john monks

    john monks

    693
    1
    Mar 9, 2012
    Can we find out more about your application?
     
  6. WSBoaBoa

    WSBoaBoa

    3
    0
    Jan 9, 2013
    The application is simply to prevent any sort of surge from affecting the secondary circuits no matter the load. From what I have read up on these devices range from 5Kv to 50Kv but wanted to know the effects of a surge beyond the rating, does the device simply burn out or does it arc over to the secondary circuit even though the power is totally isolated from one another. If it simply stops working that would work for my application.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,451
    2,809
    Jan 21, 2010
    That's not a very helpful explanation.

    optocouplers provide isolation, not really "surge protection".

    And you realise that power doesn't go through them, right?
     
  8. john monks

    john monks

    693
    1
    Mar 9, 2012
    If you want to know about isolating circuits in general I can help you.
    If you have a precise application I can be of more help but I'll try.

    Transformers are more resistant to archover or flashover than optical isolators.
    Transformers are generally faster than optical isolators.
    For example the MOC8205 has a transition time of about 5microseconds while a transformer
    may have a transition time of less than 1 nanosecond.
    But an optical fiber may have very short transition times but you have an insertion delay of
    about 1nanosecond for every 8 inches.

    Is this what you were looking for?
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    10,836
    2,443
    Nov 17, 2011
    When you exceed a component's rating all sorts of things may happen. The component (here: optoisolator) may just stop working, it may short circuit or it may go open.

    Anyway, regardless of the type of component (transformer, optoisolator etc.):
    If you exceed the specified input-output isolation voltage, chances are very high that a conductive path will be created between input and output leading to energy flowing across the isolation barrier with normally desastrous effects for the circuit and the users's safety.

    Even if the component itself withstands a higher transient (surge) than specified (manufacturers do leave some headroom to account for tolerances), an electric arc my be created around the component, over the surface of the PCB etc. conducting energy to the output side.

    You should never exceed the ratings of an isolating component. If you can't ensure this, you should consider additional layers of safety as for example:
    - double isolation: use a second tier of isolating components to ensure safety in case of a single failure of one isolation barrier
    - add voltage and/or current limiting elements: fuses, VDRs, supressor diodes, series resistors etc.
    - provide protective ground
    etc.

    Harald
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-