Mains Circuits

Discussion in 'Feedback and Suggestions' started by ronv, Dec 3, 2015.

  1. ronv

    Thread Starter AAC Fanatic!

    Nov 12, 2008
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    First let me say I don't have a problem, but rather am trying to understand the terms of service regarding mains circuits. I understand why non - isolated supplies are not allowed, but I'm not sure why an opto isolator connected to mains would be against the rules, but a transformer attached to mains is ok. Maybe someone can clear it up for me.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    All the old rules may or may not be out the window. Now that liability is assumed by an LLC or other corporate entity, the original owner and administrator is no longer personally liable for the content.
     
  3. JohnInTX

    Moderator

    Jun 26, 2012
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    I don't think something like a mains-driven opto should be off limits. As a noob-mod, I posed the same question just today. The consensus so far is that its generally OK. You can't control/monitor AC if it is not around the circuit. The key would be to safely confine it to the immediate input/output and not have mains-referenced power busses all over the board with the potential to connect to other systems.

    If its decided differently, I'll let you know.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    I think you must be talking about this -- 220VAC Detecting by PIC18FXX.

    Although the real load is at the right side of the opto isolator, but now the led of opto isolator is became a load, if you just look at the 5V at he right side, yes, it's ok, but if you look the left side, that is the led to the mains, there is about 311Vdc on the right side of the four diode bridge rectifier, the only protection is the 47K, if the damaged of led is shorted and is not broke, what will happen, that is the serious problem?

    If you insert a transformer between the four diode bridge rectifier and mains then the load didn't connected to the mains directly, if something happened then the dangerous degree will be not that serious at the right side

    Btw: the fuse is needed whatever you have the transformer or not.
     
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  5. DickCappels

    Moderator

    Aug 21, 2008
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    There is no prohibition against circuits that connect to the mains.

    Check the User Agreement which is the authority on what is allowed and what isn't, regardless of my opinion.
    http://www.allaboutcircuits.com/user-agreement/

    (begin excerpt from User Agreement)
    Restricted topics. The following topics are regularly raised however are considered "off-topic" at all times and will result in Your thread being closed without question:
    • Any kind of over-unity devices and systems
    • Devices designed to electrocute or shock another person
    • LEDs to mains
    • Phone jammers
    • Rail guns and high-energy projectile devices
    • Transformer-less power supplies
    (end excerpt from User Agreement)
     
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  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Isn't the connecting the input-side of an opto-isolator across the AC line exactly the same thing as connecting a LED across the AC line?

    Just asking....
     
  7. JohnInTX

    Moderator

    Jun 26, 2012
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    Reasonable question and one I had to think on a bit.

    Its a matter of purpose. An opto or transformer isolated input for the purpose of detecting the AC line is just that. There is AC present just like it would be on the input terminals of a transformer - isolated power supply. But such a connection implies that the user knows that the hazard is there.

    A string of LEDs to mains or transformerless power supply does not always imply the hazard i.e. you would not expect your 5Vcc supply to be referenced to the hot side of a 120V line, even though it would function. An unsuspecting or inexperienced person could do real damage to him/herself by touching in the wrong place.

    I think the distinction of expectation applies. I expect the inputs of an AC detector to have AC on them and would take steps to ensure safety. I would NOT expect my PIC demo board to have 120V on the Vcc bus nor blow my PC when I hooked up the USB. Same goes for a casual user.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Egad ! I just raised the RED flag Eh!
    Now that I got the answer I will remove the pdf so some n00b might not build it.
    I am confident around live circuits as I poke them everyday when repairing SMPS of TV's & Washing machines as such.
    But As John said this circuit is sometimes a must. Well it is for me as I building the charger. I guess it should be allowed depending on the OP's experience and confidence.
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Strickly taken, the led in the optocoupler would be a led to mains circuit, wich is not allowed in the User Agreement.
    That is why I suggested the circuit with the small transformer as detector.

    Bertus
     
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  10. Wendy

    Moderator

    Mar 24, 2008
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    I have no problem with an optoLED to mains because it is internal to the circuit, and not meant to be seen by the outside, unlike a conventional LED.

    The rule is primarily aimed at beginners.

    Too many beginners try to save a transformer by using AC with no experience with what their doing. If you have to ask for help you should not be doing it. If you know what your doing then you won't need to ask for help. A classic catch 22 situation.

    You can misuse a transformer, but it is a stand alone part that is self current limiting. In the case of a wall wart all the design is done.
     
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  11. WBahn

    Moderator

    Mar 31, 2012
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    Personally, I don't have a very big problem with the original circuit in the post in question because the intent of the (missing) transformer is to isolate the low voltage section from the high voltage section. In this case, the high voltage section is simply everything before the isolating optical interface, so the low voltage side shouldn't see high voltages in it except near the entry point -- which is very analogous to the case if there is a transformer involved. Having said that, there are failure modes that would put a few hundred voltage across the optocoupler input pins and that needs to be considered -- will that blow the optocoupler in such a way that the high voltage can make it to the output pins? I don't know. If it is a fine-pitch part that might also be enough voltage to arc between pins. But this is something that has to be considered even in transformer-isolated designs -- what are the failure modes and how can they be accommodated.

    As for the ToS, that comes down to "letter of the law" vs. "spirit of the law". As already noted, a big part of the spirit is the recognition that many people that either ask these questions (or that read the posts and act one what they read) are very inexperienced newbies. The driving spirit, as I take it, is to limit discussions involving circuits in which there is a higher than reasonable risk of dangerous voltages making their way into parts of the circuit that someone is too likely to consider "safe" and, therefore, willing to go poking around in it even while the circuit is powered.
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    Anyone can get themselves killed or hurt connecting a transformer or an optocoupler up to the mains. The act of connecting anything to the mains has danger associated with it, and just specifying that a transformer is in the circuit does not take the danger factor to zero.
    I agree with with a lot of what has been said above. The likely person to get hurt connecting to the mains is the inexperienced. So, what should our response be to an unknown person asking advice in connecting anything to the mains?
    Do we:
    - ask what their experience level is before we assist?
    - ignore them completely?
    - tell them if they have to ask they don't have enough experience and they should take up woodworking?

    For me, alarm bells go off when someone (unknown) wants to connect an optocoupler up the the mains. The designer is the one that must build an interface to the optocoupler for it to operate safely. If the designer does not have the knowledge and experience then very bad things can happen, like line voltage intermixed with low voltage signals with little physical isolation.

    If one of the well known (and experienced) members build a mains based circuit, I wouldn't bat an eye. It is mainly the people with 3 posts asking a mains related question that makes me nervous.
     
    Last edited: Dec 5, 2015
  13. Wendy

    Moderator

    Mar 24, 2008
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    The difference is in outcome. Connect an opto wrong to mains and the lights go out and lots of smoke. I've done the same with transformers.

    With power supplies and LEDs (as WBahn said) you might think the circuit safe, and electrocute someone who is not familiar with it.

    These discussions come up every couple of years, it was thoroughly discussed back when the rules were established. There is nothing wrong with bringing the subject up again, but they were thought out.
     
  14. Lestraveled

    Well-Known Member

    May 19, 2014
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    I would like clarification on the proper response we should give to people asking these kinds of question. Also, it is always healthy to occasionally reexamine issues.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    LEDs to mains for illumination is still verboten. It is up for discussion, but if you have a project doing this think again.

    Optos to mains is not a problem. They are also isolated and meant to isolate one side from the other.

    Power supplies can have active circuitry on the live AC side, but there can not be a direct connection (except via transformers) to the low voltage side.

    If you need high voltage high currents we tend to look sideways at those anyhow, but if you are an experience user (building a Tesla coil) in the judgment of the moderating staff (and we do talk things over a lot off line) we would still like to see a transformer. No paths to AC should be possible in any way.
     
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