AC polarity on fig 8 power lead

Discussion in 'General Electronics Chat' started by Copey84, Apr 26, 2016.

  1. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Hi all, have came across the figure eight AC power lead on many appliances and as it can be fitted either way polarity must not be an issue. Why is this? Could someone explain?
    Has the changing polarity of the AC power got something to do with it, going positive negative depending on frequency.
    Is it just possible to change polarity on some AC circuits but not all, I know that reverse polarity on a motor will change direction, but when it comes to electronics could AC polarity reverse cause damage?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    The plugs are polarized, and if the sockets are wired correctly then hot goes to hot and neutral goes to neutral. AFAIK things will pretty much always work, but there may be safety issues from a socket that is wired backwards. The Big Box Home Improvement stores sell a three prong gizmo that detects sockets wired backwards.

    http://www.amazon.com/Power-Gear-50542-3-Wire-Receptacle/dp/B002LZTKIA

    Now on the other end it can be connected to the appliance either way and it probably doesn't matter because transformers, used in power supplied don't care which wire is hot and which wire is neutral. AC or Universal Motors generally don't care either.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    There is no polarity on AC, the figure 8 leads are mainly for transformers in radios , dvds etc...
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    In the UK at least, any appliance with that type of lead must comply with "double insulated" specification.

    A good example is in iron cored type mains transformers - the mains and secondary windings have completely separate bobbins on the core, the outer construction is also carefully done so the wire tails from the primary can't touch the secondary if the fuse and droop.

    In days of old, the primary had a few layers of tape over it and then the secondary was wound on top of that. A burnout could burn through to the secondary and make it live.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Polarity is the wrong term. For residential service, there is a Line and a Neutral. The Line is the wire with the energy on it. The Neutral is a return line to theAC power grid, and usually is connected to Ground at the breaker box. When powering a transformer isolated power supply, it usually doesn't matter which line cord wire is connect to which AC input connector pin.

    The "figure 8" connector you have is for a device that has passed reinforced-insulation analysis and testing, and is designed such that it presents no shock hazard no matter which way the cord is plugged in.

    Even though polarity is the wrong word to describe the AC power, non-polarized is the right term for the figure-8 connector.

    ak
     
  6. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Thanks for replys, so fig8 leads will only be used on appliances that connect to transformers, and that are double insulated in UK.
    Hypothetically speaking could any AC appliance be connected live and neutral reverse without any damage being done, as the AC power is always going positive to negative?
     
  7. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Hi ak just seen your reply, basically will any AC appliance not mind which way the live and neutral are poled as the current and voltage are always changing ? Hypothetically speaking, as there may be circuits with filters to ground that would trip any circuit breakers if connected the wrong way.
     
  8. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Put another way, if I apply a AC signal to a circuit does it matter which way round it is connected, live and neutral when talking about an appliance?
    As I see it the signal is always going through both polarity, so why does it matter which way it is connected.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Not as far as appliance operation is concerned.
    It's only a possible safety consideration.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    In most cases it won't make any difference, but some manufacturers skimp on the design if they think they can get away with it.

    Something with a non-reversible mains plug may turn out to have less clearance between pcb traces on the side that should be neutral.

    They usually have to pass high voltage insulation tests - but may not perform so well in high humidity.
     
  11. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Hi all, my posts may caused some confusion. What I had hoped to do was use the fig 8 lead to power a project that I'm building at moment.
    Need a 230v supply so thought of using the fig 8 lead to feed power supply contained in a plastic enclosure, no metal parts, so no need for an earth connection, also all low voltage circuit after power supply.
    However as the lead is non-polarized there is a chance of reverse polarity, that's what got me thinking about AC loads.
    Simple solution is to mark the polarity on lead which I prob will, but just out of interest with a small module power supply including transformer and rectifier could I damage it reverse polarity?
    I kinda think it would be ok, as most of the fig 8 leads feed directly to transformers just like my power supply, just wanted a second opinion.
    I had also thought of using a switch mode power supply from a old mobile phone, but would it be ok to use it 24/7? thinking of overheating .
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    AFAIK: you just need to make sure the transformer has separate bobbins for the primary & secondary windings. That is; if you stripped out the laminations and took it all apart - you could hold the primary in one hand and the secondary in the other. The old type with one wound on top of the other with only a few layers of tape in between, could burn through and make the secondary live.

    With a double insulated transformer, it makes no difference which way round the figure=8 plug goes.

    With the old transformers with one winding on top the other - the finish of the first winding is physically close to the start of the one on top. If the bottom winding is the primary; the finish lead would be close to the secondary - so you'd probably want to make sure that was the neutral.
     
  13. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Thanks Ian makes sense to me now, wasn't confident about possible reverse polarity, will make sure transformer is up to job.
    If I decide to use a switch mode power supply would a mobile phone charger be suitable? have doubts about using 24/7 though, thinking it might over heat.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Most people don't bother unplugging phone chargers and various wall warts.

    Its your responsibility to make sure any unit you re-purpose is used within its rated current - most have some form of over current protection, but some work better than others.
     
  15. Copey84

    Thread Starter Member

    Jul 27, 2015
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    It's a Nokia charger with suitable power requirments, might use it still undecided.
    Thanks again Ian.
     
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