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Shocked from a tranformer

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by goofeedad, Aug 8, 2009.

  1. goofeedad

    goofeedad New Member

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    I'm confused, in a past thread I was told that you can't get shocked from the output of a transformer that changes 110VAC down to any VAC. Is that right? and if so why?

    Isn't AC dangerous in any form?

    If it's not asking too much maybe someone could give us newbies to the forum a quick tutorial.

    Sorry if this is not normal etiquette.
     
  2. kchriste

    kchriste New Member Forum Supporter

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    No that is not right. If the voltage is high enough, you can get shocked from the secondary of a transformer if you touch two of the leads on the secondary or if one of the leads on the secondary is connected to ground somehow and you touch the other.

    Low voltage AC is safe to touch. ie: touching 12Vac won't hurt you. Don't stick your tongue on it!
     
    Last edited: Aug 8, 2009
  3. HarveyH42

    HarveyH42 Banned

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    I hate getting shocked, so I always show the proper respect, regardless of the expected voltage. That way I don't get zapped (very often) if something goes wrong. Just a good habit to have.
     
  4. dave

    Dave New Member

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  5. goofeedad

    goofeedad New Member

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    Uhmmm...... Well, first of all, that's what I thaught. Second.... if it's the amperage that hurts not the voltage (ie. 1,000 V at 1/4amp should be ok) what does it matter what volttage it is? Is it because the power supply for an outdoor lighting system puts out such a low amperage?

    BTW, the transformer I'm useing is a 120VAC to 28VAC both primary and secondary are grounded to the same bus.

    Is it possible to get my 28VAC safe? By that I mean safer than 120VAC drawn right from the household outlet?
     
  6. be80be

    be80be Well-Known Member

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    Can kill you
    here some more
    you can read it all here http://en.wikipedia.org/wiki/Electric_shock
     
    Last edited: Aug 8, 2009
  7. be80be

    be80be Well-Known Member

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    Use a GFI plug. It takes less AC current
    to stop your heart. But I no for a fact low voltage AC can hurt you. I have worked on lines with 8800 volts have got a shock or two from 110 to 240 volt line. Almost ended up in the Hospital from a 48volt phone line that I didn't think would hurt me. I would almost say if the shock had went threw me like if i had grounded my left hand I would not be writing this now.
     
    Last edited: Aug 8, 2009
  8. BrownOut

    BrownOut Banned

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    It's Ohm's law. If your bodily tissue represents, say 1k ohms, then 28V will only push 28mA, a pretty insignificant current. You might feel a tingle, but that wouldn't be lethal. But say you're touching 120V, then you're feeling 120mA, and you're at the threshold of a lethal current. Your transformer should have an isolated secondary. That gives you another layer of safety.
     
  9. tariq7868

    tariq7868 New Member

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    you can also get a shock of about 1000-3000v from secondary,, even if you are applying only 110vac , in the case if you havnt properly connected 110v line (or if it loosened) at primary...this is due to the change in current in very short time ..even when you turn on the power there is much higher voltage generated at the secondary at first..
     
    Last edited: Aug 9, 2009
  10. Hero999

    Hero999 Banned

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    You might feel a small tingle from 28VAC, as a general rule AC is more dangerous than DC so you won't be able to feel 28VDC.
     
  11. goofeedad

    goofeedad New Member

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    My transformer should have an isolated secondary? How do I know if I have that? and how would I go about cicuiting that?
     
  12. Hero999

    Hero999 Banned

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    It depends on how the transformer is constructed.

    The only real way of testing it is to use a high voltage resistance meter (>2kV).
     
  13. marcbarker

    marcbarker New Member

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    Many transformers have good galvanic isolation, but because of capacitance between primary/secondary there is a slight leakage current which can be felt when touching the metal casing if it's not earthed. If you touch the casing with a neon screwdriver it'll light up. Maybe it's not something you get with 120 V mains voltage.

    Even though it's only a few mA, there are interesting effects with non-earthed metal case equipment (and 'double insulated' too) because of the natural leakage. If you lightly touch the metal casing with fingers and stroke lightly, you can hear a 50 Hz humming sound coming from the muscles in your hand! You can get this same effect touching Anti-Static Mats, if you place the un-earthed equipment on the mat.
     
  14. RCinFLA

    RCinFLA Well-Known Member

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    Most power transformers are isolated. An autotransformer is not.

    Usually sufficient to use ohm-meter between a primary winding lead and secondary winding lead. Should read meg-ohms.

    A GFI breaker has a current sense transformer to amplifer to make sure the same amount of current that came out of AC hot line goes back in AC neutral line. I tested one with a 4.7 meg resistor from hot to ground while a 1200 watt (10 amp) hair drier was running. When resistor touched the connections it popped open. Pretty impressive, detecting 25 uA delta with 10 amp load.
     
  15. Hero999

    Hero999 Banned

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    marcbarker,
    I've never noticed this with small transformers because they have a capacitance of only 100pF or so.

    Are you sure they're not powered from switchers?

    SMPSes have small (4.7nF) Y rated capacitors from the secondary side to primary side to mitigate EMI on the secondary, giving a leakage current of just 340µA which you won't even be able to feel. The problem is if lots devices are connected together the leakage currents add together, suppose you connect your TV to: a DVD player, a games console and a stereo system the currents will add giving 1.36mA which will give you a small shock.
     
    Last edited: Aug 10, 2009
  16. bountyhunter

    bountyhunter Well-Known Member

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    The general threshold voltage for safety is usually given as around 60V. 110VAC is about 150V (peak) with respect to the neutral, so it does bite you.

    A 28VAC transformer should be safe to touch.
     
    Last edited: Aug 10, 2009
  17. bountyhunter

    bountyhunter Well-Known Member

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    Because your skin has a certain impedance so it takes a specific amount of voltage to force that current to flow through it.
     
  18. bountyhunter

    bountyhunter Well-Known Member

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    RIP

    That is definitely not OK
     
  19. giftiger_wunsch

    giftiger_wunsch New Member

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    28mA is enough to produce a painful shock and can also cause minor burns if prolonged. A "slight tingle" is more like 1-8mA. 1k is also a very low estimate for resistance through your body. Try tightly gripping the contacts of a multimeter. It is difficult to get a true reading but you'll never get one as low at 1k.
     
    Last edited: Aug 10, 2009
  20. be80be

    be80be Well-Known Member

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    You all can play if you want But it's like I said the worst shock I have got was from a 48volt ac phone line and had it went threw me and not just threw my arm it would of got me. You don't get shocked on a dry day with idea condition
     
  21. giftiger_wunsch

    giftiger_wunsch New Member

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    Depends on the voltage but if you're dry you'll generally have a much higher resistance, so less current can flow. It stands to reason that you're less likely to be shocked if your skin is very dry.

    My friend claimed he was shocked by a 12V car battery with completely dry hands before but I have a strong suspicion he was being paranoid because he assumes something as powerful as a car battery should be able to harm him. Most likely his hands were very sweaty from working on the vehicle and he got a mild shock which he grossly exaggerated :rolleyes: He's used heavy-duty gloves to handle 12V batteries ever since :D
     

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